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The design, construction, and commissioning of the ALICE Time-Projection Chamber (TPC) is described. It is the main device for pattern recognition, tracking, and identification of charged particles in the ALICE experiment at the CERN LHC. The TPC is cylindrical in shape with a volume close to 90 m3 and is operated in a 0.5 T solenoidal magnetic field parallel to its axis.
In this paper we describe in detail the design considerations for this detector for operation in the extreme multiplicity environment of central Pb–Pb collisions at LHC energy. The implementation of the resulting requirements into hardware (field cage, read-out chambers, electronics), infrastructure (gas and cooling system, laser-calibration system), and software led to many technical innovations which are described along with a presentation of all the major components of the detector, as currently realized. We also report on the performance achieved after completion of the first round of stand-alone calibration runs and demonstrate results close to those specified in the TPC Technical Design Report.
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.
The polarization of the Λ and ¯Λ hyperons along the beam (z) direction, Pz, has been measured in Pb-Pb collisions at √sNN=5.02 TeV recorded with ALICE at the Large Hadron Collider (LHC). The main contribution to Pz comes from elliptic flow-induced vorticity and can be characterized by the second Fourier sine coefficient Pz,s2=⟨Pzsin(2φ−2Ψ2)⟩, where φ is thhyperon azimuthal emission angle and Ψ2 is the elliptic flow plane angle. We report the measurement of Pz,s2 for different collision centralities and in the 30%–50% centrality interval as a function of the hyperon transverse momentum and rapidity. The Pz,s2 is positive similarly as measured by the STAR Collaboration in Au-Au collisions at √sNN=200 GeV, with somewhat smaller amplitude in the semicentral collisions. This is the first experimental evidence of a nonzero hyperon Pz in Pb-Pb collisions at the LHC. The comparison of the measured Pz,s2 with the hydrodynamic model calculations shows sensitivity to the competing contributions from thermal and the recently found shear-induced vorticity, as well as to whether the polarization is acquired at the quark-gluon plasma or the hadronic phase.
Correction to: ALICE Collaboration. Direct observation of the dead-cone effect in quantum chromodynamics. Nature 605, 440–446 (2022). https://doi.org/10.1038/s41586-022-04572-w
Themultiplicity dependence of the pseudorapidity density of charged particles in proton–proton (pp) collisions at centre-of-mass energies √s = 5.02, 7 and 13 TeV measured by ALICE is reported. The analysis relies on track segments measured in the midrapidity range (|η| < 1.5). Results are presented for inelastic events having at least one charged particle produced in the pseudorapidity interval |η| < 1. The multiplicity dependence of the pseudorapidity density of charged particles is measured with mid- and forward rapidity multiplicity estimators, the latter being less affected by autocorrelations.Adetailed comparison with predictions from the PYTHIA 8 and EPOS LHC event generators is also presented. The results can be used to constrain models for particle production as a function of multiplicity in pp collisions.
The first measurement of the production of pions, kaons, (anti-)protons and φ mesons at midrapidity in Xe–Xe collisions at √sNN = 5.44 TeV is presented. Transverse momentum (pT) spectra and pT-integrated yields are extracted in several centrality intervals bridging from p–Pb to mid-central Pb–Pb collisions in terms of final-state multiplicity. The study of Xe–Xe and Pb–Pb collisions allows systems at similar charged-particle multiplicities but with different initial geometrical eccentricities to be investigated. A detailed comparison of the spectral shapes in the two systems reveals an opposite behaviour for radial and elliptic flow. In particular, this study shows that the radial flow does not depend on the colliding system when compared at similar charged-particle multiplicity. In terms of hadron chemistry, the previously observed smooth evolution of particle ratios with multiplicity from small to large collision systems is also found to hold in Xe–Xe. In addition, our results confirm that two remarkable features of particle production at LHC energies are also valid in the collision of medium-sized nuclei: the lower proton-to-pion ratio with respect to the thermal model expectations and the increase of the φ-to-pion ratio with increasing final-state multiplicity.
We present the first measurement of event-by-event fluctuations in the kaon sector in Pb – Pb collisions at √sNN = 2.76 TeV with the ALICE detector at the LHC. The robust fluctuation correlator νdyn is used to evaluate the magnitude of fluctuations of the relative yields of neutral and charged kaons, as well as the relative yields of charged kaons, as a function of collision centrality and selected kinematic ranges. While the correlator νdyn[K+,K−] exhibits a scaling approximately in inverse proportion of the charged particle multiplicity, νdyn[K0 S ,K±] features a significant deviation from such scaling. Within uncertainties, the value of νdyn[K0 S ,K±] is independent of the selected transverse momentum interval, while it exhibits a pseudorapidity dependence. The results are compared with HIJING, AMPT and EPOS–LHC predictions, and are further discussed in the context of the possible production of disoriented chiral condensates in central Pb – Pb collisions.
The production of baryons and K0 S mesons (V0 particles) was measured in p–Pb collisions at √sNN = 5.02 TeV and pp collisions at √s = 7 TeV with ALICE at the LHC. The production of these strange particles is studied separately for particles associated with hard scatterings and the underlying event to shed light on the baryon-to-meson ratio enhancement observed at intermediate transverse momentum (pT) in high multiplicity pp and p–Pb collisions. Hard scatterings are selected on an eventby-event basis with jets reconstructed with the anti-kT algorithm using charged particles. The production of strange particles associated with jets pch T, jet > 10 and pch T, jet > 20 GeV/c in p–Pb collisions, and with jet pch T, jet > 10 GeV/c in pp collisions is reported as a function of pT. Its dependence on angular distance from the jet axis, R(V0, jet), for jets with pch T, jet > 10 GeV/c in p–Pb collisions is reported as well. The pT-differential production spectra of strange particles associated with jets are found to be harder compared to that in the underlying event and both differ from the inclusive measurements. In events containing a jet, the density of the V0 particles in the underlying event is found to be larger than the density in the minimum bias events. The /K0 S ratio associated with jets in p–Pb collisions is consistent with the ratio in pp collisions and follows the expectation of jets fragmenting in vacuum. On the other hand, this ratio within jets is consistently lower than the one obtained in the underlying event and it does not show the characteristic enhancement of baryons at intermediate pT often referred to as “baryon anomaly” in the inclusive measurements.
Neutral pion (π0) and η meson production cross sections were measured up to unprecedentedly high transverse momenta (pT) in p–Pb collisions at √sNN = 8.16 TeV. The mesons were reconstructed via their two-photon decay channel in the rapidity interval −1.3 < y < 0.3 in the ranges of 0.4 < pT < 200 GeV/c and 1.0 < pT < 50 GeV/c, respectively. The respective nuclear modification factor (RpPb) is presented for pT up to of 200 and 30 GeV/c, where the former was achieved by extending the π0 measurement in pp collisions at √s = 8 TeV using the merged cluster technique. The values of RpPb are below unity for pT < 10 GeV/c, while they are consistent with unity for pT > 10 GeV/c, leaving essentially no room for final state energy loss. The new data provide strong constraints for nuclear parton distribution and fragmentation functions over a broad kinematic range and are compared to model predictions as well as previous results at √sNN = 5.02 TeV.
The production of ϒ mesons in Pb–Pb collisions at a centre-of-mass energy per nucleon pair √sNN = 5.02 TeV is measured with the muon spectrometer of the ALICE detector at the LHC. The yields as well as the nuclear modification factors are determined in the forward rapidity region 2.5 < y < 4.0, as a function of rapidity, transverse momentum and collision centrality. The results show that the production of the ϒ(1S) meson is suppressed by a factor of about three with respect to the production in proton–proton collisions. For the first time, a significant signal for the ϒ(2S) meson is observed at forward rapidity, indicating a suppression stronger by about a factor 2–3 with respect to the ground state. The measurements are compared with transport, hydrodynamic, comover and statistical hadronisation model calculations.
This work presents new constraints on the existence and the binding energy of a possible – bound state, the H-dibaryon, derived from – femtoscopic measurements by the ALICE collaboration. The results are obtained from a new measurement using the femtoscopy technique in pp collisions at √s = 13 TeV and p–Pb collisions at √sNN = 5.02 TeV, combined with previously published results from pp collisions at √s = 7 TeV. The – scattering parameter space, spanned by the inverse scattering length f −1 0 and the effective range d0, is constrained by comparing the measured – correlation function with calculations obtained within the Lednický model. The data are compatible with hypernuclei results and lattice computations, both predicting a shallow attractive interaction, and permit to test different theoretical approaches describing the – interaction. The region in the (f −1 0 ,d0) plane which would accommodate a – bound state is substantially restricted compared to previous studies. The binding energy of the possible – bound state is estimated within an effective-range expansion approach and is found to be B = 3.2+1.6 −2.4(stat)+1.8 −1.0(syst) MeV.
In this letter, the production of deuterons and anti-deuterons in pp collisions at √s = 7 TeV is studied as a function of the charged-particle multiplicity density at mid-rapidity with the ALICE detector at the LHC. Production yields are measured at mid-rapidity in five multiplicity classes and as a function of the deuteron transverse momentum (pT). The measurements are discussed in the context of hadron–coalescence models. The coalescence parameter B2, extracted from the measured spectra of (anti-)deuterons and primary (anti-)protons, exhibits no significant pT-dependence for pT < 3 GeV/c, in agreement with the expectations of a simple coalescence picture. At fixed transverse momentum per nucleon, the B2 parameter is found to decrease smoothly from low multiplicity pp to Pb–Pb collisions, in qualitative agreement with more elaborate coalescence models. The measured mean transverse momentum of (anti-)deuterons in pp is not reproduced by the Blast-Wave model calculations that simultaneously describe pion, kaon and proton spectra, in contrast to central Pb–Pb collisions. The ratio between the pT-integrated yield of deuterons to protons, d/p, is found to increase with the chargedparticle multiplicity, as observed in inelastic pp collisions at different centre-of-mass energies. The d/p ratios are reported in a wide range, from the lowest to the highest multiplicity values measured in pp collisions at the LHC.
Pion-kaon femtoscopy and the lifetime of the hadronic phase in Pb-Pb collisions at √sNN = 2.76 TeV
()
In this paper, the first femtoscopic analysis of pion–kaon correlations at the LHC is reported. The analysis was performed on the Pb–Pb collision data at √sNN = 2.76 TeV recorded with the ALICE detector. The non-identical particle correlations probe the spatio-temporal separation between sources of different particle species as well as the average source size of the emitting system. The sizes of the pion and kaon sources increase with centrality, and pions are emitted closer to the centre of the system and/or later than kaons. This is naturally expected in a system with strong radial flow and is qualitatively reproduced by hydrodynamic models. ALICE data on pion–kaon emission asymmetry are consistent with (3+1)-dimensional viscous hydrodynamics coupled to a statistical hadronisation model, resonance propagation, and decay code THERMINATOR 2 calculation, with an additional time delay between 1 and 2 fm/c for kaons. The delay can be interpreted as evidence for a significant hadronic rescattering phase in heavy-ion collisions at the LHC.
We report on the measurement of the size of the particle-emitting source from two-baryon correlations with ALICE in high-multiplicity pp collisions at √s = 13 TeV. The source radius is studied with low relative momentum p–p, p–p, p–, and p– pairs as a function of the pair transverse mass mT considering for the first time in a quantitative way the effect of strong resonance decays. After correcting for this effect, the radii extracted for pairs of different particle species agree. This indicates that protons, antiprotons, s, and s originate from the same source. Within the measured mT range (1.1–2.2) GeV/c2 the invariant radius of this common source varies between 1.3 and 0.85 fm. These results provide a precise reference for studies of the strong hadron–hadron interactions and for the investigation of collective properties in small colliding systems.
Multiplicity dependence of inclusive J/ψ production at midrapidity in pp collisions at √s = 13 TeV
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Measurements of the inclusive J/ψ yield as a function of charged-particle pseudorapidity density dNch/dη in pp collisions at √s = 13 TeV with ALICE at the LHC are reported. The J/ψ meson yield is measured at midrapidity (|y| < 0.9) in the dielectron channel, for events selected based on the charged-particle multiplicity at midrapidity (|η| < 1) and at forward rapidity (−3.7 < η < −1.7 and 2.8 < η < 5.1); both observables are normalized to their corresponding averages in minimum bias events. The increase of the normalized J/ψ yield with normalized dNch/dη is significantly stronger than linear and dependent on the transverse momentum. The data are compared to theoretical predictions, which describe the observed trends well, albeit not always quantitatively.
ϒ production in p–Pb interactions is studied at the centre-of-mass energy per nucleon–nucleon collision √sNN = 8.16 TeV with the ALICE detector at the CERN LHC. The measurement is performed reconstructing bottomonium resonances via their dimuon decay channel, in the centre-of-mass rapidity intervals 2.03 < ycms < 3.53 and −4.46 < ycms < −2.96, down to zero transverse momentum. In this work, results on the ϒ(1S) production cross section as a function of rapidity and transverse momentum are presented. The corresponding nuclear modification factor shows a suppression of the ϒ(1S) yields with respect to pp collisions, both at forward and backward rapidity. This suppression is stronger in the low transverse momentum region and shows no significant dependence on the centrality of the interactions. Furthermore, the ϒ(2S) nuclear modification factor is evaluated, suggesting a suppression similar to that of the ϒ(1S). A first measurement of the ϒ(3S) has also been performed. Finally, results are compared with previous ALICE measurements in p–Pb collisions at √sNN = 5.02 TeV and with theoretical calculations.
The striking similarities that have been observed between high-multiplicity proton-proton (pp) collisions and heavy-ion collisions can be explored through multiplicity-differential measurements of identified hadrons in pp collisions. With these measurements, it is possible to study mechanisms such as collective flow that determine the shapes of hadron transverse momentum (pT) spectra, to search for possible modifications of the yields of short-lived hadronic resonances due to scattering effects in an extended hadron-gas phase, and to investigate different explanations provided by phenomenological models for enhancement of strangeness production with increasing multiplicity. In this paper, these topics are addressed through measurements of the K∗(892)0 and φ(1020) mesons at midrapidity in pp collisions at √s = 13 TeV as a function of the charged-particle multiplicity. The results include the pT spectra, pT-integrated yields, mean transverse momenta, and the ratios of the yields of these resonances to those of longer-lived hadrons. Comparisons with results from other collision systems and energies, as well as predictions from phenomenological models, are also discussed.
The measurement of the deuteron and anti-deuteron production in the rapidity range −1 < y < 0 as a function of transverse momentum and event multiplicity in p–Pb collisions at √sNN = 5.02 TeV is presented. (Anti-)deuterons are identified via their specific energy loss dE/dx and via their time-offlight. Their production in p–Pb collisions is compared to pp and Pb–Pb collisions and is discussed within the context of thermal and coalescence models. The ratio of integrated yields of deuterons to protons (d/p) shows a significant increase as a function of the charged-particle multiplicity of the event starting from values similar to those observed in pp collisions at low multiplicities and approaching those observed in Pb–Pb collisions at high multiplicities. The mean transverse particle momenta are extracted from the deuteron spectra and the values are similar to those obtained for p and particles. Thus, deuteron spectra do not follow mass ordering. This behaviour is in contrast to the trend observed for non-composite particles in p–Pb collisions. In addition, the production of the rare 3He and 3He nuclei has been studied. The spectrum corresponding to all non-single diffractive p-Pb collisions is obtained in the rapidity window −1 < y < 0 and the pT-integrated yield dN/dy is extracted. It is found that the yields of protons, deuterons, and 3He, normalised by the spin degeneracy factor, follow an exponential decrease with mass number.
We present a measurement of azimuthal correlations between inclusive J/ψ and charged hadrons in p–Pb collisions recorded with the ALICE detector at the CERN LHC. The J/ψ are reconstructed at forward (p-going, 2.03<y<3.53) and backward (Pb-going, −4.46<y<−2.96) rapidity via their μ+μ− decay channel, while the charged hadrons are reconstructed at mid-rapidity (|η|<1.8). The correlations are expressed in terms of associated charged-hadron yields per J/ψ trigger. A rapidity gap of at least 1.5 units is required between the trigger J/ψ and the associated charged hadrons. Possible correlations due to collective effects are assessed by subtracting the associated per-trigger yields in the low-multiplicity collisions from those in the high-multiplicity collisions. After the subtraction, we observe a strong indication of remaining symmetric structures at Δφ≈0 and Δφ≈π, similar to those previously found in two-particle correlations at middle and forward rapidity. The corresponding second-order Fourier coefficient (v2) in the transverse momentum interval between 3 and 6 GeV/c is found to be positive with a significance of about 5σ. The obtained results are similar to the J/ψ v2 coefficients measured in Pb–Pb collisions at sNN=5.02 TeV, suggesting a common mechanism at the origin of the J/ψ v2.
A measurement of the production of prompt +c baryons in Pb–Pb collisions at √sNN = 5.02 TeV with the ALICE detector at the LHC is reported. The +c and − c were reconstructed at midrapidity (|y| < 0.5) via the hadronic decay channel +c → pK0 S (and charge conjugate) in the transverse momentum and centrality intervals 6 < pT < 12 GeV/c and 0–80%. The +c /D0 ratio, which is sensitive to the charm quark hadronisation mechanisms in the medium, is measured and found to be larger than the ratio measured in minimum-bias pp collisions at √s = 7 TeV and in p–Pb collisions at √sNN = 5.02 TeV. In particular, the values in p–Pb and Pb–Pb collisions differ by about two standard deviations of the combined statistical and systematic uncertainties in the common pT interval covered by the measurements in the two collision systems. The + c /D0 ratio is also compared with model calculations including different implementations of charm quark hadronisation. The measured ratio is reproduced by models implementing a pure coalescence scenario, while adding a fragmentation contribution leads to an underestimation. The + c nuclear modification factor, RAA, is also presented. The measured values of the RAA of + c , D+ s and non-strange D mesons are compatible within the combined statistical and systematic uncertainties. They show, however, a hint of a hierarchy (RD0 AA < RD+ s AA < R+ c AA ), conceivable with a contribution from coalescence mechanisms to charm hadron formation in the medium.
Measurements of the production of muons from heavy-flavour hadron decays in Pb–Pb collisions at √sNN = 5.02 and 2.76 TeV using the ALICE detector at the LHC are reported. The nuclear modification factor RAA at √sNN = 5.02 TeV is measured at forward rapidity (2.5 < y < 4) as a function of transverse momentum pT in central, semi-central, and peripheral collisions over a wide pT interval, 3 < pT < 20 GeV/c, in which muons from beauty-hadron decays are expected to take over from charm as the dominant source at high pT (pT > 7 GeV/c). The RAA shows an increase of the suppression of the yields of muons from heavy-flavour hadron decays with increasing centrality. A suppression by a factor of about three is observed in the 10% most central collisions. The RAA at √sNN = 5.02 TeV is similar to that at √sNN = 2.76 TeV. The precise RAA measurements have the potential to distinguish between model predictions implementing different mechanisms of parton energy loss in the high-density medium formed in heavy-ion collisions. They place important constraints for the understanding of the heavy-quark interaction with the hot and dense QCD medium.
The elliptic and triangular flow coefficients v2 and v3 of prompt D0, D+, and D∗+ mesons were measured at midrapidity (|y| < 0.8) in Pb–Pb collisions at the centre-of-mass energy per nucleon pair of √sNN = 5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decays in the transverse momentum interval 1 < pT < 36 GeV/c in central (0–10%) and semi-central (30–50%) collisions. Compared to pions, protons, and J/ψ mesons, the average D-meson vn harmonics are compatible within uncertainties with a mass hierarchy for pT 3 GeV/c, and are similar to those of charged pions for higher pT. The coupling of the charm quark to the light quarks in the underlying medium is further investigated with the application of the event-shape engineering (ESE) technique to the D-meson v2 and pT-differential yields. The D-meson v2 is correlated with average bulk elliptic flow in both central and semi-central collisions. Within the current precision, the ratios of per-event Dmeson yields in the ESE-selected and unbiased samples are found to be compatible with unity. All the measurements are found to be reasonably well described by theoretical calculations including the effects of charm-quark transport and the recombination of charm quarks with light quarks in a hydrodynamically expanding medium.
Inclusive ϒ(1S) and ϒ(2S) production have been measured in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair √sNN = 5.02 TeV, using the ALICE detector at the CERN LHC. The ϒ mesons are reconstructed in the centre-of-mass rapidity interval 2.5 < y < 4 and in the transversemomentum range pT < 15 GeV/c, via their decays to muon pairs. In this Letter, we present results on the inclusive ϒ(1S) nuclear modification factor RAA as a function of collision centrality, transverse momentum and rapidity. The ϒ(1S) and ϒ(2S) RAA, integrated over the centrality range 0–90%, are 0.37± 0.02(stat) ± 0.03(syst) and 0.10 ± 0.04(stat) ± 0.02(syst), respectively, leading to a ratio Rϒ(2S) AA /Rϒ(1S) AA of 0.28±0.12(stat)±0.06(syst). The observed ϒ(1S) suppression increases with the centrality of the collision and no significant variation is observed as a function of transverse momentum and rapidity.
The multiplicity dependence of jet production in pp collisions at the centre-of-mass energy of s√=13 TeV is studied for the first time. Jets are reconstructed from charged particles using the anti-kT algorithm with resolution parameters R varying from 0.2 to 0.7. The jets are measured in the pseudorapidity range |ηjet|<0.9−R and in the transverse momentum range 5<pchT,jet<140 GeV/c. The multiplicity intervals are categorised by the ALICE forward detector V0. The pT differential cross section of charged-particle jets are compared to leading order (LO) and next-to-leading order (NLO) perturbative quantum chromodynamics (pQCD) calculations. It is found that the data are better described by the NLO calculation, although the NLO prediction overestimates the jet cross section below 20 GeV/c. The cross section ratios for different R are also measured and compared to model calculations. These measurements provide insights into the angular dependence of jet fragmentation. The jet yield increases with increasing self-normalised charged-particle multiplicity. This increase shows only a weak dependence on jet transverse momentum and resolution parameter at the highest multiplicity. While such behaviour is qualitatively described by the present version of PYTHIA, quantitative description may require implementing new mechanisms for multi-particle production in hadronic collisions.
The study of the production of nuclei and antinuclei in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. In this paper, the production of protons, deuterons and 3He and their charge conjugates at midrapidity is studied as a function of the charged-particle multiplicity in inelastic pp collisions at s√=5.02 TeV using the ALICE detector. Within the uncertainties, the yields of nuclei in pp collisions at s√=5.02 TeV are compatible with those in pp collisions at different energies and to those in p–Pb collisions when compared at similar multiplicities. The measurements are compared with the expectations of coalescence and Statistical Hadronisation Models. The results suggest a common formation mechanism behind the production of light nuclei in hadronic interactions and confirm that they do not depend on the collision energy but on the number of produced particles.
The production of π±, K±, K0S, K∗(892)0, p, ϕ(1020), Λ, Ξ−, Ω−, and their antiparticles was measured in inelastic proton–proton (pp) collisions at a center-of-mass energy of s√ = 13 TeV at midrapidity (|y|<0.5) as a function of transverse momentum (pT) using the ALICE detector at the CERN LHC. Furthermore, the single-particle pT distributions of K0S, Λ, and Λ¯¯¯¯ in inelastic pp collisions at s√=7 TeV are reported here for the first time. The pT distributions are studied at midrapidity within the transverse momentum range 0≤pT≤20 GeV/c, depending on the particle species. The pT spectra, integrated yields, and particle yield ratios are discussed as a function of collision energy and compared with measurements at lower s√ and with results from various general-purpose QCD-inspired Monte Carlo models. A hardening of the spectra at high pT with increasing collision energy is observed, which is similar for all particle species under study. The transverse mass and xT≡2pT/s√ scaling properties of hadron production are also studied. As the collision energy increases from s√ = 7–13 TeV, the yields of non- and single-strange hadrons normalized to the pion yields remain approximately constant as a function of s√, while ratios for multi-strange hadrons indicate enhancements. The pT-differential cross sections of π±, K± and p (p¯¯¯) are compared with next-to-leading order perturbative QCD calculations, which are found to overestimate the cross sections for π± and p (p¯¯¯) at high pT.
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.
A measurement of beauty hadron production at mid-rapidity in proton-lead collisions at a nucleon–nucleon centre-of-mass energy sNN−−−√=5.02 TeV is presented. The semi-inclusive decay channel of beauty hadrons into J/ψ is considered, where the J/ψ mesons are reconstructed in the dielectron decay channel at mid-rapidity down to transverse momenta of 1.3 GeV/c. The bb¯ production cross section at mid-rapidity, dσbb¯/dy, and the total cross section extrapolated over full phase space, σbb¯, are obtained. This measurement is combined with results on inclusive J/ψ production to determine the prompt J/ψ cross sections. The results in p–Pb collisions are then scaled to expectations from pp collisions at the same centre-of-mass energy to derive the nuclear modification factor RpPb, and compared to models to study possible nuclear modifications of the production induced by cold nuclear matter effects. RpPb is found to be smaller than unity at low pT for both J/ψ coming from beauty hadron decays and prompt J/ψ.
This paper presents the measurements of π±, K±, p and p¯¯¯ transverse momentum (pT) spectra as a function of charged-particle multiplicity density in proton–proton (pp) collisions at s√ = 13 TeV with the ALICE detector at the LHC. Such study allows us to isolate the center-of-mass energy dependence of light-flavour particle production. The measurements reported here cover a pT range from 0.1 to 20 GeV/c and are done in the rapidity interval |y|<0.5. The pT-differential particle ratios exhibit an evolution with multiplicity, similar to that observed in pp collisions at s√ = 7 TeV, which is qualitatively described by some of the hydrodynamical and pQCD-inspired models discussed in this paper. Furthermore, the pT-integrated hadron-to-pion yield ratios measured in pp collisions at two different center-of-mass energies are consistent when compared at similar multiplicities. This also extends to strange and multi-strange hadrons, suggesting that, at LHC energies, particle hadrochemistry scales with particle multiplicity the same way under different collision energies and colliding systems.
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 invariant differential cross section of inclusive ω(782) meson production at midrapidity (|y|<0.5) in pp collisions at s√=7TeV was measured with the ALICE detector at the LHC over a transverse momentum range of 2<pT<17GeV/c. The ω meson was reconstructed via its ω→π+π−π0 decay channel. The measured ω production cross section is compared to various calculations: PYTHIA 8.2 Monash 2013 describes the data, while PYTHIA 8.2 Tune 4C overestimates the data by about 50%. A recent NLO calculation, which includes a model describing the fragmentation of the whole vector-meson nonet, describes the data within uncertainties below 6GeV/c, while it overestimates the data by up to 50% for higher pT. The ω/π0 ratio is in agreement with previous measurements at lower collision energies and the PYTHIA calculations. In addition, the measurement is compatible with transverse mass scaling within the measured pT range and the ratio is constant with Cω/π0=0.67±0.03~(stat)~±0.04~(sys)~ above a transverse momentum of 2.5GeV/c.
First results on K/π, p/π and K/p fluctuations are obtained with the ALICE detector at the CERN LHC as a function of centrality in Pb--Pb collisions at sNN−−−√=2.76 TeV. The observable νdyn, which is defined in terms of the moments of particle multiplicity distributions, is used to quantify the magnitude of dynamical fluctuations of relative particle yields and also provides insight into the correlation between particle pairs. This study is based on a novel experimental technique, called the Identity Method, which allows one to measure the moments of multiplicity distributions in case of incomplete particle identification. The results for p/π show a change of sign in νdyn from positive to negative towards more peripheral collisions. For central collisions, the results follow the smooth trend of the data at lower energies and νdyn exhibits a change in sign for p/π and K/p.
The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity (|y|<0.5) are measured in proton-proton collisions at s√ = 13 TeV as a function of the charged particle multiplicity, using the ALICE detector at the LHC. The production rates of K0S, Λ, Ξ, and Ω increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV.
Neutral pion and η meson invariant differential yields were measured in non-single diffractive p–Pb collisions at sNN−−−√ = 5.02 TeV with the ALICE experiment at the CERN LHC. The analysis combines results from three complementary photon measurements, utilizing the PHOS and EMCal calorimeters and the Photon Conversion Method. The invariant differential yields of π0 and η meson inclusive production are measured near mid-rapidity in a broad transverse momentum range of 0.3<pT<20 GeV/c and 0.7<pT<20 GeV/c, respectively. The measured η/π0 ratio increases with pT and saturates for pT > 4 GeV/c at 0.483±0.015stat±0.015sys. A deviation from mT scaling is observed for pT< 2 GeV/c. The measured η/π0 ratio is consistent with previous measurements from proton-nucleus and pp collisions over the full pT range. The measured η/π0 ratio at high pT also agrees within uncertainties with measurements from nucleus–nucleus collisions. The π0 and η yields in p–Pb relative to the scaled pp interpolated reference, RpPb, are presented for 0.3<pT< 20 GeV/c and 0.7<pT< 20 GeV/c, respectively. The results are compared with theoretical model calculations. The values of RpPb are consistent with unity for transverse momenta above 2 GeV/c. These results support the interpretation that the suppressed yield of neutral mesons measured in Pb–Pb collisions at LHC energies is due to parton energy loss in the hot QCD medium.
The energy deposited at very forward rapidities (very forward energy) is a powerful tool for characterising proton fragmentation in pp and p-Pb collisions. The correlation of very forward energy with particle production at midrapidity provides direct insights into the initial stages and the subsequent evolution of the collision. Furthermore, the correlation with the production of particles with large transverse momenta at midrapidity provides information complementary to the measurements of the underlying event, which are usually interpreted in the framework of models implementing centrality-dependent multiple parton interactions.
Results about very forward energy, measured by the ALICE zero degree calorimeters (ZDCs), and its dependence on the activity measured at midrapidity in pp collisions at s√ = 13 TeV and in p-Pb collisions at sNN−−−√ = 8.16 TeV are discussed. The measurements performed in pp collisions are compared with the expectations of three hadronic interaction event generators: PYTHIA 6 (Perugia 2011 tune), PYTHIA 8 (Monash tune), and EPOS LHC. These results provide new constraints on the validity of models in describing the beam remnants at very forward rapidities, where perturbative QCD cannot be used.
ϕ meson measurements provide insight into strangeness production, which is one of the key observables for the hot medium formed in high-energy heavy-ion collisions. ALICE measured ϕ production through its decay in muon pairs in Pb–Pb collisions at sNN−−−√=2.76 TeV in the intermediate transverse momentum range 2<pT<5 GeV/c and in the rapidity interval 2.5<y<4. The ϕ yield was measured as a function of the transverse momentum and collision centrality. The nuclear modification factor was obtained as a function of the average number of participating nucleons. Results were compared with the ones obtained via the kaon decay channel in the same pT range at midrapidity. The values of the nuclear modification factor in the two rapidity regions are in agreement within uncertainties.
The production of prompt D0, D+, and D*+ mesons was measured at midrapidity (|y| < 0.5) in Pb–Pb collisions at the centre-of-mass energy per nucleon–nucleon pair sNN−−−√ = 5.02 TeV with the ALICE detector at the LHC. The D mesons were reconstructed via their hadronic decay channels and their production yields were measured in central (0–10%) and semicentral (30–50%) collisions. The measurement was performed up to a transverse momentum (pT) of 36 or 50 GeV/c depending on the D meson species and the centrality interval. For the first time in Pb–Pb collisions at the LHC, the yield of D0 mesons was measured down to pT = 0, which allowed a model-independent determination of the pT-integrated yield per unit of rapidity (dN/dy). A maximum suppression by a factor 5 and 2.5 was observed with the nuclear modification factor (RAA) of prompt D mesons at pT = 6–8 GeV/c for the 0–10% and 30–50% centrality classes, respectively. The D-meson RAA is compared with that of charged pions, charged hadrons, and J/ψ mesons as well as with theoretical predictions. The analysis of the agreement between the measured RAA, elliptic (v2) and triangular (v3) flow, and the model predictions allowed us to constrain the charm spatial diffusion coefficient Ds. Furthermore the comparison of RAA and v2 with different implementations of the same models provides an important insight into the role of radiative energy loss as well as charm quark recombination in the hadronisation mechanisms.
The production of J/ψ is measured at midrapidity (|y| < 0.9) in proton-proton collisions at s√ = 5.02 and 13 TeV, through the dielectron decay channel, using the ALICE detector at the Large Hadron Collider. The data sets used for the analyses correspond to integrated luminosities of Lint = 19.4 ± 0.4 nb−1 and Lint = 32.2 ± 0.5 nb−1 at s√ = 5.02 and 13 TeV, respectively. The fraction of non-prompt J/ψ mesons, i.e. those originating from the decay of beauty hadrons, is measured down to a transverse momentum pT = 2 GeV/c (1 GeV/c) at s√ = 5.02 TeV (13 TeV). The pT and rapidity (y) differential cross sections, as well as the corresponding values integrated over pT and y, are carried out separately for prompt and non-prompt J/ψ mesons. The results are compared with measurements from other experiments and theoretical calculations based on quantum chromodynamics (QCD). The shapes of the pT and y distributions of beauty quarks predicted by state-of-the-art perturbative QCD models are used to extrapolate an estimate of the bb¯¯¯ pair cross section at midrapidity and in the total phase space. The total bb¯¯¯ cross sections are found to be σbb¯¯¯ = 541 ± 45 (stat.) ± 69 (syst.)+10−12 (extr.) μb and σbb¯¯¯ = 218±37 (stat.)±31 (syst.)+8.2−9.1 (extr.) μb at s√ = 13 and 5.02 TeV, respectively. The value obtained from the combination of ALICE and LHCb measurements in pp collisions at s√ = 13 TeV is also provided.
Understanding the production mechanism of light (anti)nuclei is one of the key challenges of nuclear physics and has important consequences for astrophysics, since it provides an input for indirect dark-matter searches in space. In this paper, the latest results about the production of light (anti)nuclei in pp collisions at s√ = 13 TeV are presented, focusing on the comparison with the predictions of coalescence and thermal models. For the first time, the coalescence parameters B2 for deuterons and B3 for helions are compared with parameter-free theoretical predictions that are directly constrained by the femtoscopic measurement of the source radius in the same event class. A fair description of the data with a Gaussian wave function is observed for both deuteron and helion, supporting the coalescence mechanism for the production of light (anti)nuclei in pp collisions. This method paves the way for future investigations of the internal structure of more complex nuclear clusters, including the hypertriton.
The pT-differential production cross section of prompt Λ +c charmed baryons was measured with the ALICE detector at the Large Hadron Collider (LHC) in pp collisions at s√=7 TeV and in p-Pb collisions at sNN−−−√=5.02 TeV at midrapidity. The Λ +c and Λ¯¯¯¯c¯¯¯ were reconstructed in the hadronic decay modes Λ +c → pK−π+, Λ +c → pK 0S and in the semileptonic channel Λ +c → e+νeΛ (and charge conjugates). The measured values of the Λ +c/D0 ratio, which is sensitive to the c-quark hadronisation mechanism, and in particular to the production of baryons, are presented and are larger than those measured previously in different colliding systems, centre-of-mass energies, rapidity and pT intervals, where the Λ +c production process may differ. The results are compared with the expectations obtained from perturbative Quantum Chromodynamics calculations and Monte Carlo event generators. Neither perturbative QCD calculations nor Monte Carlo models reproduce the data, indicating that the fragmentation of heavy-flavour baryons is not well understood. The first measurement at the LHC of the Λ +c nuclear modification factor, RpPb, is also presented. The RpPb is found to be consistent with unity and with that of D mesons within the uncertainties, and consistent with a theoretical calculation that includes cold nuclear matter effects and a calculation that includes charm quark interactions with a deconfined medium.
The measurement of azimuthal correlations of charged particles is presented for Pb-Pb collisions at sNN−−−√=2.76 TeV and p-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the CERN Large Hadron Collider. These correlations are measured for the second, third and fourth order flow vector in the pseudorapidity region |η| < 0.8 as a function of centrality and transverse momentum p T using two observables, to search for evidence of p T-dependent flow vector fluctuations. For Pb-Pb collisions at 2.76 TeV, the measurements indicate that p T-dependent fluctuations are only present for the second order flow vector. Similar results have been found for p-Pb collisions at 5.02 TeV. These measurements are compared to hydrodynamic model calculations with event-by-event geometry fluctuations in the initial state to constrain the initial conditions and transport properties of the matter created in Pb–Pb and p–Pb collisions.
Measurement of Z-boson production in p-Pb collisions at sNN−−−√ = 8.16 TeV and Pb-Pb collisions at sNN−−−√ = 5.02 TeV is reported. It is performed in the dimuon decay channel, through the detection of muons with pseudorapidity −4 < ημ < −2.5 and transverse momentum pμT > 20 GeV/c in the laboratory frame. The invariant yield and nuclear modification factor are measured for opposite-sign dimuons with invariant mass 60 < mμμ < 120 GeV/c2 and rapidity 2.5 < yμμcms < 4. They are presented as a function of rapidity and, for the Pb-Pb collisions, of centrality as well. The results are compared with theoretical calculations, both with and without nuclear modifications to the Parton Distribution Functions (PDFs). In p-Pb collisions the center-of-mass frame is boosted with respect to the laboratory frame, and the measurements cover the backward (−4.46 < yμμcms < −2.96) and forward (2.03 < yμμcms < 3.53) rapidity regions. For the p-Pb collisions, the results are consistent within experimental and theoretical uncertainties with calculations that include both free-nucleon and nuclear-modified PDFs. For the Pb-Pb collisions, a 3.4σ deviation is seen in the integrated yield between the data and calculations based on the free-nucleon PDFs, while good agreement is found once nuclear modifications are considered.
The pT-differential non-linear flow modes, v4,22, v5,32, v6,33 and v6,222 for π±, K±, K0S , p + p¯¯¯, Λ + Λ¯¯¯¯ and ϕ-meson have been measured for the first time at sNN−−−√ = 5.02 TeV in Pb-Pb collisions with the ALICE detector at the Large Hadron Collider. The results were obtained with a multi-particle technique, correlating the identified hadrons with reference charged particles from a different pseudorapidity region. These non-linear observables probe the contribution from the second and third order initial spatial anisotropy coefficients to higher flow harmonics. All the characteristic features observed in previous pT-differential anisotropic flow measurements for various particle species are also present in the non-linear flow modes, i.e. increase of magnitude with increasing centrality percentile, mass ordering at low pT and particle type grouping in the intermediate pT range. Hydrodynamical calculations (iEBE-VISHNU) that use different initial conditions and values of shear and bulk viscosity to entropy density ratios are confronted with the data at low transverse momenta. These calculations exhibit a better agreement with the anisotropic flow coefficients than the non-linear flow modes. These observations indicate that non-linear flow modes can provide additional discriminatory power in the study of initial conditions as well as new stringent constraints to hydrodynamical calculations.
This article reports measurements characterizing the Underlying Event (UE) associated with hard scatterings at midrapidity (|η| < 0.8) in pp collisions at s√ = 13 TeV. The hard scatterings are identified by the leading particle, the charged particle with the highest transverse momentum (pleadingT) in the event. Charged-particle number-densities and summed transverse-momentum densities are measured in different azimuthal regions defined with respect to the leading particle direction: Toward, Transverse, and Away. The Toward and Away regions contain the fragmentation products of the hard scatterings in addition to the UE contribution, whereas particles in the Transverse region are expected to originate predominantly from the UE. The study is performed as a function of pleadingT with three different pT thresholds for the associated particles, ptrackT > 0.15, 0.5, and 1.0 GeV/c. The charged-particle density in the Transverse region rises steeply for low values of pleadingT and reaches a plateau. The results confirm the trend that the charged-particle density in the Transverse region shows a stronger increase with s√ than the inclusive charged-particle density at midrapidity. The UE activity is increased by approximately 20% when going from 7 TeV to 13 TeV pp collisions. The plateau in the Transverse region (5 <pleadingT< 40 GeV/c) is further characterized by the probability distribution of its charged-particle multiplicity normalized to its average value (relative transverse activity, RT) and the mean transverse momentum as a function of RT. Experimental results are compared to model calculations using PYTHIA 8 and EPOS LHC. The overall agreement between models and data is within 30%. These measurements provide new insights on the interplay between hard scatterings and the associated UE in pp collisions.
We report the measured transverse momentum (pT) spectra of primary charged particles from pp, p-Pb and Pb-Pb collisions at a center-of-mass energy sNN−−−√=5.02 TeV in the kinematic range of 0.15 < pT< 50 GeV/c and |η| < 0.8. A significant improvement of systematic uncertainties motivated the reanalysis of data in pp and Pb-Pb collisions at sNN−−−√=2.76 TeV, as well as in p-Pb collisions at sNN−−−√=5.02 TeV, which is also presented. Spectra from Pb-Pb collisions are presented in nine centrality intervals and are compared to a reference spectrum from pp collisions scaled by the number of binary nucleon-nucleon collisions. For central collisions, the pT spectra are suppressed by more than a factor of 7 around 6–7 GeV/c with a significant reduction in suppression towards higher momenta up to 30 GeV/c. The nuclear modification factor RpPb, constructed from the pp and p-Pb spectra measured at the same collision energy, is consistent with unity above 8 GeV/c. While the spectra in both pp and Pb-Pb collisions are substantially harder at sNN−−−√=5.02 TeV compared to 2.76 TeV, the nuclear modification factors show no significant collision energy dependence. The obtained results should provide further constraints on the parton energy loss calculations to determine the transport properties of the hot and dense QCD matter.
The inclusive J/ψ production in Pb–Pb collisions at the center-of-mass energy per nucleon pair sNN−−−√ = 5.02 TeV, measured with the ALICE detector at the CERN LHC, is reported. The J/ψ meson is reconstructed via the dimuon decay channel at forward rapidity (2.5 < y < 4) down to zero transverse momentum. The suppression of the J/ψ yield in Pb–Pb collisions with respect to binary-scaled pp collisions is quantified by the nuclear modification factor (RAA). The RAA at sNN−−−√ = 5.02 TeV is presented and compared with previous measurements at sNN−−−√ = 2.76 TeV as a function of the centrality of the collision, and of the J/ψ transverse momentum and rapidity. The inclusive J/ψ RAA shows a suppression increasing toward higher transverse momentum, with a steeper dependence for central collisions. The modification of the J/ψ average transverse momentum and average squared transverse momentum is also studied. Comparisons with the results of models based on a transport equation and on statistical hadronization are carried out.
Production cross sections of muons from semi-leptonic decays of charm and beauty hadrons were measured at forward rapidity (2.5 < y < 4) in proton-proton (pp) collisions at a centre-of-mass energy s√ = 5.02 TeV with the ALICE detector at the CERN LHC. The results were obtained in an extended transverse momentum interval, 2 < pT< 20 GeV/c, and with an improved precision compared to previous measurements performed in the same rapidity interval at centre-of-mass energies s√ = 2.76 and 7 TeV. The pT- and y-differential production cross sections as well as the pT-differential production cross section ratios between different centre-of-mass energies and different rapidity intervals are described, within experimental and theoretical uncertainties, by predictions based on perturbative QCD.
The second (v2) and third (v3) flow harmonic coefficients of J/ψ mesons are measured at forward rapidity (2.5 < y < 4.0) in Pb-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the LHC. Results are obtained with the scalar product method and reported as a function of transverse momentum, pT, for various collision centralities. A positive value of J/ψ v3 is observed with 3.7σ significance. The measurements, compared to those of prompt D0 mesons and charged particles at mid-rapidity, indicate an ordering with vn(J/ψ) < vn(D0) < vn(h±) (n = 2, 3) at low and intermediate pT up to 6 GeV/c and a convergence with v2(J/ψ) ≈ v2(D0) ≈ v2(h±) at high pT above 6–8 GeV/c. In semi-central collisions (5–40% and 10–50% centrality intervals) at intermediate pT between 2 and 6 GeV/c, the ratio v3/v2 of J/ψ mesons is found to be significantly lower (4.6σ) with respect to that of charged particles. In addition, the comparison to the prompt D0-meson ratio in the same pT interval suggests an ordering similar to that of the v2 and v3 coefficients. The J/ψ v2 coefficient is further studied using the Event Shape Engineering technique. The obtained results are found to be compatible with the expected variations of the eccentricity of the initial-state geometry.
The inclusive charged particle transverse momentum distribution is measured in proton–proton collisions at s=900 GeV at the LHC using the ALICE detector. The measurement is performed in the central pseudorapidity region (|η|<0.8) over the transverse momentum range 0.15<pT<10 GeV/c. The correlation between transverse momentum and particle multiplicity is also studied. Results are presented for inelastic (INEL) and non-single-diffractive (NSD) events. The average transverse momentum for |η|<0.8 is 〈pT〉INEL=0.483±0.001 (stat.)±0.007 (syst.) GeV/c and 〈pT〉NSD=0.489±0.001 (stat.)±0.007 (syst.) GeV/c, respectively. The data exhibit a slightly larger 〈pT〉 than measurements in wider pseudorapidity intervals. The results are compared to simulations with the Monte Carlo event generators PYTHIA and PHOJET.
Results on the production of 4He and Image 1 nuclei in Pb–Pb collisions at √sNN=2.76TeV in the rapidity range |y|<1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0–10% central events are found to be dN/dyHe4=(0.8±0.4(stat)±0.3(syst))×10−6 and Image 2, respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (Tchem=156MeV) as for light hadrons. The measured ratio of Image 3 is 1.4±0.8(stat)±0.5(syst).
Transverse momentum (pT ) spectra of charged particles at mid-pseudorapidity in Xe–Xe collisions at √sNN=5.44TeV measured with the ALICE apparatus at the Large Hadron Collider are reported. The kinematic range 0.15<pT<50GeV/c and |η|<0.8 is covered. Results are presented in nine classes of collision centrality in the 0–80% range. For comparison, a pp reference at the collision energy of √s=5.44 TeV is obtained by interpolating between existing pp measurements at √s=5.02 and 7 TeV. The nuclear modification factors in central Xe–Xe collisions and Pb–Pb collisions at a similar center-of-mass energy of √sNN=5.02 TeV, and in addition at 2.76 TeV, at analogous ranges of charged particle multiplicity density 〈dNch/dη〉 show a remarkable similarity at pT>10 GeV/c. The centrality dependence of the ratio of the average transverse momentum 〈pT〉 in Xe–Xe collisions over Pb–Pb collision at √s=5.02 TeV is compared to hydrodynamical model calculations.
The production of muons from heavy-flavour hadron decays in p–Pb collisions at t √sNN=5.02 TeV was studied for 2<pT<16 GeV/c with the ALICE detector at the CERN LHC. The measurement was performed at forward (p-going direction) and backward (Pb-going direction) rapidity, in the ranges of rapidity in the centre-of-mass system (cms) 2.03<ycms<3.53 and −4.46<ycms<−2.96, respectively. The production cross sections and nuclear modification factors are presented as a function of transverse momentum (pT). At forward rapidity, the nuclear modification factor is compatible with unity while at backward rapidity, in the interval 2.5<pT<3.5 GeV/c, it is above unity by more than 2σ. The ratio of the forward-to-backward production cross sections is also measured in the overlapping interval 2.96<|ycms|<3.53 and is smaller than unity by 3.7σ in 2.5<pT<3.5 GeV/c. The data are described by model calculations including cold nuclear matter effects.
We present measurements of the elliptic (v2), triangular (v3) and quadrangular (v4) anisotropic azimuthal flow over a wide range of pseudorapidities (−3.5<η<5). The measurements are performed with Pb–Pb collisions at √sNN=2.76 TeV using the ALICE detector at the Large Hadron Collider (LHC). The flow harmonics are obtained using two- and four-particle correlations from nine different centrality intervals covering central to peripheral collisions. We find that the shape of vn(η) is largely independent of centrality for the flow harmonics n=2–4, however the higher harmonics fall off more steeply with increasing |η|. We assess the validity of extended longitudinal scaling of v2 by comparing to lower energy measurements, and find that the higher harmonic flow coefficients are proportional to the charged particle densities at larger pseudorapidities. Finally, we compare our measurements to both hydrodynamical and transport models, and find they both have challenges when it comes to describing our data.
The production of charged pions, kaons and (anti)protons has been measured at mid-rapidity (−0.5<y<0) in p–Pb collisions at √sNN=5.02 TeV using the ALICE detector at the LHC. Exploiting particle identification capabilities at high transverse momentum (pT), the previously published pT spectra have been extended to include measurements up to 20 GeV/c for seven event multiplicity classes. The pT spectra for pp collisions at s=7 TeV, needed to interpolate a pp reference spectrum, have also been extended up to 20 GeV/c to measure the nuclear modification factor (RpPb) in non-single diffractive p–Pb collisions. At intermediate transverse momentum (2<pT<10 GeV/c) the proton-to-pion ratio increases with multiplicity in p–Pb collisions, a similar effect is not present in the kaon-to-pion ratio. The pT dependent structure of such increase is qualitatively similar to those observed in pp and heavy-ion collisions. At high pT (>10 GeV/c), the particle ratios are consistent with those reported for pp and Pb–Pb collisions at the LHC energies. At intermediate pT the (anti)proton RpPb shows a Cronin-like enhancement, while pions and kaons show little or no nuclear modification. At high pT the charged pion, kaon and (anti)proton RpPb are consistent with unity within statistical and systematic uncertainties.
The production of prompt D0, Ds+, and Λc+ hadrons, and their ratios, Ds+/D0 and Λc+/D0, are measured in proton–proton collisions at √s=13 TeV at midrapidity (|y|<0.5) with the ALICE detector at the LHC. The measurements are performed as a function of the charm-hadron transverse momentum (pT) in intervals of charged-particle multiplicity, measured with two multiplicity estimators covering different pseudorapidity regions. While the strange to non-strange Ds+/D0 ratio indicates no significant multiplicity dependence, the baryon-to-meson pT-differential Λc+/D0 ratio shows a multiplicity-dependent enhancement, with a significance of 5.3σ for 1<pT<12 GeV/c, comparing the highest multiplicity interval with respect to the lowest one. The measurements are compared with a theoretical model that explains the multiplicity dependence by a canonical treatment of quantum charges in the statistical hadronisation approach, and with predictions from event generators that implement colour reconnection mechanisms beyond the leading colour approximation to model the hadronisation process. The Λc+/D0 ratios as a function of pT present a similar shape and magnitude as the Λ/KS0 ratios in comparable multiplicity intervals, suggesting a potential common mechanism for light- and charm-hadron formation, with analogous multiplicity dependence. The pT-integrated ratios, extrapolated down to pT=0, do not show a significant dependence on multiplicity within the uncertainties.
Multiplicity (Nch) distributions and transverse momentum (pT) spectra of inclusive primary charged particles in the kinematic range of |η|<0.8 and 0.15 GeV/c<pT<10 GeV/c are reported for pp, p–Pb, Xe–Xe and Pb–Pb collisions at centre-of-mass energies per nucleon pair ranging from √sNN=2.76 TeV up to 13 TeV. A sequential two-dimensional unfolding procedure is used to extract the correlation between the transverse momentum of primary charged particles and the charged-particle multiplicity of the corresponding collision. This correlation sharply characterises important features of the final state of a collision and, therefore, can be used as a stringent test of theoretical models. The multiplicity distributions as well as the mean and standard deviation derived from the pT spectra are compared to state-of-the-art model predictions. Providing these fundamental observables of bulk particle production consistently across a wide range of collision energies and system sizes can serve as an important input for tuning Monte Carlo event generators.
We present the first systematic comparison of the charged-particle pseudorapidity densities for three widely different collision systems, pp, pPb, and PbPb, at the top energy of the Large Hadron Collider (√sNN=5.02TeV) measured over a wide pseudorapidity range (−3.5<η<5), the widest possible among the four experiments at that facility. The systematic uncertainties are minimised since the measurements are recorded by the same experimental apparatus (ALICE). The distributions for pPb and PbPb collisions are determined as a function of the centrality of the collisions, while results from pp collisions are reported for inelastic events with at least one charged particle at midrapidity. The charged-particle pseudorapidity densities are, under simple and robust assumptions, transformed to charged-particle rapidity densities. This allows for the calculation and the presentation of the evolution of the width of the rapidity distributions and of a lower bound on the Bjorken energy density, as a function of the number of participants in all three collision systems. We find a decreasing width of the particle production, and roughly a smooth ten fold increase in the energy density, as the system size grows, which is consistent with a gradually higher dense phase of matter.
The pseudorapidity (η) and transverse-momentum (pT) distributions of charged particles produced in proton–proton collisions are measured at the centre-of-mass energy √s=13 TeV. The pseudorapidity distribution in |η|<1.8 is reported for inelastic events and for events with at least one charged particle in |η|<1. The pseudorapidity density of charged particles produced in the pseudorapidity region |η|<0.5 is 5.31±0.18 and 6.46±0.19 for the two event classes, respectively. The transverse-momentum distribution of charged particles is measured in the range 0.15<pT<20 GeV/c and |η|<0.8 for events with at least one charged particle in |η|<1. The evolution of the transverse momentum spectra of charged particles is also investigated as a function of event multiplicity. The results are compared with calculations from PYTHIA and EPOS Monte Carlo generators.
W±-boson production in p–Pb collisions at √sNN = 8.16 TeV and Pb–Pb collisions at √sNN = 5.02 TeV
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The production of the W± bosons measured in p–Pb collisions at a centreof-mass energy per nucleon–nucleon collision √sNN = 8.16 TeV and Pb–Pb collisions at √sNN = 5.02 TeV with ALICE at the LHC is presented. The W± bosons are measured via their muonic decay channel, with the muon reconstructed in the pseudorapidity region −4 < ηµ lab < −2.5 with transverse momentum p µ T > 10 GeV/c. While in Pb–Pb collisions the measurements are performed in the forward (2.5 < yµ cms < 4) rapidity region, in p–Pb collisions, where the centre-of-mass frame is boosted with respect to the laboratory frame, the measurements are performed in the backward (−4.46 < yµ cms < −2.96) and forward (2.03 < yµ cms < 3.53) rapidity regions. The W− and W+ production cross sections, leptoncharge asymmetry, and nuclear modification factors are evaluated as a function of the muon rapidity. In order to study the production as a function of the p–Pb collision centrality, the production cross sections of the W− and W+ bosons are combined and normalised to the average number of binary nucleon–nucleon collision hNcolli. In Pb–Pb collisions, the same measurements are presented as a function of the collision centrality. Study of the binary scaling of the W±-boson cross sections in p–Pb and Pb–Pb collisions is also reported. The results are compared with perturbative QCD calculations, with and without nuclear modifications of the Parton Distribution Functions (PDFs), as well as with available data at the LHC. Significant deviations from the theory expectations are found in the two collision systems, indicating that the measurements can provide additional constraints for the determination of nuclear PDFs and in particular of the light-quark distributions.
Hadronic resonances are used to probe the hadron gas produced in the late stage of heavy-ion collisions since they decay on the same timescale, of the order of 1 to 10 fm/c, as the decoupling time of the system. In the hadron gas, (pseudo)elastic scatterings among the products of resonances that decayed before the kinetic freeze-out and regeneration processes counteract each other, the net effect depending on the resonance lifetime, the duration of the hadronic phase, and the hadronic cross sections at play. In this context, the Σ(1385)± particle is of particular interest as models predict that regeneration dominates over rescattering despite its relatively short lifetime of about 5.5 fm/c. The first measurement of the Σ(1385)± resonance production at midrapidity in Pb-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector is presented in this Letter. The resonances are reconstructed via their hadronic decay channel, Λπ, as a function of the transverse momentum (pT) and the collision centrality. The results are discussed in comparison with the measured yield of pions and with expectations from the statistical hadronization model as well as commonly employed event generators, including PYTHIA8/Angantyr and EPOS3 coupled to the UrQMD hadronic cascade afterburner. None of the models can describe the data. For Σ(1385)±, a similar behaviour as K∗(892)0 is observed in data unlike the predictions of EPOS3 with afterburner.
An excess of J/ψ yield at very low transverse momentum (pT<0.3 GeV/c), originating from coherent photoproduction, is observed in peripheral and semicentral hadronic Pb–Pb collisions at a center-of-mass energy per nucleon pair of sNN=5.02 TeV. The measurement is performed with the ALICE detector via the dimuon decay channel at forward rapidity (2.5<y<4). The nuclear modification factor at very low pT and the coherent photoproduction cross section are measured as a function of centrality down to the 10% most central collisions. These results extend the previous study at sNN=2.76 TeV, confirming the clear excess over hadronic production in the pT range 0−0.3 GeV/c and the centrality range 70–90%, and establishing an excess with a significance greater than 5σ also in the 50–70% and 30–50% centrality ranges. The results are compared with earlier measurements at sNN=2.76 TeV and with different theoretical predictions aiming at describing how coherent photoproduction occurs in hadronic interactions with nuclear overlap.
The transverse momentum (pT) differential yields of (anti-)3He and (anti-)3H measured in p-Pb collisions at sNN−−−√ = 5.02 TeV with ALICE at the Large Hadron Collider (LHC) are presented. The ratios of the pT-integrated yields of (anti-)3He and (anti-)3H to the proton yields are reported, as well as the pT dependence of the coalescence parameters B3 for (anti-)3He and (anti-)3H. For (anti-)3He, the results obtained in four classes of the mean charged-particle multiplicity density are also discussed. These results are compared to predictions from a canonical statistical hadronization model and coalescence approaches. An upper limit on the total yield of 4He¯ is determined.
One of the key challenges for nuclear physics today is to understand from first principles the effective interaction between hadrons with different quark content. First successes have been achieved using techniques that solve the dynamics of quarks and gluons on discrete space-time lattices1,2. Experimentally, the dynamics of the strong interaction have been studied by scattering hadrons off each other. Such scattering experiments are difficult or impossible for unstable hadrons3,4,5,6 and so high-quality measurements exist only for hadrons containing up and down quarks7. Here we demonstrate that measuring correlations in the momentum space between hadron pairs8,9,10,11,12 produced in ultrarelativistic proton–proton collisions at the CERN Large Hadron Collider (LHC) provides a precise method with which to obtain the missing information on the interaction dynamics between any pair of unstable hadrons. Specifically, we discuss the case of the interaction of baryons containing strange quarks (hyperons). We demonstrate how, using precision measurements of proton–omega baryon correlations, the effect of the strong interaction for this hadron–hadron pair can be studied with precision similar to, and compared with, predictions from lattice calculations13,14. The large number of hyperons identified in proton–proton collisions at the LHC, together with accurate modelling15 of the small (approximately one femtometre) inter-particle distance and exact predictions for the correlation functions, enables a detailed determination of the short-range part of the nucleon-hyperon interaction.
The study of the strength and behaviour of the antikaon-nucleon (K¯¯¯¯N) interaction constitutes one of the key focuses of the strangeness sector in low-energy Quantum Chromodynamics (QCD). In this letter a unique high-precision measurement of the strong interaction between kaons and protons, close and above the kinematic threshold, is presented. The femtoscopic measurements of the correlation function at low pair-frame relative momentum of (K+ p ⊕ K− p¯¯¯) and (K− p ⊕ K+ p¯¯¯) pairs measured in pp collisions at s√ = 5, 7 and 13 TeV are reported. A structure observed around a relative momentum of 58 MeV/c in the measured correlation function of (K− p ⊕ K+ p¯¯¯) with a significance of 4.4. σ constitutes the first experimental evidence for the opening of the (K¯¯¯¯0n⊕K0n¯¯¯) isospin breaking channel due to the mass difference between charged and neutral kaons. The measured correlation functions have been compared to Jülich and Kyoto models in addition to the Coulomb potential. The high-precision data at low relative momenta presented in this work prove femtoscopy to be a powerful complementary tool to scattering experiments and provide new constraints above the K¯¯¯¯N threshold for low-energy QCD chiral models.
Two-particle transverse momentum differential correlators, recently measured in Pb-Pb collisions at LHC energies, provide an additional tool to gain insights into particle production mechanisms and infer transport properties, such as the ratio of shear viscosity to entropy density, of the medium created in Pb-Pb collisions. The longitudinal long-range correlations and the large azimuthal anisotropy measured at low transverse momenta in small collision systems, namely pp and p-Pb, at LHC energies resemble manifestations of collective behaviour. This suggests that locally equilibrated matter may be produced in these small collision systems, similar to what is observed in Pb-Pb collisions. In this work, the same two-particle transverse momentum differential correlators are exploited in pp and p-Pb collisions at s√=7 TeV and sNN−−−√=5.02 TeV, respectively, to seek evidence for viscous effects. Specifically, the strength and shape of the correlators are studied as a function of the produced particle multiplicity to identify evidence for longitudinal broadening that might reveal the presence of viscous effects in these smaller systems. The measured correlators and their evolution from pp and p-Pb to Pb-Pb collisions are additionally compared to predictions from Monte Carlo event generators, and the potential presence of viscous effects is discussed.
Production of K0S, Λ (Λ), Ξ±, and Ω± in jets and in the underlying event in pp and p–Pb collisions
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The production of strange hadrons (K0S, Λ, Ξ±, and Ω±), baryon-to-meson ratios (Λ/K0S, Ξ/K0S, and Ω/K0S), and baryon-to-baryon ratios (Ξ/Λ, Ω/Λ, and Ω/Ξ) associated with jets and the underlying event were measured as a function of transverse momentum (pT) in pp collisions at s√=13 TeV and p-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector at the LHC. The inclusive production of the same particle species and the corresponding ratios are also reported. The production of multi-strange hadrons, Ξ± and Ω±, and their associated particle ratios in jets and in the underlying event are measured for the first time. In both pp and p-Pb collisions, the baryon-to-meson and baryon-to-baryon yield ratios measured in jets differ from the inclusive particle production for low and intermediate hadron pT (0.6−6 GeV/c). Ratios measured in the underlying event are in turn similar to those measured for inclusive particle production. In pp collisions, the particle production in jets is compared with PYTHIA 8 predictions with three colour-reconnection implementation modes. None of them fully reproduces the data in the measured hadron pT region. The maximum deviation is observed for Ξ± and Ω±, which reaches a factor of about six. In p-Pb collisions, there is no significant event-multiplicity dependence for particle production in jets, in contrast to what is observed in the underlying event. The presented measurements provide novel constraints on hadronisation and its Monte Carlo description. In particular, they demonstrate that the fragmentation of jets alone is insufficient to describe the strange and multi-strange particle production in hadronic collisions at LHC energies.
Three-body nuclear forces play an important role in the structure of nuclei and hypernuclei and are also incorporated in models to describe the dynamics of dense baryonic matter, such as in neutron stars. So far, only indirect measurements anchored to the binding energies of nuclei can be used to constrain the three-nucleon force, and if hyperons are considered, the scarce data on hypernuclei impose only weak constraints on the three-body forces. In this work, we present the first direct measurement of the p−p−p and p−p−Λ systems in terms of three-particle mixed moments carried out for pp collisions at s√ = 13 TeV. Three-particle cumulants are extracted from the normalised mixed moments by applying the Kubo formalism, where the three-particle interaction contribution to these moments can be isolated after subtracting the known two-body interaction terms. A negative cumulant is found for the p−p−p system, hinting to the presence of a residual three-body effect while for p−p−Λ the cumulant is consistent with zero. This measurement demonstrates the accessibility of three-baryon correlations at the LHC.
The transverse-momentum (pT) spectra of K∗(892)0 and ϕ(1020) measured with the ALICE detector up to pT = 16 GeV/c in the rapidity range −1.2<y<0.3, in p-Pb collisions at the center-of-mass energy per nucleon-nucleon collision sNN−−−√ = 5.02 TeV are presented as a function of charged particle multiplicity and rapidity. The measured pT distributions show a dependence on both multiplicity and rapidity at low pT whereas no significant dependence is observed at high pT. A rapidity dependence is observed in the pT-integrated yield (dN/dy), whereas the mean transverse momentum (⟨pT⟩) shows a flat behavior as a function of rapidity. The rapidity asymmetry (Yasym) at low pT ( < 5 GeV/c) is more significant for higher multiplicity classes. At high pT, no significant rapidity asymmetry is observed in any of the multiplicity classes. Both K∗(892)0 and ϕ(1020) show similar Yasym. The nuclear modification factor (QCP) as a function of pT shows a Cronin-like enhancement at intermediate pT, which is more prominent at higher rapidities (Pb-going direction) and in higher multiplicity classes. At high pT (> 5 GeV/c), the QCP values are greater than unity and no significant rapidity dependence is observed.
This Letter reports on the first measurements of transverse momentum dependent flow angle Ψn and flow magnitude vn fluctuations, determined using new four-particle correlators. The measurements are performed for various centralities in Pb-Pb collisions at a centre-of-mass energy per nucleon pair of sNN−−−√ = 5.02 TeV with ALICE at the CERN Large Hadron Collider. Both flow angle and flow magnitude fluctuations are observed in the presented centrality ranges and are strongest in the most central collisions and for a transverse momentum pT>2 GeV/c. Comparison with theoretical models, including iEBE-VISHNU, MUSIC, and AMPT, show that the measurements exhibit unique sensitivities to the initial state of heavy-ion collisions.
This letter reports measurements which characterize the underlying event associated with hard scatterings at mid-pseudorapidity (|η|<0.8) in pp, p−Pb and Pb−Pb collisions at centre-of-mass energy per nucleon pair, sNN−−−√=5.02 TeV. The measurements are performed with ALICE at the LHC. Different multiplicity classes are defined based on the event activity measured at forward rapidities. The hard scatterings are identified by the leading particle defined as the charged particle with the largest transverse momentum (pT) in the collision and having 8<pT<15 GeV/c. The pT spectra of associated particles (0.5≤pT<6 GeV/c) are measured in different azimuthal regions defined with respect to the leading particle direction: toward, transverse, and away. The associated charged particle yields in the transverse region are subtracted from those of the away and toward regions. The remaining jet-like yields are reported as a function of the multiplicity measured in the transverse region. The measurements show a suppression of the jet-like yield in the away region and an enhancement of high-pT associated particles in the toward region in central Pb−Pb collisions, as compared to minimum-bias pp collisions. These observations are consistent with previous measurements that used two-particle correlations, and with an interpretation in terms of parton energy loss in a high-density quark gluon plasma. These yield modifications vanish in peripheral Pb−Pb collisions and are not observed in either high-multiplicity pp or p−Pb collisions.
The production of K∗(892)0 and ϕ(1020) mesons in proton-proton (pp) and lead-lead (Pb-Pb) collisions at sNN−−−√=5.02 TeV has been measured using the ALICE detector at the Large Hadron Collider (LHC). The transverse momentum (pT) distributions of K∗(892)0 and ϕ(1020) mesons have been measured at midrapidity (|y|<0.5) up to pT=20 GeV/c in inelastic pp collisions and for several Pb-Pb collision centralities. The collision centrality and collision energy dependence of the average transverse momenta agree with the radial flow scenario observed with stable hadrons, showing that the effect is stronger for more central collisions and higher collision energies. The K∗0/K ratio is found to be suppressed in Pb-Pb collisions relative to pp collisions: this indicates a loss of the measured K∗(892)0 signal due to rescattering of its decay products in the hadronic phase. In contrast, for the longer-lived ϕ(1020) mesons, no such suppression is observed. The nuclear modification factors (RAA) of K∗(892)0 and ϕ(1020) mesons are calculated using pp reference spectra at the same collision energy. In central Pb-Pb collisions for pT>8 GeV/c, the RAA values of K∗(892)0 and ϕ(1020) are below unity and observed to be similar to those of pions, kaons, and (anti)protons. The RAA values at high pT (>~8 GeV/c) for K∗(892)0 and ϕ(1020) mesons are in agreement within uncertainties for sNN−−−√=5.02 and 2.76 TeV.
The Transition Radiation Detector (TRD) was designed and built to enhance the capabilities of the ALICE detector at the Large Hadron Collider (LHC). While aimed at providing electron identification and triggering, the TRD also contributes significantly to the track reconstruction and calibration in the central barrel of ALICE. In this paper the design, construction, operation, and performance of this detector are discussed. A pion rejection factor of up to 410 is achieved at a momentum of 1 GeV/c in p–Pb collisions and the resolution at high transverse momentum improves by about 40% when including the TRD information in track reconstruction. The triggering capability is demonstrated both for jet, light nuclei, and electron selection.
Neutral pion and η meson production in the transverse momentum range 1 < pT < 20 GeV/c have been measured at mid-rapidity by the ALICE experiment at the Large Hadron Collider (LHC) in central and semi-central Pb-Pb collisions at sNN−−−−√ = 2.76 TeV. These results were obtained using the photon conversion method as well as the PHOS and EMCal detectors. The results extend the upper pT reach of the previous ALICE π0 measurements from 12 GeV/c to 20 GeV/c and present the first measurement of η meson production in heavy-ion collisions at the LHC. The η/π0 ratio is similar for the two centralities and reaches at high pT a plateau value of 0.457 ± 0.013stat ± 0.018syst. A suppression of similar magnitude for π0 and η meson production is observed in Pb-Pb collisions with respect to their production in pp collisions scaled by the number of binary nucleon-nucleon collisions. We discuss the results in terms of NLO pQCD predictions and hydrodynamic models. The measurements show a stronger suppression with respect to what was observed at lower center-of-mass energies in the pT range 6 < pT < 10 GeV/c. At pT < 3 GeV/c, hadronization models describe the π0 results while for the η some tension is observed.
The production of the ρ(770)0 meson has been measured at mid-rapidity (|y|<0.5) in pp and centrality differential Pb-Pb collisions at sNN−−−√ = 2.76 TeV with the ALICE detector at the Large Hadron Collider. The particles have been reconstructed in the ρ(770)→π+π− decay channel in the transverse momentum (pT) range 0.5−11 GeV/c. A centrality dependent suppression of the ratio of the integrated yields 2ρ(770)0/(π++π−) is observed. The ratio decreases by ∼40% from pp to central Pb-Pb collisions. A study of the pT-differential 2ρ(770)0/(π++π−) ratio reveals that the suppression occurs at low transverse momenta, pT<2 GeV/c. At higher momentum, particle ratios measured in heavy-ion and pp collisions are consistent. The observed suppression is very similar to that previously measured for the K∗(892)0/K ratio and is consistent with EPOS3 predictions that may imply that rescattering in the hadronic phase is a dominant mechanism for the observed suppression.
The production yield of the Λ(1520) baryon resonance is measured at mid-rapidity in Pb-Pb collisions at sNN−−−√ = 2.76 TeV with the ALICE detector at the LHC. The measurement is performed in the Λ(1520)→pK− (and charge conjugate) hadronic decay channel as a function of the transverse momentum (pT) and collision centrality. The pT-integrated production rate of Λ(1520) relative to Λ in central collisions is suppressed by about a factor of 2 with respect to peripheral collisions. This is the first observation of the suppression of a baryonic resonance at the LHC and the first 3σ evidence of Λ(1520) suppression within a single collision system. The measured Λ(1520)/Λ ratio in central collisions is smaller than the value predicted by the statistical hadronisation model calculations. The shape of the measured pT distribution and the centrality dependence of the suppression are reproduced by the EPOS3 Monte Carlo event generator. The measurement adds further support to the formation of a dense hadronic phase in the final stages of the evolution of the fireball created in heavy-ion collisions, lasting long enough to cause a significant reduction in the observable yield of short-lived resonances.
We present measurements of two-particle differential number correlation functions R2 and transverse momentum correlation functions P2, obtained from p-Pb collisions at 5.02 TeV and Pb-Pb collisions at 2.76 TeV. The results are obtained using charged particles in the pseudorapidity range |η|< 1.0, and transverse momentum range 0.2<pT<2.0 GeV/c as a function of pair separation in pseudorapidity, |Δη|, azimuthal angle, Δφ, and for several charged-particle multiplicity classes. Measurements are carried out for like-sign and unlike-sign charged-particle pairs separately and combined to obtain charge-independent and charge-dependent correlation functions. We study the evolution of the width of the near-side peak of these correlation functions with collision centrality. Additionally, we study Fourier decompositions of the correlators in Δφ as a function of the pair separation |Δη|. Significant differences in the dependence of their harmonic coefficients on multiplicity classes are found. These differences can be exploited, in theoretical models, to obtain further insight into charged-particle production and transport in heavy-ion collisions. Moreover, an upper limit of non-flow contributions to flow coefficients vn measured in Pb-Pb collisions based on the relative strength of Fourier coefficients measured in p-Pb interactions is estimated.
Invariant differential yields of deuterons and anti-deuterons in pp collisions at s√ = 0.9, 2.76 and 7 TeV and the yields of tritons, 3He nuclei and their anti-nuclei at s√ = 7 TeV have been measured with the ALICE detector at the CERN Large Hadron Collider. The measurements cover a wide transverse momentum (pT) range in the rapidity interval |y|<0.5, extending both the energy and the pT reach of previous measurements up to 3 GeV/c for A=2 and 6 GeV/c for A=3. The coalescence parameters of (anti-)deuterons and 3He¯¯¯¯¯¯ nuclei exhibit an increasing trend with pT and are found to be compatible with measurements in pA collisions at low pT and lower energies. The integrated yields decrease by a factor of about 1000 for each increase of the mass number with one (anti-)nucleon. Furthermore, the deuteron-to-proton ratio is reported as a function of the average charged particle multiplicity at different center-of-mass energies.
The correlations between event-by-event fluctuations of anisotropic flow harmonic amplitudes have been measured in Pb-Pb collisions at sNN−−−√ = 2.76 TeV with the ALICE detector at the LHC. The results are reported in terms of multiparticle correlation observables dubbed Symmetric Cumulants. These observables are robust against biases originating from nonflow effects. The centrality dependence of correlations between the higher order harmonics (the quadrangular v4 and pentagonal v5 flow) and the lower order harmonics (the elliptic v2 and triangular v3 flow) is presented. The transverse momentum dependence of correlations between v3 and v2 and between v4 and v2 is also reported. The results are compared to calculations from viscous hydrodynamics and A Multi-Phase Transport ({AMPT}) model calculations. The comparisons to viscous hydrodynamic models demonstrate that the different order harmonic correlations respond differently to the initial conditions and the temperature dependence of the ratio of shear viscosity to entropy density (η/s). A small average value of η/s is favored independent of the specific choice of initial conditions in the models. The calculations with the AMPT initial conditions yield results closest to the measurements. Correlations between the magnitudes of v2, v3 and v4 show moderate pT dependence in mid-central collisions. Together with existing measurements of individual flow harmonics, the presented results provide further constraints on the initial conditions and the transport properties of the system produced in heavy-ion collisions.
The correlations of identical charged kaons were measured in p-Pb collisions at sNN−−−√=5.02 TeV by the ALICE experiment at the LHC. The femtoscopic invariant radii and correlation strengths were extracted from one-dimensional kaon correlation functions and were compared with those obtained in pp and Pb-Pb collisions at s√=7 TeV and sNN−−−√=2.76 TeV, respectively. The presented results also complement the identical-pion femtoscopic data published by the ALICE collaboration. The extracted radii increase with increasing charged-particle multiplicity and decrease with increasing pair transverse momentum. At comparable multiplicities, the radii measured in p-Pb collisions are found to be close to those observed in pp collisions. The obtained femtoscopic parameters are reproduced by the EPOS 3 hadronic interaction model and disfavor models with large initial size or strong collective expansion at low multiplicities.
We report on the first femtoscopic measurement of baryon pairs, such as p−p, p−Λ, and Λ−Λ, measured by ALICE at the Large Hadron Collider (LHC) in proton-proton collisions at √s=7TeV. This study demonstrates the feasibility of such measurements in pp collisions at ultrarelativistic energies. The femtoscopy method is employed to constrain the hyperon-nucleon and hyperon-hyperon interactions, which are still rather poorly understood. A new method to evaluate the influence of residual correlations induced by the decays of resonances and experimental impurities is hereby presented. The p−p, p−Λ, and Λ−Λ correlation functions were fitted simultaneously with the help of a new tool developed specifically for the femtoscopy analysis in small colliding systems: Correlation Analysis Tool using the Schrödinger equation (CATS). Within the assumption that in pp collisions the three particle pairs originate from a common source, its radius is found to be equal to r0=1.125±0.018(stat)+0.058−0.035(syst) fm. The sensitivity of the measured p−Λ correlation is tested against different scattering parameters, which are defined by the interaction among the two particles, but the statistics is not sufficient yet to discriminate among different models. The measurement of the Λ−Λ correlation function constrains the phase space spanned by the effective range and scattering length of the strong interaction. Discrepancies between the measured scattering parameters and the resulting correlation functions at LHC and RHIC energies are discussed in the context of various models.
Comprehensive results on the production of unidentified charged particles, π±, K±, K0S, K*(892)0, p, p¯¯¯, ϕ(1020), Λ, Λ¯¯¯¯, Ξ−, Ξ¯¯¯¯+, Ω− and Ω¯¯¯¯+ hadrons in proton-proton (pp) collisions at s√ = 7 TeV at midrapidity (|y|<0.5) as a function of charged-particle multiplicity density are presented. In order to avoid auto-correlation biases, the actual transverse momentum (pT) spectra of the particles under study and the event activity are measured in different rapidity windows. In the highest multiplicity class, the charged-particle density reaches about 3.5 times the value measured in inelastic collisions. While the yield of protons normalized to pions remains approximately constant as a function of multiplicity, the corresponding ratios of strange hadrons to pions show a significant enhancement that increases with increasing strangeness content. Furthermore, all identified particle to pion ratios are shown to depend solely on charged-particle multiplicity density, regardless of system type and collision energy. The evolution of the spectral shapes with multiplicity and hadron mass shows patterns that are similar to those observed in p-Pb and Pb-Pb collisions at LHC energies. The obtained pT distributions and yields are compared to expectations from QCD-based pp event generators as well as to predictions from thermal and hydrodynamic models. These comparisons indicate that traces of a collective, equilibrated system are already present in high-multiplicity pp collisions.
At the Large Hadron Collider at CERN in Geneva, Switzerland, atomic nuclei are collided at ultra-relativistic energies. Many final-state particles are produced in each collision and their properties are measured by the ALICE detector. The detector signals induced by the produced particles are digitized leading to data rates that are in excess of 48 GB/s. The ALICE High Level Trigger (HLT) system pioneered the use of FPGA- and GPU-based algorithms to reconstruct charged-particle trajectories and reduce the data size in real time. The results of the reconstruction of the collision events, available online, are used for high level data quality and detector-performance monitoring and real-time time-dependent detector calibration. The online data compression techniques developed and used in the ALICE HLT have more than quadrupled the amount of data that can be stored for offline event processing.
Mid-rapidity production of π±, K± and (p¯)p measured by the ALICE experiment at the LHC, in Pb-Pb and inelastic pp collisions at sNN−−−√ = 5.02 TeV, is presented. The invariant yields are measured over a wide transverse momentum (pT) range from hundreds of MeV/c up to 20 GeV/c. The results in Pb-Pb collisions are presented as a function of the collision centrality, in the range 0−90%. The comparison of the pT-integrated particle ratios, i.e. proton-to-pion (p/π) and kaon-to-pion (K/π) ratios, with similar measurements in Pb-Pb collisions at sNN−−−√ = 2.76 TeV show no significant energy dependence. Blast-wave fits of the pT spectra indicate that in the most central collisions radial flow is slightly larger at 5.02 TeV with respect to 2.76 TeV. Particle ratios (p/π, K/π) as a function of pT show pronounced maxima at pT ≈ 3 GeV/c in central Pb-Pb collisions. At high pT, particle ratios at 5.02 TeV are similar to those measured in pp collisions at the same energy and in Pb-Pb collisions at sNN−−−√ = 2.76 TeV. Using the pp reference spectra measured at the same collision energy of 5.02 TeV, the nuclear modification factors for the different particle species are derived. Within uncertainties, the nuclear modification factor is particle species independent for high pT and compatible with measurements at sNN−−−√ = 2.76 TeV. The results are compared to state-of-the-art model calculations, which are found to describe the observed trends satisfactorily.
The first measurement at the LHC of charge-dependent directed flow (v1) relative to the spectator plane is presented for Pb-Pb collisions at sNN−−−√ = 5.02 TeV. Results are reported for charged hadrons and D0 mesons for the transverse momentum intervals pT>0.2 GeV/c and 3<pT< 6 GeV/c in the 5-40% and 10-40% centrality classes, respectively. The difference between the positively and negatively charged hadron v1 has a positive slope as a function of pseudorapidity η, dΔv1/dη=[1.68 ± 0.49 (stat.) ± 0.41 (syst.)] ×10−4. The same measurement for D0 and D¯0 mesons yields a positive value dΔv1/dη= [4.9 ± 1.7 (stat.) ± 0.6 (syst.)]×10−1, which is about three orders of magnitude larger than the one of the charged hadrons. These measurements can provide new insights into the effects of the strong electromagnetic field and the initial tilt of matter created in non-central heavy-ion collisions on the dynamics of light (u, d, and s) and heavy (c) quarks. The large difference between the observed Δv1 of charged hadrons and D0 mesons may reflect different sensitivity of the charm and light quarks to the early time dynamics of a heavy-ion collision. These observations challenge some of the recent theoretical calculations, which predicted a negative and an order of magnitude smaller value of dΔv1/dη for both light-flavour and charmed hadrons.
The first measurements of the scattering parameters of ΛK pairs in all three charge combinations (ΛK+, ΛK−, and ΛK0S) are presented. The results are achieved through a femtoscopic analysis of ΛK correlations in Pb-Pb collisions at sNN−−−√ = 2.76 TeV recorded by ALICE at the LHC. The femtoscopic correlations result from strong final-state interactions, and are fit with a parametrization allowing for both the characterization of the pair emission source and the measurement of the scattering parameters for the particle pairs. Extensive studies with the THERMINATOR 2 event generator provide a good description of the non-femtoscopic background, which results mainly from collective effects, with unprecedented precision. Furthermore, together with HIJING simulations, this model is used to account for contributions from residual correlations induced by feed-down from particle decays. The extracted scattering parameters indicate that the strong force is repulsive in the ΛK+ interaction and attractive in the ΛK− interaction. The data hint that the and ΛK0S interaction is attractive, however the uncertainty of the result does not permit such a decisive conclusion. The results suggest an effect arising either from different quark-antiquark interactions between the pairs (ss¯ in ΛK+ and uu¯¯¯ in ΛK−) or from different net strangeness for each system (S = 0 for ΛK+, and S = −2 for ΛK−). Finally, the ΛK systems exhibit source radii larger than expected from extrapolation from identical particle femtoscopic studies. This effect is interpreted as resulting from the separation in space-time of the single-particle Λ and K source distributions.
Correction to: Nature https://doi.org/10.1038/s41586-020-3001-6Published online 09 December 2020
In Fig. 1c of this Article, owing to an error during the production process, the equation incorrectly began ‘C(k*, r*) = …’ instead of ‘C(k*) = …’. In addition, in affiliation 71 ‘Dipartimento di Fisica dell’Università degli studi di Bari Aldo Moro’ has been corrected to read ‘Dipartimento di Fisica dell’Università degli studi di Cagliari’. The original Article has been corrected online.
A measurement of dielectron production in proton-proton (pp) collisions at s√=13 TeV, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron invariant mass mee and pair transverse momentum pT,ee that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of mee, pT,ee, and event multiplicity dNch/dη. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured η/π0 ratio in pp and proton-nucleus (p-A) collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at 0.15<mee<0.6 GeV/c2 and for pT,ee<0.4 GeV/c indicates an enhancement of soft dielectrons, reminiscent of the 'anomalous' soft-photon and -dilepton excess in hadron-hadron collisions reported by several experiments under different experimental conditions. The enhancement factor over the hadronic cocktail amounts to 1.61±0.13(stat.)±0.17(syst.,data)±0.34(syst.,cocktail) in the ALICE acceptance. Acceptance-corrected excess spectra in mee and pT,ee are extracted and compared with calculations of dielectron production from hadronic bremsstrahlung and thermal radiation within a hadronic many-body approach.
Λ+c production and baryon-to-meson ratios in pp and p–Pb collisions at √sNN = 5.02 TeV at the LHC
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The prompt production of the charm baryon Λ+c and the Λ+c/D0 production ratios were measured at midrapidity with the ALICE detector in pp and p-Pb collisions at sNN−−−√=5.02TeV. These new measurements show a clear decrease of the Λ+c/D0 ratio with increasing transverse momentum (pT) in both collision systems in the range 2<pT<12 GeV/c, exhibiting similarities with the light-flavour baryon-to-meson ratios p/π and Λ/K0S. At low pT, predictions that include additional colour-reconnection mechanisms beyond the leading-colour approximation; assume the existence of additional higher-mass charm-baryon states; or include hadronisation via coalescence can describe the data, while predictions driven by charm-quark fragmentation processes measured in e+e− and e−p collisions significantly underestimate the data. The results presented in this letter provide significant evidence that the established assumption of universality (colliding-system independence) of parton-to-hadron fragmentation is not sufficient to describe charm-baryon production in hadronic collisions at LHC energies.
The production cross section of prompt Λ+c charm baryons was measured with the ALICE detector at the LHC at midrapidity in proton-proton (pp) and proton-lead (p-Pb) collisions at a centre-of-mass energy per nucleon pair of sNN−−−√=5.02 TeV. The Λ+c and Λ¯¯¯¯−c baryons were reconstructed in the hadronic decay channels Λ+c→pK−π+ and Λ+c→pK0S and respective charge conjugates. The measured differential cross sections as a function of transverse momentum (pT) and the pT-integrated Λ+c production cross section in pp and in p-Pb collisions are presented. The Λ+c nuclear modification factor (RpPb), calculated from the cross sections in pp and in p-Pb collisions, is presented and compared with the RpPb of D mesons. The Λ+c/D0 ratio is also presented and compared with the light-flavour baryon-to-meson ratios p/π and Λ/K0S, and measurements from other LHC experiments. The results are compared to predictions from model calculations and Monte Carlo event generators.
The W and Z boson production was measured via the muonic decay channel in proton-lead collisions at sNN−−−√=5.02 TeV at the Large Hadron Collider with the ALICE detector. The measurement covers backward (−4.46<ycms<−2.96) and forward (2.03<ycms<3.53) rapidity regions, corresponding to Pb-going and p-going directions, respectively. The Z-boson production cross section, with dimuon invariant mass of 60<mμμ<120 GeV/c2 and muon transverse momentum (pμT) larger than 20 GeV/c, is measured. The production cross section and charge asymmetry of muons from W-boson decays with pμT>10 GeV/c are determined. The results are compared to theoretical calculations both with and without including the nuclear modification of the parton distribution functions. The W-boson production is also studied as a function of the collision centrality: the cross section of muons from W-boson decays is found to scale with the average number of binary nucleon-nucleon collisions within uncertainties.
The production of π±, K±, and (p¯¯¯)p is measured in pp collisions at s√=13 TeV in different topological regions. Particle transverse momentum (pT) spectra are measured in the ``toward'', ``transverse'', and ``away'' angular regions defined with respect to the direction of the leading particle in the event. While the toward and away regions contain the fragmentation products of the near-side and away-side jets, respectively, the transverse region is dominated by particles from the Underlying Event (UE). The relative transverse activity classifier, RT=NT/⟨NT⟩, is used to group events according to their UE activity, where NT is the measured charged-particle multiplicity per event in the transverse region and ⟨NT⟩ is the mean value over all the analysed events. The first measurements of identified particle pT spectra as a function of RT in the three topological regions are reported. The yield of high transverse momentum particles relative to the RT-integrated measurement decreases with increasing RT in both the toward and away regions, indicating that the softer UE dominates particle production as RT increases and validating that RT can be used to control the magnitude of the UE. Conversely, the spectral shapes in the transverse region harden significantly with increasing RT. This hardening follows a mass ordering, being more significant for heavier particles. The pT-differential particle ratios (p+p¯¯¯)/(π++π−) and (K++K−)/(π++π−) in the low UE limit (RT→0) approach expectations from Monte Carlo generators such as PYTHIA 8 with Monash 2013 tune and EPOS LHC, where the jet-fragmentation models have been tuned to reproduce e+e− results.
The production of prompt charmed mesons D0, D+ and D∗+, and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at the centre-of-mass energy per nucleon pair, sNN−−−√, of 2.76 TeV. The production yields for rapidity |y|<0.5 are presented as a function of transverse momentum, pT, in the interval 1-36 GeV/c for the centrality class 0-10% and in the interval 1-16 GeV/c for the centrality class 30-50%. The nuclear modification factor RAA was computed using a proton-proton reference at s√=2.76 TeV, based on measurements at s√=7 TeV and on theoretical calculations. A maximum suppression by a factor of 5-6 with respect to binary-scaled pp yields is observed for the most central collisions at pT of about 10 GeV/c. A suppression by a factor of about 2-3 persists at the highest pT covered by the measurements. At low pT (1-3 GeV/c), the RAA has large uncertainties that span the range 0.35 (factor of about 3 suppression) to 1 (no suppression). In all pT intervals, the RAA is larger in the 30-50% centrality class compared to central collisions. The D-meson RAA is also compared with that of charged pions and, at large pT, charged hadrons, and with model calculations.
We report on two-particle charge-dependent correlations in pp, p-Pb, and Pb-Pb collisions as a function of the pseudorapidity and azimuthal angle difference, Δη and Δφ respectively. These correlations are studied using the balance function that probes the charge creation time and the development of collectivity in the produced system. The dependence of the balance function on the event multiplicity as well as on the trigger and associated particle transverse momentum (pT) in pp, p-Pb, and Pb-Pb collisions at sNN−−−√=7, 5.02, and 2.76 TeV, respectively, are presented. In the low transverse momentum region, for 0.2<pT<2.0 GeV/c, the balance function becomes narrower in both Δη and Δφ directions in all three systems for events with higher multiplicity. The experimental findings favor models that either incorporate some collective behavior (e.g. AMPT) or different mechanisms that lead to effects that resemble collective behavior (e.g. PYTHIA8 with color reconnection). For higher values of transverse momenta the balance function becomes even narrower but exhibits no multiplicity dependence, indicating that the observed narrowing with increasing multiplicity at low pT is a feature of bulk particle production.
In ultraperipheral collisions (UPCs) of relativistic nuclei without overlap of nuclear densities, the two nuclei are excited by the Lorentz-contracted Coulomb fields of their collision partners. In these UPCs, the typical nuclear excitation energy is below a few tens of MeV, and a small number of nucleons are emitted in electromagnetic dissociation (EMD) of primary nuclei, in contrast to complete nuclear fragmentation in hadronic interactions. The cross sections of emission of given numbers of neutrons in UPCs of 208Pb nuclei at sNN−−−√=5.02 TeV were measured with the neutron zero degree calorimeters (ZDCs) of the ALICE detector at the LHC, exploiting a similar technique to that used in previous studies performed at sNN−−−√=2.76 TeV. In addition, the cross sections for the exclusive emission of one, two, three, four, and five forward neutrons in the EMD, not accompanied by the emission of forward protons, and thus mostly corresponding to the production of 207,206,205,204,203Pb, respectively, were measured for the first time. The predictions from the available models describe the measured cross sections well. These cross sections can be used for evaluating the impact of secondary nuclei on the LHC components, in particular, on superconducting magnets, and also provide useful input for the design of the Future Circular Collider (FCC-hh).
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 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.
A newly developed observable for correlations between symmetry planes, which characterize the direction of the anisotropic emission of produced particles, is measured in Pb-Pb collisions at sNN−−−√=2.76 TeV with ALICE. This so-called Gaussian Estimator allows for the first time the study of these quantities without the influence of correlations between different flow amplitudes. The centrality dependence of various correlations between two, three and four symmetry planes is presented. The ordering of magnitude between these symmetry plane correlations is discussed and the results of the Gaussian Estimator are compared with measurements of previously used estimators. The results utilizing the new estimator lead to significantly smaller correlations than reported by studies using the Scalar Product method. Furthermore, the obtained symmetry plane correlations are compared to state-of-the-art hydrodynamic model calculations for the evolution of heavy-ion collisions. While the model predictions provide a qualitative description of the data, quantitative agreement is not always observed, particularly for correlators with significant non-linear response of the medium to initial state anisotropies of the collision system. As these results provide unique and independent information, their usage in future Bayesian analysis can further constrain our knowledge on the properties of the QCD matter produced in ultrarelativistic heavy-ion collisions.
We report about the properties of the underlying event measured with ALICE at the LHC in pp and p−Pb collisions at sNN−−−√=5.02 TeV. The event activity, quantified by charged-particle number and summed-pT densities, is measured as a function of the leading-particle transverse momentum (ptrigT). These quantities are studied in three azimuthal-angle regions relative to the leading particle in the event: toward, away, and transverse. Results are presented for three different pT thresholds (0.15, 0.5, and 1 GeV/c) at mid-pseudorapidity (|η|<0.8). The event activity in the transverse region, which is the most sensitive to the underlying event, exhibits similar behaviour in both pp and p−Pb collisions, namely, a steep increase with ptrigT for low ptrigT, followed by a saturation at ptrigT≈5 GeV/c. The results from pp collisions are compared with existing measurements at other centre-of-mass energies. The quantities in the toward and away regions are also analyzed after the subtraction of the contribution measured in the transverse region. The remaining jet-like particle densities are consistent in pp and p−Pb collisions for ptrigT>10 GeV/c, whereas for lower ptrigT values the event activity is slightly higher in p−Pb than in pp collisions. The measurements are compared with predictions from the PYTHIA 8 and EPOS LHC Monte Carlo event generators.