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We present results on transverse momentum (pT) and rapidity (y) differential production cross sections, mean transverse momentum and mean transverse momentum square of inclusive J/ψ and ψ(2S) at forward rapidity (2.5 < y < 4) as well as ψ(2S)-to-J/ψ cross section ratios. These quantities are measured in pp collisions at center of mass energies s√=5.02 and 13 TeV with the ALICE detector. Both charmonium states are reconstructed in the dimuon decay channel, using the muon spectrometer. A comprehensive comparison to inclusive charmonium cross sections measured at s√=2.76, 7 and 8 TeV is performed. A comparison to non-relativistic quantum chromodynamics and fixed-order next-to-leading logarithm calculations, which describe prompt and non-prompt charmonium production respectively, is also presented. A good description of the data is obtained over the full pT range, provided that both contributions are summed. In particular, it is found that for pT > 15 GeV/c the non-prompt contribution reaches up to 50% of the total charmonium yield.
The transverse momentum distributions of the strange and double-strange hyperon resonances (Σ(1385)±,Ξ(1530)0) produced in p–Pb collisions at √sNN = 5.02 TeV were measured in the rapidity range −0.5<yCMS<0 for event classes corresponding to different charged-particle multiplicity densities, ⟨dNch/dηlab⟩. The mean transverse momentum values are presented as a function of ⟨dNch/dηlab⟩, as well as a function of the particle masses and compared with previous results on hyperon production. The integrated yield ratios of excited to ground-state hyperons are constant as a function of ⟨dNch/dηlab⟩. The equivalent ratios to pions exhibit an increase with ⟨dNch/dηlab⟩, depending on their strangeness content.
The production of K∗(892)0 and ϕ(1020) mesons has been measured in p–Pb collisions at √sNN = 5.02 TeV. K∗0 and ϕ are reconstructed via their decay into charged hadrons with the ALICE detector in the rapidity range - 0.5 < y < 0. The transverse momentum spectra, measured as a function of the multiplicity, have a pT range from 0 to 15 GeV/c for K∗0 and from 0.3 to 21 GeV/c for ϕ. Integrated yields, mean transverse momenta and particle ratios are reported and compared with results in pp collisions at √s = 7 TeV and Pb–Pb collisions at √sNN = 2.76 TeV. In Pb–Pb and p–Pb collisions, K∗0 and ϕ probe the hadronic phase of the system and contribute to the study of particle formation mechanisms by comparison with other identified hadrons. For this purpose, the mean transverse momenta and the differential proton-to-ϕ ratio are discussed as a function of the multiplicity of the event. The short-lived K∗0 is measured to investigate re-scattering effects, believed to be related to the size of the system and to the lifetime of the hadronic phase.
We present the charged-particle pseudorapidity density in Pb–Pb collisions at √sNN = 5.02 TeV in centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from −3.5 to 5, which is sufficient for reliable estimates of the total number of charged particles produced in the collisions. For the most central (0–5%) collisions we find 21 400 ± 1 300, while for the most peripheral (80–90%) we find 230 ± 38. This corresponds to an increase of (27 ± 4)% over the results at √sNN = 2.76 TeV previously reported by ALICE. The energy dependence of the total number of charged particles produced in heavy-ion collisions is found to obey a modified power-law like behaviour. The chargedparticle pseudorapidity density of the most central collisions is compared to model calculations — none of which fully describes the measured distribution. We also present an estimate of the rapidity density of charged particles. The width of that distribution is found to exhibit a remarkable proportionality to the beam rapidity, independent of the collision energy from the top SPS to LHC energies.
The production of electrons from heavy-flavour hadron decays was measured as a function of transverse momentum (pT) in minimum-bias p–Pb collisions at √sNN = 5.02 TeV using the ALICE detector at the LHC. The measurement covers the pT interval 0.5 < pT < 12 GeV/c and the rapidity range −1.065 < ycms < 0.135 in the centre-of-mass reference frame. The contribution of electrons from background sources was subtracted using an invariant mass approach. The nuclear modification factor RpPb was calculated by comparing the pT-differential invariant cross section in p–Pb collisions to a pp reference at the same centre-of-mass energy, which was obtained by interpolating measurements at √s = 2.76 TeV and √s = 7 TeV. The RpPb is consistent with unity within uncertainties of about 25%, which become larger for pT below 1 GeV/c. The measurement shows that heavy-flavour production is consistent with binary scaling, so that a suppression in the high-pT yield in Pb–Pb collisions has to be attributed to effects induced by the hot medium produced in the final state. The data in p–Pb collisions are described by recent model calculations that include cold nuclear matter effects.
Two-particle angular correlations were measured in pp collisions at s√=7 TeV for pions, kaons, protons, and lambdas, for all particle/anti-particle combinations in the pair. Data for mesons exhibit an expected peak dominated by effects associated with mini-jets and are well reproduced by general purpose Monte Carlo generators. However, for baryon–baryon and anti-baryon–anti-baryon pairs, where both particles have the same baryon number, a near-side anti-correlation structure is observed instead of a peak. This effect is interpreted in the context of baryon production mechanisms in the fragmentation process. It currently presents a challenge to Monte Carlo models and its origin remains an open question.
We have identified a mistake in how Fig. 1 is referenced in the text of the article Eur. Phys. J. C 77 (2017) no. 8, 569 which affected three paragraphs of the results section. The corrected three paragraphs as well as the unmodified accompanying figure are reproduced in this document with the correct labeling.
In addition, an editing issue led to a missing acknowledgements section. The missing section is reproduced at the end of this document in the manner in which it should have appeared in the published article.
The invariant differential cross sections for inclusive π0 and η mesons at midrapidity were measured in pp collisions at s√=2.76 TeV for transverse momenta 0.4<pT<40 GeV/c and 0.6<pT<20 GeV/c, respectively, using the ALICE detector. This large range in pT was achieved by combining various analysis techniques and different triggers involving the electromagnetic calorimeter (EMCal). In particular, a new single-cluster, shower-shape based method was developed for the identification of high-pT neutral pions, which exploits that the showers originating from their decay photons overlap in the EMCal. Above 4 GeV/c, the measured cross sections are found to exhibit a similar power-law behavior with an exponent of about 6.3. Next-to-leading-order perturbative QCD calculations differ from the measured cross sections by about 30% for the π0, and between 30–50% for the η meson, while generator-level simulations with PYTHIA 8.2 describe the data to better than 10–30%, except at pT<1 GeV/c. The new data can therefore be used to further improve the theoretical description of π0 and η meson production.
The azimuthal correlations of D mesons with charged particles were measured with the ALICE apparatus in pp collisions at s√=7 TeV and p–Pb collisions at sNN−−−√=5.02 TeV at the Large Hadron Collider. D0, D+, and D∗+ mesons and their charge conjugates with transverse momentum 3<pT<16 GeV/c and rapidity in the nucleon-nucleon centre-of-mass system |ycms|<0.5 (pp collisions) and −0.96<ycms<0.04 (p–Pb collisions) were correlated to charged particles with pT>0.3 GeV/c. The yield of charged particles in the correlation peak induced by the jet containing the D meson and the peak width are compatible within uncertainties in the two collision systems. The data are described within uncertainties by Monte-Carlo simulations based on PYTHIA, POWHEG, and EPOS 3 event generators.
The transverse momentum distributions of the strange and double-strange hyperon resonances (Σ(1385)±, Ξ(1530)0) produced in p–Pb collisions at sNN−−−√=5.02 TeV were measured in the rapidity range −0.5<yCMS<0 for event classes corresponding to different charged-particle multiplicity densities, ⟨dNch/dηlab⟩. The mean transverse momentum values are presented as a function of ⟨dNch/dηlab⟩, as well as a function of the particle masses and compared with previous results on hyperon production. The integrated yield ratios of excited to ground-state hyperons are constant as a function of ⟨dNch/dηlab⟩. The equivalent ratios to pions exhibit an increase with ⟨dNch/dηlab⟩, depending on their strangeness content.
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Collective behavior has been observed in high-energy heavy-ion collisions for several decades. Collectivity is driven by the high particle multiplicities that are produced in these collisions. At the Large Hadron Collider (LHC), features of collectivity have also been seen in high-multiplicity proton-proton collisions that can attain particle multiplicities comparable to peripheral Pb-Pb collisions. One of the possible signatures of collective behavior is the decrease of femtoscopic radii extracted from pion and kaon pairs emitted from high-multiplicity collisions with increasing pair transverse momentum. This decrease can be described in terms of an approximate transverse mass scaling. In the present work, femtoscopic analyses are carried out by the ALICE collaboration on charged pion and kaon pairs produced in pp collisions at s√=13 TeV from the LHC to study possible collectivity in pp collisions. The event-shape analysis method based on transverse sphericity is used to select for spherical versus jet-like events, and the effects of this selection on the femtoscopic radii for both charged pion and kaon pairs are studied. This is the first time this selection method has been applied to charged kaon pairs. An approximate transverse-mass scaling of the radii is found in all multiplicity ranges studied when the difference in the Lorentz boost for pions and kaons is taken into account. This observation does not support the hypothesis of collective expansion of hot and dense matter that should only occur in high-multiplicity events. A possible alternate explanation of the present results is based on a scenario of common emission conditions for pions and kaons in pp collisions for the multiplicity ranges studied.
This Letter presents the most precise measurement to date of the matter/antimatter imbalance at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair sNN−−−√=5.02 TeV. Using the Statistical Hadronization framework, it is possible to obtain the value of the electric charge and baryon chemical potentials, μQ=−0.18±0.90 MeV and μB=0.71±0.45 MeV, with unprecedented precision. A centrality-differential study of the antiparticle-to-particle yield ratios of charged pions, protons, Ω-baryons, and light (hyper)nuclei is performed. These results indicate that the system created in Pb-Pb collisions at the LHC is on average baryon-free and electrically neutral at midrapidity.
K+K− pairs may be produced in photonuclear collisions, either from the decays of photoproduced ϕ(1020) mesons, or directly as non-resonant K+K− pairs. Measurements of K+K− photoproduction probe the couplings between the ϕ(1020) and charged kaons with photons and nuclear targets. We present the first measurement of coherent photoproduction of K+K− pairs on lead ions in ultra-peripheral collisions using the ALICE detector, including the first investigation of direct K+K− production. There is significant K+K− production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range 1.1<MKK<1.4 GeV/c2 above the ϕ(1020) resonance, for rapidity |yKK|<0.8 and pT,KK<0.1 GeV/c, the measured coherent photoproduction cross section is dσ/dy = 3.37 ± 0.61 (stat.) ± 0.15 (syst.) mb. The centre-of-mass energy per nucleon of the photon-nucleus (Pb) system WγPb,n ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for ϕ(1020) photoproduction alone. The mass spectrum is fit to a cocktail consisting of ϕ(1020) decays, direct K+K− photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct K+K− photoproduction are presented.
Studying strangeness and baryon production mechanisms through angular correlations between charged
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The angular correlations between charged Ξ baryons and associated identified hadrons (pions, kaons, protons, Λ baryons, and Ξ baryons) are measured in pp collisions at s√=13 TeV with the ALICE detector to give insight into the particle production mechanisms and balancing of quantum numbers on the microscopic level. In particular, the distribution of strangeness is investigated in the correlations between the doubly-strange Ξ baryon and mesons and baryons that contain a single strange quark, K and Λ. As a reference, the results are compared to Ξπ and Ξp correlations, where the associated mesons and baryons do not contain a strange valence quark. These measurements are expected to be sensitive to whether strangeness is produced through string breaking or in a thermal production scenario. Furthermore, the multiplicity dependence of the correlation functions is measured to look for the turn-on of additional particle production mechanisms with event activity. The results are compared to predictions from the string-breaking model PYTHIA 8, including tunes with baryon junctions and rope hadronisation enabled, the cluster hadronisation ly or qualitatively by the Monte Carlo models, no one model can match all features of the data. These results provide stringent constraints on the strangeness and baryon number production mechanisms in pp collisions.
The Chiral Magnetic Wave (CMW) phenomenon is essential to provide insights into the strong interaction in QCD, the properties of the quark-gluon plasma, and the topological characteristics of the early universe, offering a deeper understanding of fundamental physics in high-energy collisions. Measurements of the charge-dependent anisotropic flow coefficients are studied in Pb-Pb collisions at center-of-mass energy per nucleon-nucleon collision sNN−−−√= 5.02 TeV to probe the CMW. In particular, the slope of the normalized difference in elliptic (v2) and triangular (v3) flow coefficients of positively and negatively charged particles as a function of their event-wise normalized number difference, is reported for inclusive and identified particles. The slope rNorm3 is found to be larger than zero and to have a magnitude similar to rNorm2, thus pointing to a large background contribution for these measurements. Furthermore, rNorm2 can be described by a blast wave model calculation that incorporates local charge conservation. In addition, using the event shape engineering technique yields a fraction of CMW (fCMW) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit for fCMW, and in the 10-60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level.
The first measurement of the e+e− pair production at midrapidity and low invariant mass in central Pb−Pb collisions at sNN−−−√=5.02 TeV at the LHC is presented. The yield of e+e− pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass (mee) and pair transverse momentum (pT,ee) ranges mee<3.5 GeV/c2 and pT,ee<8 GeV/c. For 0.18<mee<0.5 GeV/c2 the ratio of data to the cocktail of hadronic contributions without ρ mesons amounts to 1.42±0.12 (stat.)±0.17 (syst.)±0.12 (cocktail) and 1.44±0.12 (stat.)±0.17 (syst.)+0.17−0.21 (cocktail), including or not including medium effects in the estimation of the heavy-flavor background, respectively. It is consistent with predictions from two different models for an additional contribution of thermal e+e− pairs from the hadronic and partonic phases. In the intermediate-mass range (1.2<mee<2.6 GeV/c2), the pair transverse impact parameter of the e+e− pairs (DCAee) is used for the first time in Pb−Pb collisions to separate displaced dielectrons from heavy-flavor hadron decays from a possible (thermal) contribution produced at the interaction point. The data are consistent with a suppression of e+e− pairs from cc¯¯ and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb−Pb collisions at sNN−−−√=5.02 TeV is reported via the study of virtual direct photons in the transverse momentum range 1<pT<5 GeV/c. A model including prompt photons, as well as photons from the pre-equilibrium and fluid-dynamic phases, can reproduce the result, while being at the upper edge of the data uncertainties.
The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum (high pT) hadron trigger in proton−proton and central Pb−Pb collisions at sNN−−−√=5.02 TeV. A data-driven statistical method is used to mitigate the large uncorrelated background in central Pb−Pb collisions. Recoil jet distributions are reported for jet resolution parameter R=0.2, 0.4, and 0.5 in the range 7<pT,jet<140 GeV/c and trigger−recoil jet azimuthal separation π/2<Δφ<π. The measurements exhibit a marked medium-induced jet yield enhancement at low pT and at large azimuthal deviation from Δφ∼π. The enhancement is characterized by its dependence on Δφ, which has a slope that differs from zero by 4.7σ. Comparisons to model calculations incorporating different formulations of jet quenching are reported. These comparisons indicate that the observed yield enhancement arises from the response of the QGP medium to jet propagation.
The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum (high pT) hadron trigger in proton−proton and central Pb−Pb collisions at sNN−−−√=5.02 TeV. A data-driven statistical method is used to mitigate the large uncorrelated background in central Pb−Pb collisions. Recoil jet distributions are reported for jet resolution parameter R=0.2, 0.4, and 0.5 in the range 7<pT,jet<140 GeV/c and trigger−recoil jet azimuthal separation π/2<Δφ<π. The measurements exhibit a marked medium-induced jet yield enhancement at low pT and at large azimuthal deviation from Δφ∼π. The enhancement is characterized by its dependence on Δφ, which has a slope that differs from zero by 4.7σ. Comparisons to model calculations incorporating different formulations of jet quenching are reported. These comparisons indicate that the observed yield enhancement arises from the response of the QGP medium to jet propagation.
The ALICE Collaboration reports measurements of the semi-inclusive distribution of charged-particle jets recoiling from a high transverse momentum (high pT) charged hadron, in pp and central Pb−Pb collisions at center of mass energy per nucleon−nucleon collision sNN−−−√=5.02 TeV. The large uncorrelated background in central Pb−Pb collisions is corrected using a data-driven statistical approach, which enables precise measurement of recoil jet distributions over a broad range in pT,chjet and jet resolution parameter R. Recoil jet yields are reported for R=0.2, 0.4, and 0.5 in the range 7<pT,chjet<140 GeV/c and π/2<Δφ<π, where Δφ is the azimuthal angular separation between hadron trigger and recoil jet. The low pT,chjet reach of the measurement explores unique phase space for studying jet quenching, the interaction of jets with the quark-gluonnplasma generated in high-energy nuclear collisions. Comparison of pT,chjet distributions from pp and central Pb−Pb collisions probes medium-induced jet energy loss and intra-jet broadening, while comparison of their acoplanarity distributions explores in-medium jet scattering and medium response. The measurements are compared to theoretical calculations incorporating jet quenching.
The ALICE Collaboration reports measurements of the semi-inclusive distribution of charged-particle jets recoiling from a high transverse momentum (high pT) charged hadron, in pp and central Pb−Pb collisions at center of mass energy per nucleon−nucleon collision sNN−−−√=5.02 TeV. The large uncorrelated background in central Pb−Pb collisions is corrected using a data-driven statistical approach, which enables precise measurement of recoil jet distributions over a broad range in pT,chjet and jet resolution parameter R. Recoil jet yields are reported for R=0.2, 0.4, and 0.5 in the range 7<pT,chjet<140 GeV/c and π/2<Δφ<π, where Δφ is the azimuthal angular separation between hadron trigger and recoil jet. The low pT,chjet reach of the measurement explores unique phase space for studying jet quenching, the interaction of jets with the quark-gluonnplasma generated in high-energy nuclear collisions. Comparison of pT,chjet distributions from pp and central Pb−Pb collisions probes medium-induced jet energy loss and intra-jet broadening, while comparison of their acoplanarity distributions explores in-medium jet scattering and medium response. The measurements are compared to theoretical calculations incorporating jet quenching.
The first measurements of skewness and kurtosis of mean transverse momentum (⟨pT⟩) fluctuations are reported in Pb−Pb collisions at sNN−−−√ = 5.02 TeV, Xe−Xe collisions at sNN−−−√ = 5.44 TeV and pp collisions at s√=5.02 TeV using the ALICE detector. The measurements are carried out as a function of system size ⟨dNch/dη⟩1/3|η|<0.5, using charged particles with transverse momentum (pT) and pseudorapidity (η), in the range 0.2<pT<3.0 GeV/c and |η|<0.8, respectively. In Pb−Pb and Xe−Xe collisions, positive skewness is observed in the fluctuations of ⟨pT⟩ for all centralities, which is significantly larger than what would be expected in the scenario of independent particle emission. This positive skewness is considered a crucial consequence of the hydrodynamic evolution of the hot and dense nuclear matter created in heavy-ion collisions. Furthermore, similar observations of positive skewness for minimum bias pp collisions are also reported here. Kurtosis of ⟨pT⟩ fluctuations is found to be in good agreement with the kurtosis of Gaussian distribution, for most central Pb−Pb collisions. Hydrodynamic model calculations with MUSIC using Monte Carlo Glauber initial conditions are able to explain the measurements of both skewness and kurtosis qualitatively from semicentral to central collisions in Pb--Pb system. Color reconnection mechanism in PYTHIA8 model seems to play a pivotal role in capturing the qualitative behavior of the same measurements in pp collisions.
Deuterons are atomic nuclei composed of a neutron and a proton held together by the strong interaction. Unbound ensembles composed of a deuteron and a third nucleon have been investigated in the past using scattering experiments and they constitute a fundamental reference in nuclear physics to constrain nuclear interactions and the properties of nuclei. In this work K+−d and p−d femtoscopic correlations measured by the ALICE Collaboration in proton−proton (pp) collisions at s√=13 TeV at the Large Hadron Collider (LHC) are presented. It is demonstrated that correlations in momentum space between deuterons and kaons or protons allow us to study three-hadron systems at distances comparable with the proton radius. The analysis of the K+−d correlation shows that the relative distances at which deuterons and proton/kaons are produced are around 2 fm. The analysis of the p−d correlation shows that only a full three-body calculation that accounts for the internal structure of the deuteron can explain the data. In particular, the sensitivity of the observable to the short-range part of the interaction is demonstrated. These results indicate that correlations involving light nuclei in pp collisions at the LHC will also provide access to any three-body systems in the strange and charm sectors.
The ALICE Collaboration reports a search for jet quenching effects in high-multiplicity (HM) proton−proton collisions at s√ = 13 TeV, using the semi-inclusive azimuthal-difference distribution Δφ of charged-particle jets recoiling from a high transverse momentum (high-pT,trig) trigger hadron. Jet quenching may broaden the Δφ distribution measured in HM events compared to that in minimum bias (MB) events. The measurement employs a pT,trig-differential observable for data-driven suppression of the contribution of multiple partonic interactions, which is the dominant background. While azimuthal broadening is indeed observed in HM compared to MB events, similar broadening for HM events is observed for simulations based on the PYTHIA 8 Monte Carlo generator, which does not incorporate jet quenching. We elucidate the origin of the broadening by comparing biases induced by HM selection in the data and simulations, and discuss its implications for the study of jet quenching in small collision systems.
The first measurements of K∗(892)0 resonance production as a function of charged-particle multiplicity in Xe−Xe collisions at sNN−−−√= 5.44 TeV and pp collisions at s√= 5.02 TeV using the ALICE detector are presented. The resonance is reconstructed at midrapidity (|y|<0.5) using the hadronic decay channel K∗0→K±π∓. Measurements of transverse-momentum integrated yield, mean transverse-momentum, nuclear modification factor of K∗0, and yield ratios of resonance to stable hadron (K∗0/K) are compared across different collision systems (pp, p−Pb, Xe−Xe, and Pb−Pb) at similar collision energies to investigate how the production of K∗0 resonances depends on the size of the system formed in these collisions. The hadronic rescattering effect is found to be independent of the size of colliding systems and mainly driven by the produced charged-particle multiplicity, which is a proxy of the volume of produced matter at the chemical freeze-out. In addition, the production yields of K∗0 in Xe−Xe collisions are utilized to constrain the dependence of the kinetic freeze-out temperature on the system size using HRG-PCE model.
The production yields of the Σ(1385)± and Ξ(1530)0 resonances are measured in pp collisions at s√=13 TeV with ALICE. The measurements are performed as a function of the charged particle multiplicity ⟨dNch/dη⟩, which is related to the energy density produced in the collision. The results include transverse momentum (pT) distributions, pT-integrated yields, mean transverse momenta of Σ(1385)± and Ξ(1530)0, as well as ratios of the pT-integrated resonance yields relative to yields of other hadron species. The Σ(1385)±/π± and Ξ(1530)0/π± yield ratios are consistent with the trend of the enhancement of strangeness production from low to high multiplicity pp collisions, which was previously observed for strange and multi-strange baryons. The yield ratio between the measured resonances and the long-lived baryons with the same strangeness content exhibits a hint of a mild increasing trend at low multiplicity, despite too large uncertainties to exclude the flat behaviour. The results are compared to predictions from models such as EPOS-LHC and PYTHIA 8 with Rope shoving. The latter provides the best description of the multiplicity dependence of the Σ(1385)± and Ξ(1530)0 production in pp collisions at s√=13 TeV.
Measurements of the production cross sections of prompt D0, D+, D∗+, D+s, Λ+c, and Ξ+c charm hadrons at midrapidity in proton−proton collisions at s√=13 TeV with the ALICE detector are presented. The D-meson cross sections as a function of transverse momentum (pT) are provided with improved precision and granularity. The ratios of pT-differential meson production cross sections based on this publication and on measurements at different rapidity and collision energy provide a constraint on gluon parton distribution functions at low values of Bjorken-x (10−5−10−4). The measurements of Λ+c (Ξ+c) baryon production extend the measured pT intervals down to pT=0(3)~GeV/c. These measurements are used to determine the charm-quark fragmentation fractions and the cc¯¯ production cross section at midrapidity (|y|<0.5) based on the sum of the cross sections of the weakly-decaying ground-state charm hadrons D0, D+, D+s, Λ+c, Ξ0c and, for the first time, Ξ+c, and of the strongly-decaying J/psi mesons. The first measurements of Ξ+c and Σ0,++c fragmentation fractions at midrapidity are also reported. A significantly larger fraction of charm quarks hadronising to baryons is found compared to e+e− and ep collisions. The cc¯¯ production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations.
Measurements of the production cross sections of prompt D0, D+, D∗+, D+s, Λ+c, and Ξ+c charm hadrons at midrapidity in proton−proton collisions at s√=13 TeV with the ALICE detector are presented. The D-meson cross sections as a function of transverse momentum (pT) are provided with improved precision and granularity. The ratios of pT-differential meson production cross sections based on this publication and on measurements at different rapidity and collision energy provide a constraint on gluon parton distribution functions at low values of Bjorken-x (10−5−10−4). The measurements of Λ+c (Ξ+c) baryon production extend the measured pT intervals down to pT=0(3)~GeV/c. These measurements are used to determine the charm-quark fragmentation fractions and the cc¯¯ production cross section at midrapidity (|y|<0.5) based on the sum of the cross sections of the weakly-decaying ground-state charm hadrons D0, D+, D+s, Λ+c, Ξ0c and, for the first time, Ξ+c, and of the strongly-decaying J/psi mesons. The first measurements of Ξ+c and Σ0,++c fragmentation fractions at midrapidity are also reported. A significantly larger fraction of charm quarks hadronising to baryons is found compared to e+e− and ep collisions. The cc¯¯ production cross section at midrapidity is found to be at the upper bound of state-of-the-art perturbative QCD calculations.
Study of flavor dependence of the baryon-to-meson ratio in proton–proton collisions at √s= 13 TeV
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The production cross sections of D0 and Λ+c hadrons originating from beauty-hadron decays (i.e. non-prompt) were measured for the first time at midrapidity (|y|<0.5) by the ALICE Collaboration in proton-proton collisions at a center-of-mass energy s√=13 TeV. They are described within uncertainties by perturbative QCD calculations employing the fragmentation fractions of beauty quarks to baryons measured at forward rapidity by the LHCb Collaboration. The bb¯¯¯ production cross section per unit of rapidity at midrapidity, estimated from these measurements, is dσbb¯¯¯/dy||y|<0.5=83.1±3.5(stat.)±5.4(syst.)+12.3−3.2(extrap.)μb. The baryon-to-meson ratios are computed to investigate the hadronization mechanism of beauty quarks. The non-prompt Λ+c/D0 production ratio has a similar trend to the one measured for the promptly produced charmed particles and to the p/π+ and Λ/K0S ratios, suggesting a similar baryon-formation mechanism among light, strange, charm, and beauty hadrons. The pT-integrated non-prompt Λc/D0 ratio is found to be significantly higher than the one measured in e+e− collisions.
Results on the transverse spherocity dependence of light-flavor particle production (π, K, p, ϕ, K∗0, K0S, Λ, Ξ) at midrapidity in high-multiplicity pp collisions at s√=13 TeV were obtained with the ALICE apparatus. The transverse spherocity estimator (SpT=1O) categorizes events by their azimuthal topology. Utilizing narrow selections on SpT=1O, it is possible to contrast particle production in collisions dominated by many soft initial interactions with that observed in collisions dominated by one or more hard scatterings. Results are reported for two multiplicity estimators covering different pseudorapidity regions. The SpT=1O estimator is found to effectively constrain the hardness of the events when the midrapidity (|η|<0.8) estimator is used. The production rates of strange particles are found to be slightly higher for soft isotropic topologies, and severely suppressed in hard jet-like topologies. These effects are more pronounced for hadrons with larger mass and strangeness content, and observed when the topological selection is done within a narrow multiplicity interval. This demonstrates that an important aspect of the universal scaling of strangeness enhancement with final-state multiplicity is that high-multiplicity collisions are dominated by soft, isotropic processes. On the contrary, strangeness production in events with jet-like processes is significantly reduced. The results presented in this article are compared with several QCD-inspired Monte Carlo event generators. Models that incorporate a two-component phenomenology, either through mechanisms accounting for string density, or thermal production, are able to describe the observed strangeness enhancement as a function of SpT=1O.
Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at s√=13 TeV and p−Pb collisions at sNN−−−√=5.02 TeV. The correlation functions are measured as a function of relative azimuthal angle Δφ and pseudorapidity separation Δη for pairs of primary charged particles within the pseudorapidity interval |η|<0.9 and the transverse-momentum interval 1<pT<4 GeV/c. Flow coefficients are extracted for the long-range correlations (1.6<|Δη|<1.8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.
Modification of charged-particle jets in event-shape engineered Pb–Pb collisions at √sNN = 5.02 TeV
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Charged-particle jet yields have been measured in semicentral Pb−Pb collisions at center-of-mass energy per nucleon-nucleon collision sNN−−−√=5 TeV with the ALICE detector at the LHC. These yields are reported as a function of the jet transverse momentum, and further classified by their angle with respect to the event plane and the event shape, characterized by ellipticity, in an effort to study the path-length dependence of jet quenching. Jets were reconstructed at midrapidity from charged-particle tracks using the anti-kT algorithm with resolution parameters R= 0.2 and 0.4, with event-plane angle and event-shape values determined using information from forward scintillating detectors. The results presented in this letter show that, in semicentral Pb−Pb collisions, there is no significant difference between jet yields in predominantly isotropic and elliptical events. However, out-of-plane jets are observed to be more suppressed than in-plane jets. Further, this relative suppression is greater for low transverse momentum (< 50 GeV/c) R= 0.2 jets produced in elliptical events, with out-of-plane to in-plane jet-yield ratios varying up to 5.2σ between different event-shape classes. These results agree with previous studies indicating that jets experience azimuthally anisotropic suppression when traversing the QGP medium, and can provide additional constraints on the path-length dependence of jet energy loss.
The pseudorapidity dependence of elliptic (v2), triangular (v3), and quadrangular (v4) flow coefficients of charged particles measured in Pb-Pb collisions at a centre-of-mass energy per nucleon pair of sNN−−−√=5.02 TeV and in Xe-Xe collisions at sNN−−−√=5.44 TeV with ALICE at the LHC are presented. The measurements are performed in the pseudorapidity range −3.5<η<5 for various centrality intervals using two- and multi-particle cumulants with the subevent method. The flow probability density function (p.d.f.) is studied with the ratio of flow coefficient v2 calculated with four- and two-particle cumulant, and suggests that the variance of flow p.d.f. is independent of pseudorapidity. The decorrelation of the flow vector in the longitudinal direction is probed using two-particle correlations. The results measured with respect to different reference regions in pseudorapidity exhibit differences, argued to be a result of saturating decorrelation effect above a certain pseudorapidity separation, in contrast to previous publications which assign this observation to non-flow effects. The results are compared to 3+1 dimensional hydrodynamic and the AMPT transport model calculations. Neither of the models is able to simultaneously describe the pseudorapidity dependence of measurements of anisotropic flow and its fluctuations. The results presented in this work highlight shortcomings in our current understanding of initial conditions and subsequent system expansion in the longitudinal direction. Therefore, they provide input for its improvement.
Measurements of inclusive charged-particle jet production in pp and p-Pb collisions at center-of-mass energy per nucleon-nucleon collision sNN−−−√=5.02 TeV and the corresponding nuclear modification factor RchjetpPb are presented, using data collected with the ALICE detector at the LHC. Jets are reconstructed in the central rapidity region |ηjet|<0.5 from charged particles using the anti-kT algorithm with resolution parameters R=0.2, 0.3, and 0.4. The pT-differential inclusive production cross section of charged-particle jets, as well as the corresponding cross-section ratios, are reported for pp and p-Pb collisions in the transverse momentum range 10<pchT,jet<140 GeV/c and 10<pchT,jet<160 GeV/c, respectively, together with the nuclear modification factor RchjetpPb in the range 10<pchT,jet<140 GeV/c. The analysis extends the pT range of the previously-reported charged-particle jet measurements by the ALICE Collaboration. The nuclear modification factor is found to be consistent with one and independent of the jet resolution parameter with the improved precision of this study, indicating that the possible influence of cold nuclear matter effects on the production cross section of charged-particle jets in p-Pb collisions at sNN−−−√=5.02 TeV is smaller than the current precision. The obtained results are in agreement with other minimum bias jet measurements available for RHIC and LHC energies, and are well reproduced by the NLO perturbative QCD POWHEG calculations with parton shower provided by PYTHIA8 as well as by JETSCAPE simulations.
n this Letter, the first measurement of the inelastic cross section for antitriton−nucleus interactions is reported, covering the momentum range of 0.8≤p<2.4 GeV/c. The measurement is carried out using data recorded with the ALICE detector in pp and Pb−Pb collisions at a centre-of-mass energy per nucleon of 13 TeV and 5.02 TeV, respectively. The detector material serves as an absorber for antitriton nuclei. The raw yield of (anti)triton nuclei measured with the ALICE apparatus is compared to the results from detailed ALICE simulations based on the GEANT4 toolkit for the propagation of (anti)particles through matter, allowing one to quantify the inelastic interaction probability in the detector material. This analysis complements the measurement of the inelastic cross section of antinuclei up to A=3 carried out by the ALICE Collaboration, and demonstrates the feasibility of the study of the isospin dependence of inelastic interaction cross section with the analysis techniques presented in this Letter.
In this Letter, the first measurement of the inelastic cross section for antitriton−nucleus interactions is reported, covering the momentum range of 0.8≤p<2.4 GeV/c. The measurement is carried out using data recorded with the ALICE detector in pp and Pb−Pb collisions at a centre-of-mass energy per nucleon of 13 TeV and 5.02 TeV, respectively. The detector material serves as an absorber for antitriton nuclei. The raw yield of (anti)triton nuclei measured with the ALICE apparatus is compared to the results from detailed ALICE simulations based on the GEANT4 toolkit for the propagation of (anti)particles through matter, allowing one to quantify the inelastic interaction probability in the detector material. This analysis complements the measurement of the inelastic cross section of antinuclei up to A=3 carried out by the ALICE Collaboration, and demonstrates the feasibility of the study of the isospin dependence of inelastic interaction cross section with the analysis techniques presented in this Letter.
The interaction between Λ baryons and kaons/antikaons is a crucial ingredient for the strangeness S=0 and S=−2 sector of the meson--baryon interaction at low energies. In particular, the ΛK¯¯¯¯ might help in understanding the origin of states such as the Ξ(1620), whose nature and properties are still under debate. Experimental data on Λ−K and Λ−K¯¯¯¯ systems are scarce, leading to large uncertainties and tension between the available theoretical predictions constrained by such data. In this Letter we present the measurements of Λ−K+⊕Λ¯¯¯¯−K− and Λ−K−⊕Λ¯¯¯¯−K+ correlations obtained in the high-multiplicity triggered data sample in pp collisions at s√=13 TeV recorded by ALICE at the LHC. The correlation function for both pairs is modeled using the Lednicky−Lyuboshits analytical formula and the corresponding scattering parameters are extracted. The Λ−K−⊕Λ¯¯¯¯−K+ correlations show the presence of several structures at relative momenta k∗ above 200 MeV/c, compatible with the Ω baryon, the Ξ(1690), and Ξ(1820) resonances decaying into Λ−K− pairs. The low k∗ region in the Λ−K−⊕Λ¯¯¯¯−K+ also exhibits the presence of the Ξ(1620) state, expected to strongly couple to the measured pair. The presented data allow to access the ΛK+ and ΛK− strong interaction with an unprecedented precision and deliver the first experimental observation of the Ξ(1620) decaying into ΛK−.
The interaction between Λ baryons and kaons/antikaons is a crucial ingredient for the strangeness S=0 and S=−2 sector of the meson--baryon interaction at low energies. In particular, the ΛK¯¯¯¯ might help in understanding the origin of states such as the Ξ(1620), whose nature and properties are still under debate. Experimental data on Λ−K and Λ−K¯¯¯¯ systems are scarce, leading to large uncertainties and tension between the available theoretical predictions constrained by such data. In this Letter we present the measurements of Λ−K+⊕Λ¯¯¯¯−K− and Λ−K−⊕Λ¯¯¯¯−K+ correlations obtained in the high-multiplicity triggered data sample in pp collisions at s√=13 TeV recorded by ALICE at the LHC. The correlation function for both pairs is modeled using the Lednicky−Lyuboshits analytical formula and the corresponding scattering parameters are extracted. The Λ−K−⊕Λ¯¯¯¯−K+ correlations show the presence of several structures at relative momenta k∗ above 200 MeV/c, compatible with the Ω baryon, the Ξ(1690), and Ξ(1820) resonances decaying into Λ−K− pairs. The low k∗ region in the Λ−K−⊕Λ¯¯¯¯−K+ also exhibits the presence of the Ξ(1620) state, expected to strongly couple to the measured pair. The presented data allow to access the ΛK+ and ΛK− strong interaction with an unprecedented precision and deliver the first experimental observation of the Ξ(1620) decaying into ΛK−.
The interaction between Λ baryons and kaons/antikaons is a crucial ingredient for the strangeness S=0 and S=−2 sector of the meson−baryon interaction at low energies. In particular, the ΛK¯¯¯¯ might help in understanding the origin of states such as the Ξ(1620), whose nature and properties are still under debate. Experimental data on Λ−K and Λ−K¯¯¯¯ systems are scarce, leading to large uncertainties and tension between the available theoretical predictions constrained by such data. In this Letter we present the measurements of Λ−K+⊕Λ¯¯¯¯−K− and Λ−K−⊕Λ¯¯¯¯−K+ correlations obtained in the high-multiplicity triggered data sample in pp collisions at s√=13 TeV recorded by ALICE at the LHC. The correlation function for both pairs is modeled using the Lednicky−Lyuboshits analytical formula and the corresponding scattering parameters are extracted. The Λ−K−⊕Λ¯¯¯¯−K+ correlations show the presence of several structures at relative momenta k∗ above 200 MeV/c, compatible with the Ω baryon, the Ξ(1690), and Ξ(1820) resonances decaying into Λ−K− pairs. The low k∗ region in the Λ−K−⊕Λ¯¯¯¯−K+ also exhibits the presence of the Ξ(1620) state, expected to strongly couple to the measured pair. The presented data allow to access the ΛK+ and ΛK− strong interaction with an unprecedented precision and deliver the first experimental observation of the Ξ(1620) decaying into ΛK−.
The first measurement of the cross section for incoherent photonuclear production of J/ψ vector meson as a function of the Mandelstam |t| variable is presented. The measurement was carried out with the ALICE detector at midrapidity, |y|<0.8, using ultra-peripheral collisions of Pb nuclei at a centre-of-mass energy per nucleon pair sNN−−−√=5.02 TeV. This rapidity interval corresponds to a Bjorken-x range (0.3−1.4)×10−3. Cross sections are reported in five |t| intervals in the range 0.04<|t|<1~GeV2 and compared to the predictions of different models. Models that ignore quantum fluctuations of the gluon density in the colliding hadron predict a |t|-dependence of the cross section much steeper than in data. The inclusion of such fluctuations in the same models provides a better description of the data.
This Letter presents the measurement of near-side associated per-trigger yields, denoted ridge yields, from the analysis of angular correlations of charged hadrons in proton-proton collisions at s√ = 13 TeV. Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of 1.4<|Δη|<1.8 and a transverse momentum of 1<pT<2 GeV/c, as a function of the charged-particle multiplicity measured at midrapidity. This study extends the measurements of the ridge yield to the low multiplicity region, where in hadronic collisions it is typically conjectured that a strongly-interacting medium is unlikely to be formed. The precision of the new results allows for the first direct quantitative comparison with the results obtained in e+e− collisions at s√ = 91 GeV, where initial-state effects such as pre-equilibrium dynamics and collision geometry are not expected to play a role. In the multiplicity range where the e+e− results have good precision, the measured ridge yields in pp collisions are substantially larger than the limits set in e+e− annihilations. Consequently, the findings presented in this Letter suggest that the processes involved in e+e− annihilations do not contribute significantly to the emergence of long-range correlations in pp collisions.
In this Letter, the first measurement of the inelastic cross section for antitriton−nucleus interactions is reported, covering the momentum range of 0.8≤p<2.4 GeV/c. The measurement is carried out using data recorded with the ALICE detector in pp and Pb−Pb collisions at a centre-of-mass energy per nucleon of 13 TeV and 5.02 TeV, respectively. The detector material serves as an absorber for antitriton nuclei. The raw yield of (anti)triton nuclei measured with the ALICE apparatus is compared to the results from detailed ALICE simulations based on the GEANT4 toolkit for the propagation of (anti)particles through matter, allowing one to quantify the inelastic interaction probability in the detector material. This analysis complements the measurement of the inelastic cross section of antinuclei up to A=3 carried out by the ALICE Collaboration, and demonstrates the feasibility of the study of the isospin dependence of inelastic interaction cross section with the analysis techniques presented in this Letter.
The femtoscopic study of pairs of identical pions is particularly suited to investigate the effective source function of particle emission, due to the resulting Bose-Einstein correlation signal. In small collision systems at the LHC, pp in particular, the majority of the pions are produced in resonance decays, which significantly affect the profile and size of the source. In this work, we explicitly model this effect in order to extract the primordial source in pp collisions at s√=13 TeV from charged π-π correlations measured by ALICE. We demonstrate that the assumption of a Gaussian primordial source is compatible with the data and that the effective source, resulting from modifications due to resonances, is approximately exponential, as found in previous measurements at the LHC. The universality of hadron emission in pp collisions is further investigated by applying the same methodology to characterize the primordial source of K-p pairs. The size of the primordial source is evaluated as a function of the transverse mass (mT) of the pairs, leading to the observation of a common scaling for both π-π and K-p, suggesting a collective effect. Further, the present results are compatible with the mT scaling of the p-p and p−Λ primordial source measured by ALICE in high multiplicity pp collisions, providing compelling evidence for the presence of a common emission source for all hadrons in small collision systems at the LHC. This will allow the determination of the source function for any hadron--hadron pairs with high precision, granting access to the properties of the possible final-state interaction among pairs of less abundantly produced hadrons, such as strange or charmed particles.
We report the first results of elliptic (v2), triangular (v3) and quadrangular flow (v4) of charged particles in Pb-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the CERN Large Hadron Collider. The measurements are performed in the central pseudorapidity region |η|<0.8 and for the transverse momentum range 0.2<pT<5 GeV/c. The anisotropic flow is measured using two-particle correlations with a pseudorapidity gap greater than one unit and with the multi-particle cumulant method. Compared to results from Pb-Pb collisions at sNN−−−√=2.76 TeV, the anisotropic flow coefficients v2, v3 and v4 are found to increase by (3.0±0.6)%, (4.3±1.4)% and (10.2±3.8)%, respectively, in the centrality range 0-50%. This increase can be attributed mostly to an increase of the average transverse momentum between the two energies. The measurements are found to be compatible with hydrodynamic model calculations. This comparison provides a unique opportunity to test the validity of the hydrodynamic picture and the power to further discriminate between various possibilities for the temperature dependence of shear viscosity to entropy density ratio of the produced matter in heavy-ion collisions at the highest energies.
We report the first results of elliptic (v2), triangular (v3) and quadrangular flow (v4) of charged particles in Pb-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the CERN Large Hadron Collider. The measurements are performed in the central pseudorapidity region |η|<0.8 and for the transverse momentum range 0.2<pT<5 GeV/c. The anisotropic flow is measured using two-particle correlations with a pseudorapidity gap greater than one unit and with the multi-particle cumulant method. Compared to results from Pb-Pb collisions at sNN−−−√=2.76 TeV, the anisotropic flow coefficients v2, v3 and v4 are found to increase by (3.0±0.6)%, (4.3±1.4)% and (10.2±3.8)%, respectively, in the centrality range 0-50%. This increase can be attributed mostly to an increase of the average transverse momentum between the two energies. The measurements are found to be compatible with hydrodynamic model calculations. This comparison provides a unique opportunity to test the validity of the hydrodynamic picture and the power to further discriminate between various possibilities for the temperature dependence of shear viscosity to entropy density ratio of the produced matter in heavy-ion collisions at the highest energies.
We report the first results of elliptic (v2), triangular (v3) and quadrangular flow (v4) of charged particles in Pb-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the CERN Large Hadron Collider. The measurements are performed in the central pseudorapidity region |η|<0.8 and for the transverse momentum range 0.2<pT<5 GeV/c. The anisotropic flow is measured using two-particle correlations with a pseudorapidity gap greater than one unit and with the multi-particle cumulant method. Compared to results from Pb-Pb collisions at sNN−−−√=2.76 TeV, the anisotropic flow coefficients v2, v3 and v4 are found to increase by (3.0±0.6)%, (4.3±1.4)% and (10.2±3.8)%, respectively, in the centrality range 0-50%. This increase can be attributed mostly to an increase of the average transverse momentum between the two energies. The measurements are found to be compatible with hydrodynamic model calculations. This comparison provides a unique opportunity to test the validity of the hydrodynamic picture and the power to further discriminate between various possibilities for the temperature dependence of shear viscosity to entropy density ratio of the produced matter in heavy-ion collisions at the highest energies.
Background: In the context of the investigation of the quark gluon plasma produced in heavy-ion collisions, hadrons containing heavy (charm or beauty) quarks play a special role for the characterization of the hot and dense medium created in the interaction. The measurement of the production of charm and beauty hadrons in proton–proton collisions, besides providing the necessary reference for the studies in heavy-ion reactions, constitutes an important test of perturbative quantum chromodynamics (pQCD) calculations. Heavy-flavor production in proton–nucleus collisions is sensitive to the various effects related to the presence of nuclei in the colliding system, commonly denoted cold-nuclear-matter effects. Most of these effects are expected to modify open-charm production at low transverse momenta (pT) and, so far, no measurement of D-meson production down to zero transverse momentum was available at mid-rapidity at the energies attained at the CERN Large Hadron Collider (LHC).
Purpose: The measurements of the production cross sections of promptly produced charmed mesons in p-Pb collisions at the LHC down to pT=0 and the comparison to the results from pp interactions are aimed at the assessment of cold-nuclear-matter effects on open-charm production, which is crucial for the interpretation of the results from Pb-Pb collisions.
Methods: The prompt charmed mesons D0,D+,D*+, and D+s were measured at mid-rapidity in p-Pb collisions at a center-of-mass energy per nucleon pair √sNN=5.02 TeV with the ALICE detector at the LHC. D mesons were reconstructed from their decays D0→K−π+,D+→K−π+π+, D*+→D0π+,D+s→ϕπ+→K−K+π+, and their charge conjugates, using an analysis method based on the selection of decay topologies displaced from the interaction vertex. In addition, the prompt D0 production cross section was measured in pp collisions at √s=7 TeV and p-Pb collisions at √sNN=5.02 TeV down to pT=0 using an analysis technique that is based on the estimation and subtraction of the combinatorial background, without reconstruction of the D0 decay vertex.
Results: The production cross section in pp collisions is described within uncertainties by different implementations of pQCD calculations down to pT=0. This allowed also a determination of the total c¯c production cross section in pp collisions, which is more precise than previous ALICE measurements because it is not affected by uncertainties owing to the extrapolation to pT=0. The nuclear modification factor RpPb(pT), defined as the ratio of the pT-differential D meson cross section in p-Pb collisions and that in pp collisions scaled by the mass number of the Pb nucleus, was calculated for the four D-meson species and found to be compatible with unity within uncertainties. The results are compared to theoretical calculations that include cold-nuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks with an expanding deconfined medium.
Conclusions: These measurements add experimental evidence that the modification of the D-meson transverse momentum distributions observed in Pb–Pb collisions with respect to pp interactions is due to strong final-state effects induced by the interactions of the charm quarks with the hot and dense partonic medium created in ultrarelativistic heavy-ion collisions. The current precision of the measurement does not allow us to draw conclusions on the role of the different cold-nuclear-matter effects and on the possible presence of additional hot-medium effects in p-Pb collisions. However, the analysis technique without decay-vertex reconstruction, applied on future larger data samples, should provide access to the physics-rich range down to pT=0.
The production cross sections of the prompt charmed mesons D0, D+, D∗+ and Ds were measured at mid-rapidity in p-Pb collisions at a centre-of-mass energy per nucleon pair sNN−−−√=5.02 TeV with the ALICE detector at the LHC. D mesons were reconstructed from their decays D0→K−π+, D+→K−π+π+, D∗+→D0π+, D+s→ϕπ+→K−K+π+, and their charge conjugates. The pT-differential production cross sections were measured at mid-rapidity in the interval 1<pT<24 GeV/c for D0, D+ and D∗+ mesons and in 2<pT<12 GeV/c for Ds mesons, using an analysis method based on the selection of decay topologies displaced from the interaction vertex. The production cross sections of the D0, D+ and D∗+ mesons were also measured in three pT intervals as a function of the rapidity ycms in the centre-of-mass system in −1.26<ycms<0.34. In addition, the prompt D0 cross section was measured in pp collisions at s√=7 TeV and p-Pb collisions at sNN−−−√=5.02 TeV down to pT=0 using an analysis technique that is based on the estimation and subtraction of the combinatorial background, without reconstruction of the D0 decay vertex. The nuclear modification factor RpPb(pT), defined as the ratio of the pT-differential D-meson cross section in p-Pb collisions and that in pp collisions scaled by the mass number of the Pb nucleus, was calculated for the four D-meson species and found to be compatible with unity within experimental uncertainties. The results are compared to theoretical calculations that include cold-nuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks with an expanding deconfined medium.
The production cross sections of the prompt charmed mesons D0, D+, D∗+ and Ds were measured at mid-rapidity in p-Pb collisions at a centre-of-mass energy per nucleon pair sNN−−−√=5.02 TeV with the ALICE detector at the LHC. D mesons were reconstructed from their decays D0→K−π+, D+→K−π+π+, D∗+→D0π+, D+s→ϕπ+→K−K+π+, and their charge conjugates. The pT-differential production cross sections were measured at mid-rapidity in the interval 1<pT<24 GeV/c for D0, D+ and D∗+ mesons and in 2<pT<12 GeV/c for Ds mesons, using an analysis method based on the selection of decay topologies displaced from the interaction vertex. The production cross sections of the D0, D+ and D∗+ mesons were also measured in three pT intervals as a function of the rapidity ycms in the centre-of-mass system in −1.26<ycms<0.34. In addition, the prompt D0 cross section was measured in pp collisions at s√=7 TeV and p-Pb collisions at sNN−−−√=5.02 TeV down to pT=0 using an analysis technique that is based on the estimation and subtraction of the combinatorial background, without reconstruction of the D0 decay vertex. The nuclear modification factor RpPb(pT), defined as the ratio of the pT-differential D-meson cross section in p-Pb collisions and that in pp collisions scaled by the mass number of the Pb nucleus, was calculated for the four D-meson species and found to be compatible with unity within experimental uncertainties. The results are compared to theoretical calculations that include cold-nuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks with an expanding deconfined medium.
The measurement of prompt D-meson production as a function of multiplicity in p-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the LHC is reported. D0, D+ and D∗+ mesons are reconstructed via their hadronic decay channels in the centre-of-mass rapidity range −0.96<ycms<0.04 and transverse momentum interval 1<pT<24 GeV/c. The multiplicity dependence of D-meson production is examined by either comparing yields in p-Pb collisions in different event classes, selected based on the multiplicity of produced particles or zero-degree energy, with those in pp collisions, scaled by the number of binary nucleon-nucleon collisions (nuclear modification factor); as well as by evaluating the per-event yields in p-Pb collisions in different multiplicity intervals normalised to the multiplicity-integrated ones (relative yields). The nuclear modification factors for D0, D+ and D∗+ are consistent with one another. The D-meson nuclear modification factors as a function of the zero-degree energy are consistent with unity within uncertainties in the measured pT regions and event classes. The relative D-meson yields, calculated in various pT intervals, increase as a function of the charged-particle multiplicity. The results are compared with the equivalent pp measurements at s√=7 TeV as well as with EPOS~3 calculations.
The measurement of prompt D-meson production as a function of multiplicity in p-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the LHC is reported. D0, D+ and D∗+ mesons are reconstructed via their hadronic decay channels in the centre-of-mass rapidity range −0.96<ycms<0.04 and transverse momentum interval 1<pT<24 GeV/c. The multiplicity dependence of D-meson production is examined by either comparing yields in p-Pb collisions in different event classes, selected based on the multiplicity of produced particles or zero-degree energy, with those in pp collisions, scaled by the number of binary nucleon-nucleon collisions (nuclear modification factor); as well as by evaluating the per-event yields in p-Pb collisions in different multiplicity intervals normalised to the multiplicity-integrated ones (relative yields). The nuclear modification factors for D0, D+ and D∗+ are consistent with one another. The D-meson nuclear modification factors as a function of the zero-degree energy are consistent with unity within uncertainties in the measured pT regions and event classes. The relative D-meson yields, calculated in various pT intervals, increase as a function of the charged-particle multiplicity. The results are compared with the equivalent pp measurements at s√=7 TeV as well as with EPOS~3 calculations.
The measurement of prompt D-meson production as a function of multiplicity in p-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the LHC is reported. D0, D+ and D∗+ mesons are reconstructed via their hadronic decay channels in the centre-of-mass rapidity range −0.96<ycms<0.04 and transverse momentum interval 1<pT<24 GeV/c. The multiplicity dependence of D-meson production is examined by either comparing yields in p-Pb collisions in different event classes, selected based on the multiplicity of produced particles or zero-degree energy, with those in pp collisions, scaled by the number of binary nucleon-nucleon collisions (nuclear modification factor); as well as by evaluating the per-event yields in p-Pb collisions in different multiplicity intervals normalised to the multiplicity-integrated ones (relative yields). The nuclear modification factors for D0, D+ and D∗+ are consistent with one another. The D-meson nuclear modification factors as a function of the zero-degree energy are consistent with unity within uncertainties in the measured pT regions and event classes. The relative D-meson yields, calculated in various pT intervals, increase as a function of the charged-particle multiplicity. The results are compared with the equivalent pp measurements at s√=7 TeV as well as with EPOS~3 calculations.
The inclusive production of the ψ(2S) charmonium state was studied as a function of centrality in p-Pb collisions at the nucleon-nucleon center of mass energy sNN−−−√ = 5.02 TeV at the CERN LHC. The measurement was performed with the ALICE detector in the center of mass rapidity ranges −4.46<ycms<−2.96 and 2.03<ycms<3.53, down to zero transverse momentum, by reconstructing the ψ(2S) decay to a muon pair. The ψ(2S) production cross section σψ(2S) is presented as a function of the collision centrality, which is estimated through the energy deposited in forward rapidity calorimeters. The relative strength of nuclear effects on the ψ(2S) and on the corresponding 1S charmonium state J/ψ is then studied by means of the double ratio of cross sections [σψ(2S)/σJ/ψ]pPb/[σψ(2S)/σJ/ψ]pp between p-Pb and pp collisions, and by the values of the nuclear modification factors for the two charmonium states. The results show a large suppression of ψ(2S) production relative to the J/ψ at backward (negative) rapidity, corresponding to the flight direction of the Pb-nucleus, while at forward (positive) rapidity the suppressions of the two states are comparable. Finally, comparisons to results from lower energy experiments and to available theoretical models are presented.
The inclusive production of the ψ(2S) charmonium state was studied as a function of centrality in p-Pb collisions at the nucleon-nucleon center of mass energy sNN−−−√ = 5.02 TeV at the CERN LHC. The measurement was performed with the ALICE detector in the center of mass rapidity ranges −4.46<ycms<−2.96 and 2.03<ycms<3.53, down to zero transverse momentum, by reconstructing the ψ(2S) decay to a muon pair. The ψ(2S) production cross section σψ(2S) is presented as a function of the collision centrality, which is estimated through the energy deposited in forward rapidity calorimeters. The relative strength of nuclear effects on the ψ(2S) and on the corresponding 1S charmonium state J/ψ is then studied by means of the double ratio of cross sections [σψ(2S)/σJ/ψ]pPb/[σψ(2S)/σJ/ψ]pp between p-Pb and pp collisions, and by the values of the nuclear modification factors for the two charmonium states. The results show a large suppression of ψ(2S) production relative to the J/ψ at backward (negative) rapidity, corresponding to the flight direction of the Pb-nucleus, while at forward (positive) rapidity the suppressions of the two states are comparable. Finally, comparisons to results from lower energy experiments and to available theoretical models are presented.
The inclusive production of the ψ(2S) charmonium state was studied as a function of centrality in p-Pb collisions at the nucleon-nucleon center of mass energy sNN−−−√ = 5.02 TeV at the CERN LHC. The measurement was performed with the ALICE detector in the center of mass rapidity ranges −4.46<ycms<−2.96 and 2.03<ycms<3.53, down to zero transverse momentum, by reconstructing the ψ(2S) decay to a muon pair. The ψ(2S) production cross section σψ(2S) is presented as a function of the collision centrality, which is estimated through the energy deposited in forward rapidity calorimeters. The relative strength of nuclear effects on the ψ(2S) and on the corresponding 1S charmonium state J/ψ is then studied by means of the double ratio of cross sections [σψ(2S)/σJ/ψ]pPb/[σψ(2S)/σJ/ψ]pp between p-Pb and pp collisions, and by the values of the nuclear modification factors for the two charmonium states. The results show a large suppression of ψ(2S) production relative to the J/ψ at backward rapidity, corresponding to the flight direction of the Pb-nucleus, while at forward rapidity the suppressions of the two states are comparable. Finally, comparisons to results from lower energy experiments and to available theoretical models are presented.
We present a Bayesian approach to particle identification (PID) within the ALICE experiment. The aim is to more effectively combine the particle identification capabilities of its various detectors. After a brief explanation of the adopted methodology and formalism, the performance of the Bayesian PID approach for charged pions, kaons and protons in the central barrel of ALICE is studied. PID is performed via measurements of specific energy loss (dE/dx) and time-of-flight. PID efficiencies and misidentification probabilities are extracted and compared with Monte Carlo simulations using high-purity samples of identified particles in the decay channels K0S→π−π+, ϕ→K−K+, and Λ→pπ− in p-Pb collisions at sNN−−−√=5.02 TeV. In order to thoroughly assess the validity of the Bayesian approach, this methodology was used to obtain corrected pT spectra of pions, kaons, protons, and D0 mesons in pp collisions at s√=7 TeV. In all cases, the results using Bayesian PID were found to be consistent with previous measurements performed by ALICE using a standard PID approach. For the measurement of D0→K−π+, it was found that a Bayesian PID approach gave a higher signal-to-background ratio and a similar or larger statistical significance when compared with standard PID selections, despite a reduced identification efficiency. Finally, we present an exploratory study of the measurement of Λ+c→pK−π+ in pp collisions at s√=7 TeV, using the Bayesian approach for the identification of its decay products.
We present a Bayesian approach to particle identification (PID) within the ALICE experiment. The aim is to more effectively combine the particle identification capabilities of its various detectors. After a brief explanation of the adopted methodology and formalism, the performance of the Bayesian PID approach for charged pions, kaons and protons in the central barrel of ALICE is studied. PID is performed via measurements of specific energy loss (dE/dx) and time-of-flight. PID efficiencies and misidentification probabilities are extracted and compared with Monte Carlo simulations using high-purity samples of identified particles in the decay channels K0S→π−π+, ϕ→K−K+, and Λ→pπ− in p-Pb collisions at sNN−−−√=5.02 TeV. In order to thoroughly assess the validity of the Bayesian approach, this methodology was used to obtain corrected pT spectra of pions, kaons, protons, and D0 mesons in pp collisions at s√=7 TeV. In all cases, the results using Bayesian PID were found to be consistent with previous measurements performed by ALICE using a standard PID approach. For the measurement of D0→K−π+, it was found that a Bayesian PID approach gave a higher signal-to-background ratio and a similar or larger statistical significance when compared with standard PID selections, despite a reduced identification efficiency. Finally, we present an exploratory study of the measurement of Λ+c→pK−π+ in pp collisions at s√=7 TeV, using the Bayesian approach for the identification of its decay products.
Measurements of charged jet production as a function of centrality are presented for p-Pb collisions recorded at sNN−−−√=5.02 TeV with the ALICE detector. Centrality classes are determined via the energy deposit in neutron calorimeters at zero degree, close to the beam direction, to minimise dynamical biases of the selection. The corresponding number of participants or binary nucleon-nucleon collisions is determined based on the particle production in the Pb-going rapidity region. Jets have been reconstructed in the central rapidity region from charged particles with the anti-kT algorithm for resolution parameters R=0.2 and R=0.4 in the transverse momentum range 20 to 120 GeV/c. The reconstructed jet momentum and yields have been corrected for detector effects and underlying-event background. In the five centrality bins considered, the charged jet production in p-Pb collisions is consistent with the production expected from binary scaling from pp collisions. The ratio of jet yields reconstructed with the two different resolution parameters is also independent of the centrality selection, demonstrating the absence of major modifications of the radial jet structure in the reported centrality classes.
Measurements of charged jet production as a function of centrality are presented for p-Pb collisions recorded at sNN−−−√=5.02 TeV with the ALICE detector. Centrality classes are determined via the energy deposit in neutron calorimeters at zero degree, close to the beam direction, to minimise dynamical biases of the selection. The corresponding number of participants or binary nucleon-nucleon collisions is determined based on the particle production in the Pb-going rapidity region. Jets have been reconstructed in the central rapidity region from charged particles with the anti-kT algorithm for resolution parameters R=0.2 and R=0.4 in the transverse momentum range 20 to 120 GeV/c. The reconstructed jet momentum and yields have been corrected for detector effects and underlying-event background. In the five centrality bins considered, the charged jet production in p-Pb collisions is consistent with the production expected from binary scaling from pp collisions. The ratio of jet yields reconstructed with the two different resolution parameters is also independent of the centrality selection, demonstrating the absence of major modifications of the radial jet structure in the reported centrality classes.
Measurements of charged jet production as a function of centrality are presented for p-Pb collisions recorded at sNN−−−√=5.02 TeV with the ALICE detector. Centrality classes are determined via the energy deposit in neutron calorimeters at zero degree, close to the beam direction, to minimise dynamical biases of the selection. The corresponding number of participants or binary nucleon-nucleon collisions is determined based on the particle production in the Pb-going rapidity region. Jets have been reconstructed in the central rapidity region from charged particles with the anti-kT algorithm for resolution parameters R=0.2 and R=0.4 in the transverse momentum range 20 to 120 GeV/c. The reconstructed jet momentum and yields have been corrected for detector effects and underlying-event background. In the five centrality bins considered, the charged jet production in p-Pb collisions is consistent with the production expected from binary scaling from pp collisions. The ratio of jet yields reconstructed with the two different resolution parameters is also independent of the centrality selection, demonstrating the absence of major modifications of the radial jet structure in the reported centrality classes.
The azimuthal correlations of D mesons and charged particles were measured with the ALICE detector in pp collisions at s√=7 TeV and p-Pb collisions at sNN−−−√=5.02 TeV at the Large Hadron Collider. D0, D+, and D∗+ mesons and their charge conjugates with transverse momentum 3<pT<16 GeV/c and rapidity in the nucleon-nucleon centre-of-mass system |ycms|<0.5 (pp collisions) and −0.96<ycms<0.04 (p-Pb collisions) were correlated to charged particles with pT>0.3 Gev/c. The properties of the correlation peak induced by the jet containing the D meson, described in terms of the yield of charged particles in the peak and peak width, are compatible within uncertainties between the two collision systems, and described by Monte-Carlo simulations based on the PYTHIA, POWHEG and EPOS 3 event generators.
The azimuthal correlations of D mesons and charged particles were measured with the ALICE detector in pp collisions at s√=7 TeV and p-Pb collisions at sNN−−−√=5.02 TeV at the Large Hadron Collider. D0, D+, and D∗+ mesons with transverse momentum 3<pT<16 GeV/c and rapidity in the nucleon-nucleon centre-of-mass system |ycms|<0.5 (pp collisions) and −0.96<ycms<0.04 (p-Pb collisions) were correlated to charged particles with pT>0.3 Gev/c. The properties of the correlation peak induced by the jet containing the D meson, described in terms of the yield of charged particles in the peak and peak width, are compatible within uncertainties between the two collision systems, and described by Monte-Carlo simulations based on the PYTHIA and POWHEG event generators.
At sufficiently high temperature and energy density, nuclear matter undergoes a transition to a phase in which quarks and gluons are not confined: the Quark-Gluon Plasma (QGP) [1]. Such an extreme state of strongly-interacting QCD (Quantum Chromo-Dynamics) matter is produced in the laboratory with high-energy collisions of heavy nuclei, where an enhanced production of strange hadrons is observed [2-6]. Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions [7], is more pronounced for multi-strange baryons. Several effects typical of heavy-ion phenomenology have been observed in high-multiplicity proton-proton (pp) collisions [8,9]. Yet, enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity pp collisions. We find that the integrated yields of strange and multi-strange particles relative to pions increases significantly with the event charged-particle multiplicity. The measurements are in remarkable agreement with p-Pb collision results [10,11] indicating that the phenomenon is related to the final system created in the collision. In high-multiplicity events strangeness production reaches values similar to those observed in Pb-Pb collisions, where a QGP is formed.
At sufficiently high temperature and energy density, nuclear matter undergoes a transition to a phase in which quarks and gluons are not confined: the quark–gluon plasma (QGP)1. Such an exotic state of strongly interacting quantum chromodynamics matter is produced in the laboratory in heavy nuclei high-energy collisions, where an enhanced production of strange hadrons is observed2,3,4,5,6. Strangeness enhancement, originally proposed as a signature of QGP formation in nuclear collisions7, is more pronounced for multi-strange baryons. Several effects typical of heavy-ion phenomenology have been observed in high-multiplicity proton–proton (pp) collisions8,9, but the enhanced production of multi-strange particles has not been reported so far. Here we present the first observation of strangeness enhancement in high-multiplicity proton–proton collisions. We find that the integrated yields of strange and multi-strange particles, relative to pions, increases significantly with the event charged-particle multiplicity. The measurements are in remarkable agreement with the p–Pb collision results10,11, indicating that the phenomenon is related to the final system created in the collision. In high-multiplicity events strangeness production reaches values similar to those observed in Pb–Pb collisions, where a QGP is formed.
We report on measurements of a charge-dependent flow using a novel three-particle correlator with ALICE in Pb-Pb collisions at the LHC, and discuss the implications for observation of local parity violation and the Chiral Magnetic Wave (CMW) in heavy-ion collisions. Charge-dependent flow is reported for different collision centralities as a function of the event charge asymmetry. While our results are in qualitative agreement with expectations based on the CMW, the nonzero signal observed in higher harmonics correlations indicates a possible significant background contribution. We also present results on a differential correlator, where the flow of positive and negative charges is reported as a function of the mean charge of the particles and their pseudorapidity separation. We argue that this differential correlator is better suited to distinguish the differences in positive and negative charges expected due to the CMW and the background effects, such as local charge conservation coupled with strong radial and anisotropic flow.
Measurements of charged-particle production in pp, p−Pb, and Pb−Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum (ptrigT) in the range 8<ptrigT<15 GeV/c. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier, RT=NTch/⟨NTch⟩, is used to group events according to their UE activity, where NTch is the charged-particle multiplicity per event in the transverse region and ⟨NTch⟩ is the mean value over the whole analysed sample. The energy dependence of the RT distributions in pp collisions at s√=2.76, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle pT spectra as a function of RT in the three azimuthal regions in pp, p−Pb, and Pb−Pb collisions at sNN−−−√=5.02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p−Pb).
In this letter, measurements of (anti)alpha production in central (0−10%) Pb−Pb collisions at a center-of-mass energy per nucleon−nucleon pair of sNN−−−√ = 5.02 TeV are presented, including the first measurement of an antialpha transverse-momentum spectrum. Owing to its large mass, (anti)alpha production yields and transverse-momentum spectra are of particular interest because they provide a stringent test of particle production models. The averaged antialpha and alpha spectrum is included into a common blast-wave fit with lighter particles, indicating that the (anti)alpha also participates in the collective expansion of the medium created in the collision. A blast-wave fit including only protons, (anti)alpha, and other light nuclei results in a similar flow velocity as the fit that includes all particles. A similar flow velocity, but a significantly larger kinetic freeze-out temperature is obtained when only protons and light nuclei are included in the fit. The coalescence parameter B4 is well described by calculations from a statistical hadronization model but significantly underestimated by calculations assuming nucleus formation via coalescence of nucleons. Similarly, the (anti)alpha-to-proton ratio is well described by the statistical hadronization model. On the other hand, coalescence calculations including approaches with different implementations of the (anti)alpha substructure tend to underestimate the data.
The multi-strange baryon yields in Pb--Pb collisions have been shown to exhibit an enhancement relative to pp reactions. In this work, Ξ and Ω production rates have been measured with the ALICE experiment as a function of transverse momentum, pT, in p-Pb collisions at a centre-of-mass energy of sNN−−−√ = 5.02 TeV. The results cover the kinematic ranges 0.6 GeV/c<pT<7.2 GeV/c and 0.8 GeV/c<pT< 5 GeV/c, for Ξ and Ω respectively, in the common rapidity interval -0.5 <yCMS< 0. Multi-strange baryons have been identified by reconstructing their weak decays into charged particles. The pT spectra are analysed as a function of event charged-particle multiplicity, which in p-Pb collisions ranges over one order of magnitude and lies between those observed in pp and Pb-Pb collisions. The measured pT distributions are compared to the expectations from a Blast-Wave model. The parameters which describe the production of lighter hadron species also describe the hyperon spectra in high multiplicity p-Pb. The yield of hyperons relative to charged pions is studied and compared with results from pp and Pb-Pb collisions. A statistical model is employed, which describes the change in the ratios with volume using a canonical suppression mechanism, in which the small volume causes a species-dependent relative reduction of hadron production. The calculations, in which the magnitude of the effect depends on the strangeness content, show good qualitative agreement with the data.
The multi-strange baryon yields in PbPb collisions have been shown to exhibit an enhancement relative to pp reactions. In this work, Ξ and Ω production rates have been measured with the ALICE experiment as a function of transverse momentum, pT, in pPb collisions at a centre-of-mass energy of sNN=5.02 TeV. The results cover the kinematic ranges 0.6 GeV/c<pT<7.2 GeV/c and 0.8 GeV/c<pT<5 GeV/c, for Ξ and Ω respectively, in the common rapidity interval −0.5<yCMS<0. Multi-strange baryons have been identified by reconstructing their weak decays into charged particles. The pT spectra are analysed as a function of event charged-particle multiplicity, which in pPb collisions ranges over one order of magnitude and lies between those observed in pp and PbPb collisions. The measured pT distributions are compared to the expectations from a Blast-Wave model. The parameters which describe the production of lighter hadron species also describe the hyperon spectra in high multiplicity pPb collisions. The yield of hyperons relative to charged pions is studied and compared with results from pp and PbPb collisions. A continuous increase in the yield ratios as a function of multiplicity is observed in pPb data, the values of which range from those measured in minimum bias pp to the ones in PbPb collisions. A statistical model qualitatively describes this multiplicity dependence using a canonical suppression mechanism, in which the small volume causes a relative reduction of hadron production dependent on the strangeness content of the hyperon.
The multi-strange baryon yields in Pb--Pb collisions have been shown to exhibit an enhancement relative to pp reactions. In this work, Ξ and Ω production rates have been measured with the ALICE experiment as a function of transverse momentum, pT, in p-Pb collisions at a centre-of-mass energy of sNN−−−√ = 5.02 TeV. The results cover the kinematic ranges 0.6 GeV/c<pT<7.2 GeV/c and 0.8 GeV/c<pT< 5 GeV/c, for Ξ and Ω respectively, in the common rapidity interval -0.5 <yCMS< 0. Multi-strange baryons have been identified by reconstructing their weak decays into charged particles. The pT spectra are analysed as a function of event charged-particle multiplicity, which in p-Pb collisions ranges over one order of magnitude and lies between those observed in pp and Pb-Pb collisions. The measured pT distributions are compared to the expectations from a Blast-Wave model. The parameters which describe the production of lighter hadron species also describe the hyperon spectra in high multiplicity p-Pb. The yield of hyperons relative to charged pions is studied and compared with results from pp and Pb-Pb collisions. A statistical model is employed, which describes the change in the ratios with volume using a canonical suppression mechanism, in which the small volume causes a species-dependent relative reduction of hadron production. The calculations, in which the magnitude of the effect depends on the strangeness content, show good qualitative agreement with the data.
Three- and four-pion Bose-Einstein correlations are presented in pp, p-Pb, and Pb-Pb collisions at the LHC. We compare our measured four-pion correlations to the expectation derived from two- and three-pion measurements. Such a comparison provides a method to search for coherent pion emission. We also present mixed-charge correlations in order to demonstrate the effectiveness of several analysis procedures such as Coulomb corrections. Same-charge four-pion correlations in pp and p-Pb appear consistent with the expectations from three-pion measurements. However, the presence of non-negligible background correlations in both systems prevent a conclusive statement. In Pb-Pb collisions, we observe a significant suppression of three- and four-pion Bose-Einstein correlations compared to expectations from two-pion measurements. There appears to be no centrality dependence of the suppression within the 0-50% centrality interval. The origin of the suppression is not clear. However, by postulating either coherent pion emission or large multibody Coulomb effects, the suppression may be explained.
Three- and four-pion Bose-Einstein correlations are presented in pp, p-Pb, and Pb-Pb collisions at the LHC. We compare our measured four-pion correlations to the expectation derived from two- and three-pion measurements. Such a comparison provides a method to search for coherent pion emission. We also present mixed-charge correlations in order to demonstrate the effectiveness of several analysis procedures such as Coulomb corrections. Same-charge four-pion correlations in pp and p-Pb appear consistent with the expectations from three-pion measurements. However, the presence of non-negligible background correlations in both systems prevent a conclusive statement. In Pb-Pb collisions, we observe a significant suppression of three- and four-pion Bose-Einstein correlations compared to expectations from two-pion measurements. There appears to be no centrality dependence of the suppression within the 0%–50% centrality interval. The origin of the suppression is not clear. However, by postulating either coherent pion emission or large multibody Coulomb effects, the suppression may be explained.
The dependence of f0(980) production on the final-state charged-particle multiplicity in p−Pb collisions at sNN−−−√=5.02 TeV is reported. The production of f0(980) is measured with the ALICE detector via the f0(980)→π+π− decay channel in a midrapidity region of −0.5<y<0. Particle yield ratios of f0(980) to π and K∗(892)0 are found to be decreasing with increasing charged-particle multiplicity. The magnitude of the suppression of the f0(980)/π and f0(980)/K∗(892)0 yield ratios is found to be dependent on the transverse momentum pT, suggesting different mechanisms responsible for the measured effects. Furthermore, the nuclear modification factor QpPb of f0(980) is measured in various multiplicity ranges. The QpPb shows a strong suppression of the f0(980) production in the pT region up to about 4 GeV/c. The results on the particle yield ratios and QpPb for f0(980) may help to understand the late hadronic phase in p−Pb collisions and the nature of the internal structure of f0(980) particle.
The first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton-proton (pp) collisions is reported. The mean charged-particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at s√ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region (|η|<0.9) using the sequential recombination anti-kT algorithm with jet resolution parameters R = 0.2, 0.3, and 0.4 for the transverse momentum (pT) interval 5−110 GeV/c. High-multiplicity events are selected by the forward V0 scintillator detectors. The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet pT in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation functions zch and ξch are measured for different jet-pT intervals. Jet-pT independent fragmentation of leading jets is observed for wider jets except at high- and low-zch. The observed "hump-backed plateau" structure in the ξch distribution indicates suppression of low-pT particles. In high-multiplicity events, an enhancement of the fragmentation probability of low-zch particles accompanied by a suppression of high-zch particles is observed compared to minimum-bias events. This behavior becomes more prominent for low-pT jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet pT. These measurements provide important constraints to models of jet fragmentation.
We present a Bayesian approach to particle identification (PID) within the ALICE experiment. The aim is to more effectively combine the particle identification capabilities of its various detectors. After a brief explanation of the adopted methodology and formalism, the performance of the Bayesian PID approach for charged pions, kaons and protons in the central barrel of ALICE is studied. PID is performed via measurements of specific energy loss (dE/dx) and time-of-flight. PID efficiencies and misidentification probabilities are extracted and compared with Monte Carlo simulations using high-purity samples of identified particles in the decay channels K0S→π−π+, ϕ→K−K+, and Λ→pπ− in p-Pb collisions at sNN−−−√=5.02 TeV. In order to thoroughly assess the validity of the Bayesian approach, this methodology was used to obtain corrected pT spectra of pions, kaons, protons, and D0 mesons in pp collisions at s√=7 TeV. In all cases, the results using Bayesian PID were found to be consistent with previous measurements performed by ALICE using a standard PID approach. For the measurement of D0→K−π+, it was found that a Bayesian PID approach gave a higher signal-to-background ratio and a similar or larger statistical significance when compared with standard PID selections, despite a reduced identification efficiency. Finally, we present an exploratory study of the measurement of Λ+c→pK−π+ in pp collisions at s√=7 TeV, using the Bayesian approach for the identification of its decay products.
The production of K∗(892)0 and ϕ(1020) mesons has been measured in p-Pb collisions at sNN−−−√ = 5.02 TeV. K∗0 and ϕ are reconstructed via their decay into charged hadrons with the ALICE detector in the rapidity range −0.5<y<0. The transverse momentum spectra, measured as a function of the multiplicity, have pT range from 0 to 15 GeV/c for K∗0 and from 0.3 to 21 GeV/c for ϕ. Integrated yields, mean transverse momenta and particle ratios are reported and compared with results in pp collisions at s√ = 7 TeV and Pb-Pb collisions at sNN−−−√ = 2.76 TeV. In Pb-Pb and p-Pb collisions, K∗0 and ϕ probe the hadronic phase of the system and contribute to the study of particle formation mechanisms by comparison with other identified hadrons. For this purpose, the mean transverse momenta and the differential proton-to-ϕ ratio are discussed as a function of the multiplicity of the event. The short-lived K∗0 is measured to investigate re-scattering effects, believed to be related to the size of the system and to the lifetime of the hadronic phase.
The production of K∗(892)0 and ϕ(1020) mesons has been measured in p-Pb collisions at sNN−−−√ = 5.02 TeV. K∗0 and ϕ are reconstructed via their decay into charged hadrons with the ALICE detector in the rapidity range −0.5<y<0. The transverse momentum spectra, measured as a function of the multiplicity, have pT range from 0 to 15 GeV/c for K∗0 and from 0.3 to 21 GeV/c for ϕ. Integrated yields, mean transverse momenta and particle ratios are reported and compared with results in pp collisions at s√ = 7 TeV and Pb-Pb collisions at sNN−−−√ = 2.76 TeV. In Pb-Pb and p-Pb collisions, K∗0 and ϕ probe the hadronic phase of the system and contribute to the study of particle formation mechanisms by comparison with other identified hadrons. For this purpose, the mean transverse momenta and the differential proton-to-ϕ ratio are discussed as a function of the multiplicity of the event. The short-lived K∗0 is measured to investigate re-scattering effects, believed to be related to the size of the system and to the lifetime of the hadronic phase.
This Letter presents the most precise measurement to date of the matter/antimatter imbalance at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair sNN−−−√=5.02 TeV. Using the Statistical Hadronization framework, it is possible to obtain the value of the electric charge and baryon chemical potentials, μQ=−0.18±0.90 MeV and μB=0.71±0.45 MeV, with unprecedented precision. A centrality-differential study of the antiparticle-to-particle yield ratios of charged pions, protons, Ω-baryons, and light (hyper)nuclei is performed. These results indicate that the system created in Pb-Pb collisions at the LHC is on average baryon-free and electrically neutral at midrapidity.
The two-particle momentum correlation functions between charm mesons (D∗± and D±) and charged light-flavor mesons (π± and K±) in all charge-combinations are measured for the first time by the ALICE Collaboration in high-multiplicity proton-proton collisions at a center-of-mass energy of s√=13 TeV. For DK and D∗K pairs, the experimental results are in agreement with theoretical predictions of the residual strong interaction based on quantum chromodynamics calculations on the lattice and chiral effective field theory. In the case of Dπ and D∗π pairs, tension between the calculations including strong interactions and the measurement is observed. For all particle pairs, the data can be adequately described by Coulomb interaction only, indicating a shallow interaction between charm and light-flavor mesons. Finally, the scattering lengths governing the residual strong interaction of the Dπ and D∗π systems are determined by fitting the experimental correlation functions with a model that employs a Gaussian potential. The extracted values are small and compatible with zero.
The first measurements of femtoscopic correlations with the particle pair combinations π±K0S in pp collisions at s√=13 TeV at the Large Hadron Collider (LHC) are reported by the ALICE experiment. Using the femtoscopic approach, it is shown that it is possible to study the elusive K∗0(700) particle that has been considered a tetraquark candidate for over forty years. Boson source parameters and final-state interaction parameters are extracted by fitting a model assuming a Gaussian source to the experimentally measured two-particle correlation functions. The final-state interaction is modeled through a resonant scattering amplitude, defined in terms of a mass and a coupling parameter, decaying into a π±K0S pair. The extracted mass and Breit-Wigner width, derived from the coupling parameter, of the final-state interaction are found to be consistent with previous measurements of the K∗0(700). The small value and increasing behavior of the correlation strength with increasing source size support the hypothesis that the K∗0(700) is a four-quark state, i.e. a tetraquark state. This latter trend is also confirmed via a simple geometric model that assumes a tetraquark structure of the K∗0(700) resonance.
The first measurements of femtoscopic correlations with the particle pair combinations π±K0S in pp collisions at s√=13 TeV at the Large Hadron Collider (LHC) are reported by the ALICE experiment. Using the femtoscopic approach, it is shown that it is possible to study the elusive K∗0(700) particle that has been considered a tetraquark candidate for over forty years. Boson source parameters and final-state interaction parameters are extracted by fitting a model assuming a Gaussian source to the experimentally measured two-particle correlation functions. The final-state interaction is modeled through a resonant scattering amplitude, defined in terms of a mass and a coupling parameter, decaying into a π±K0S pair. The extracted mass and Breit-Wigner width, derived from the coupling parameter, of the final-state interaction are found to be consistent with previous measurements of the K∗0(700). The small value and increasing behavior of the correlation strength with increasing source size support the hypothesis that the K∗0(700) is a four-quark state, i.e. a tetraquark state. This latter trend is also confirmed via a simple geometric model that assumes a tetraquark structure of the K∗0(700) resonance.
Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the "ridge" phenomenon, were discovered in heavy-ion collisions, and later found in pp and p−Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small-collision systems. In this Letter, measurements of the long-range correlations in p−Pb collisions at sNN−−−√=5.02 TeV are extended to a pseudorapidity gap of Δη∼8 between particles using the ALICE, forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of Δη∼8 for the first time in p−Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small-collision systems such as p−Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient, v2({\eta}), is extracted from the long-range correlations. The v2(η) results are presented for a wide pseudorapidity range of −3.1<η<4.8 in various centrality classes in p−Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small-collision systems, the v2(η) measurements are compared to hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small-collision systems.
Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the "ridge" phenomenon, were discovered in heavy-ion collisions, and later found in pp and p−Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small-collision systems. In this Letter, measurements of the long-range correlations in p−Pb collisions at sNN−−−√=5.02 TeV are extended to a pseudorapidity gap of Δη∼8 between particles using the ALICE, forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of Δη∼8 for the first time in p−Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small-collision systems such as p−Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient, v2({\eta}), is extracted from the long-range correlations. The v2(η) results are presented for a wide pseudorapidity range of −3.1<η<4.8 in various centrality classes in p−Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small-collision systems, the v2(η) measurements are compared to hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small-collision systems.
Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the "ridge" phenomenon, were discovered in heavy-ion collisions, and later found in pp and p−Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small-collision systems. In this Letter, measurements of the long-range correlations in p−Pb collisions at sNN−−−√=5.02 TeV are extended to a pseudorapidity gap of Δη∼8 between particles using the ALICE, forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of Δη∼8 for the first time in p−Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small-collision systems such as p−Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient, v2({\eta}), is extracted from the long-range correlations. The v2(η) results are presented for a wide pseudorapidity range of −3.1<η<4.8 in various centrality classes in p−Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small-collision systems, the v2(η) measurements are compared to hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small-collision systems.
The elliptic flow (v2) of D0 mesons from beauty-hadron decays (non-prompt D0) was measured in midcentral (30-50%) Pb-Pb collisions at a centre-of-mass energy per nucleon pair sNN−−−√ = 5.02 TeV with the ALICE detector at the LHC. The D0 mesons were reconstructed at midrapidity (|y|<0.8) from their hadronic decay D0→K−π+, in the transverse momentum interval 2<pT<12 GeV/c. The result indicates a positive v2 for non-prompt D0 mesons with a significance of 2.7σ. The non-prompt D0-meson v2 is lower than that of prompt non-strange D mesons with 3.2σ significance in 2<pT<8 GeV/c, and compatible with the v2 of beauty-decay electrons. Theoretical calculations of beauty-quark transport in a hydrodynamically expanding medium describe the measurement within uncertainties.
The elliptic flow (v2) of D0 mesons from beauty-hadron decays (non-prompt D0) was measured in midcentral (30-50%) Pb-Pb collisions at a centre-of-mass energy per nucleon pair sNN−−−√ = 5.02 TeV with the ALICE detector at the LHC. The D0 mesons were reconstructed at midrapidity (|y|<0.8) from their hadronic decay D0→K−π+, in the transverse momentum interval 2<pT<12 GeV/c. The result indicates a positive v2 for non-prompt D0 mesons with a significance of 2.7σ. The non-prompt D0-meson v2 is lower than that of prompt non-strange D mesons with 3.2σ significance in 2<pT<8 GeV/c, and compatible with the v2 of beauty-decay electrons. Theoretical calculations of beauty-quark transport in a hydrodynamically expanding medium describe the measurement within uncertainties.
Measurements of charged-particle production in pp, p−Pb, and Pb−Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum (ptrigT) in the range 8<ptrigT<15 GeV/c. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier, RT=NTch/⟨NTch⟩, is used to group events according to their UE activity, where NTch is the charged-particle multiplicity per event in the transverse region and ⟨NTch⟩ is the mean value over the whole analysed sample. The energy dependence of the RT distributions in pp collisions at s√=2.76, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle pT spectra as a function of RT in the three azimuthal regions in pp, p−Pb, and Pb−Pb collisions at sNN−−−√=5.02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p−Pb).
Measurements of charged-particle production in pp, p−Pb, and Pb−Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum (ptrigT) in the range 8<ptrigT<15 GeV/c. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier, RT=NTch/⟨NTch⟩, is used to group events according to their UE activity, where NTch is the charged-particle multiplicity per event in the transverse region and ⟨NTch⟩ is the mean value over the whole analysed sample. The energy dependence of the RT distributions in pp collisions at s√=2.76, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle pT spectra as a function of RT in the three azimuthal regions in pp, p−Pb, and Pb−Pb collisions at sNN−−−√=5.02 TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p−Pb).
The Chiral Magnetic Wave (CMW) phenomenon is essential to provide insights into the strong interaction in QCD, the properties of the quark-gluon plasma, and the topological characteristics of the early universe, offering a deeper understanding of fundamental physics in high-energy collisions. Measurements of the charge-dependent anisotropic flow coefficients are studied in Pb-Pb collisions at center-of-mass energy per nucleon-nucleon collision sNN−−−√= 5.02 TeV to probe the CMW. In particular, the slope of the normalized difference in elliptic (v2) and triangular (v3) flow coefficients of positively and negatively charged particles as a function of their event-wise normalized number difference, is reported for inclusive and identified particles. The slope rNorm3 is found to be larger than zero and to have a magnitude similar to rNorm2, thus pointing to a large background contribution for these measurements. Furthermore, rNorm2 can be described by a blast wave model calculation that incorporates local charge conservation. In addition, using the event shape engineering technique yields a fraction of CMW (fCMW) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit for fCMW, and in the 10-60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level.
The Chiral Magnetic Wave (CMW) phenomenon is essential to provide insights into the strong interaction in QCD, the properties of the quark-gluon plasma, and the topological characteristics of the early universe, offering a deeper understanding of fundamental physics in high-energy collisions. Measurements of the charge-dependent anisotropic flow coefficients are studied in Pb-Pb collisions at center-of-mass energy per nucleon-nucleon collision sNN−−−√= 5.02 TeV to probe the CMW. In particular, the slope of the normalized difference in elliptic (v2) and triangular (v3) flow coefficients of positively and negatively charged particles as a function of their event-wise normalized number difference, is reported for inclusive and identified particles. The slope rNorm3 is found to be larger than zero and to have a magnitude similar to rNorm2, thus pointing to a large background contribution for these measurements. Furthermore, rNorm2 can be described by a blast wave model calculation that incorporates local charge conservation. In addition, using the event shape engineering technique yields a fraction of CMW (fCMW) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit for fCMW, and in the 10-60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level.
The cross section for coherent photonuclear production of J/ψ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of sNN−−−√=5.02 TeV. Cross sections are presented in five different J/ψ rapidity ranges within |y|<4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17<WγPb,n<920 GeV, where WγPb,n is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1×10−5<x<3.3×10−2. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.
The cross section for coherent photonuclear production of J/ψ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of sNN−−−√=5.02 TeV. Cross sections are presented in five different J/ψ rapidity ranges within |y|<4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17<WγPb,n<920 GeV, where WγPb,n is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1×10−5<x<3.3×10−2. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.
The cross section for coherent photonuclear production of J/ψ is presented as a function of the electromagnetic dissociation (EMD) of Pb. The measurement is performed with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair of sNN−−−√=5.02 TeV. Cross sections are presented in five different J/ψ rapidity ranges within |y|<4, with the J/ψ reconstructed via its dilepton decay channels. In some events the J/ψ is not accompanied by EMD, while other events do produce neutrons from EMD at beam rapidities either in one or the other beam direction, or in both. The cross sections in a given rapidity range and for different configurations of neutrons from EMD allow for the extraction of the energy dependence of this process in the range 17<WγPb,n<920 GeV, where WγPb,n is the centre-of-mass energy per nucleon of the γPb system. This range corresponds to a Bjorken-x interval spanning about three orders of magnitude: 1.1×10−5<x<3.3×10−2. In addition to the ultra-peripheral and photonuclear cross sections, the nuclear suppression factor is obtained. These measurements point to a strong depletion of the gluon distribution in Pb nuclei over a broad, previously unexplored, energy range. These results, together with previous ALICE measurements, provide unprecedented information to probe quantum chromodynamics at high energies.
The transverse momentum distributions of the strange and double-strange hyperon resonances (Σ(1385)±, Ξ(1530)0) produced in p-Pb collisions at sNN−−−√=5.02 TeV were measured in the rapidity range −0.5<yCMS<0 for event classes corresponding to different charged-particle multiplicity densities, ⟨dNch/dηlab⟩. The mean transverse momentum values are presented as a function of ⟨dNch/dηlab⟩, as well as a function of the particle masses and compared with previous results on hyperon production. The integrated yield ratios of excited to ground-state hyperons are constant as a function of ⟨dNch/dηlab⟩. The equivalent ratios to pions exhibit an increase with ⟨dNch/dηlab⟩, depending on their strangeness content.
The transverse momentum distributions of the strange and double-strange hyperon resonances (Σ(1385)±, Ξ(1530)0) produced in p-Pb collisions at sNN−−−√=5.02 TeV were measured in the rapidity range −0.5<yCMS<0 for event classes corresponding to different charged-particle multiplicity densities, ⟨dNch/dηlab⟩. The mean transverse momentum values are presented as a function of ⟨dNch/dηlab⟩, as well as a function of the particle masses and compared with previous results on hyperon production. The integrated yield ratios of excited to ground-state hyperons are constant as a function of ⟨dNch/dηlab⟩. The equivalent ratios to pions exhibit an increase with ⟨dNch/dηlab⟩, depending on their strangeness content.
The pT-differential production cross sections of non-prompt D0, D+, and D+s mesons originating from beauty-hadron decays are measured in proton−proton collisions at a centre-of-mass energy s√ of 13 TeV. The measurements are performed at midrapidity, |y|<0.5, with the data sample collected by ALICE from 2016 to 2018. The results are in agreement with predictions from several perturbative QCD calculations. The fragmentation fraction of beauty quarks to strange mesons divided by the one to non-strange mesons, fs/(fu+fd), is found to be 0.114±0.016 (stat.)±0.006 (syst.)±0.003 (BR)±0.003 (extrap.). This value is compatible with previous measurements at lower centre-of-mass energies and in different collision systems in agreement with the assumption of universality of fragmentation functions. In addition, the dependence of the non-prompt D meson production on the centre-of-mass energy is investigated by comparing the results obtained at s√=5.02 and 13 TeV, showing a hardening of the non-prompt D-meson pT-differential production cross section at higher s√. Finally, the bb¯¯¯ production cross section per unit of rapidity at midrapidity is calculated from the non-prompt D0, D+, D+s, and Λ+c hadron measurements, obtaining dσ/dy=75.2±3.2 (stat.)±5.2 (syst.)+12.3−3.2 (extrap.) μb.
The two-particle momentum correlation functions between charm mesons (D∗± and D±) and charged light-flavor mesons (π± and K±) in all charge-combinations are measured for the first time by the ALICE Collaboration in high-multiplicity proton–proton collisions at a center-of-mass energy of √s = 13 TeV. For DK and D∗K pairs, the experimental results are in agreement with theoretical predictions of the residual strong interaction based on quantum chromodynamics calculations on the lattice and chiral effective field theory. In the case of Dπ and D∗π pairs, tension between the calculations including strong interactions and the measurement is observed. For all particle pairs, the data can be adequately described by Coulomb interaction only, indicating a shallow interaction between charm and light-flavor mesons. Finally, the scattering lengths governing the residual strong interaction of the Dπ and D∗π systems are determined by fitting the experimental correlation functions with a model that employs a Gaussian potential. The extracted values are small and compatible with zero.
Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at s√=13 TeV and p−Pb collisions at sNN−−−√=5.02 TeV. The correlation functions are measured as a function of relative azimuthal angle Δφ and pseudorapidity separation Δη for pairs of primary charged particles within the pseudorapidity interval |η|<0.9 and the transverse-momentum interval 1<pT<4 GeV/c. Flow coefficients are extracted for the long-range correlations (1.6<|Δη|<1.8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.
Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at √sNN = 13 TeV and p–Pb collisions at √s = 5.02 TeV. The correlation functions are measured as a function of relative azimuthal angle ∆φ and pseudorapidity separation ∆η for pairs of primary charged particles within the pseudorapidity interval |η| < 0.9 and the transverse-momentum interval 1 < pT < 4 GeV/c. Flow coefficients are extracted for the long-range correlations (1.6 < |∆η| < 1.8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.