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We present results on transverse momentum (pT) and rapidity (y) differential production cross sections, mean transverse momentum and mean transverse momentum square of inclusive J/ψ and ψ(2S) at forward rapidity (2.5 < y < 4) as well as ψ(2S)-to-J/ψ cross section ratios. These quantities are measured in pp collisions at center of mass energies s√=5.02 and 13 TeV with the ALICE detector. Both charmonium states are reconstructed in the dimuon decay channel, using the muon spectrometer. A comprehensive comparison to inclusive charmonium cross sections measured at s√=2.76, 7 and 8 TeV is performed. A comparison to non-relativistic quantum chromodynamics and fixed-order next-to-leading logarithm calculations, which describe prompt and non-prompt charmonium production respectively, is also presented. A good description of the data is obtained over the full pT range, provided that both contributions are summed. In particular, it is found that for pT > 15 GeV/c the non-prompt contribution reaches up to 50% of the total charmonium yield.
The transverse momentum distributions of the strange and double-strange hyperon resonances (Σ(1385)±,Ξ(1530)0) produced in p–Pb collisions at √sNN = 5.02 TeV were measured in the rapidity range −0.5<yCMS<0 for event classes corresponding to different charged-particle multiplicity densities, ⟨dNch/dηlab⟩. The mean transverse momentum values are presented as a function of ⟨dNch/dηlab⟩, as well as a function of the particle masses and compared with previous results on hyperon production. The integrated yield ratios of excited to ground-state hyperons are constant as a function of ⟨dNch/dηlab⟩. The equivalent ratios to pions exhibit an increase with ⟨dNch/dηlab⟩, depending on their strangeness content.
Two-particle angular correlations were measured in pp collisions at s√=7 TeV for pions, kaons, protons, and lambdas, for all particle/anti-particle combinations in the pair. Data for mesons exhibit an expected peak dominated by effects associated with mini-jets and are well reproduced by general purpose Monte Carlo generators. However, for baryon–baryon and anti-baryon–anti-baryon pairs, where both particles have the same baryon number, a near-side anti-correlation structure is observed instead of a peak. This effect is interpreted in the context of baryon production mechanisms in the fragmentation process. It currently presents a challenge to Monte Carlo models and its origin remains an open question.
We have identified a mistake in how Fig. 1 is referenced in the text of the article Eur. Phys. J. C 77 (2017) no. 8, 569 which affected three paragraphs of the results section. The corrected three paragraphs as well as the unmodified accompanying figure are reproduced in this document with the correct labeling.
In addition, an editing issue led to a missing acknowledgements section. The missing section is reproduced at the end of this document in the manner in which it should have appeared in the published article.
The invariant differential cross sections for inclusive π0 and η mesons at midrapidity were measured in pp collisions at s√=2.76 TeV for transverse momenta 0.4<pT<40 GeV/c and 0.6<pT<20 GeV/c, respectively, using the ALICE detector. This large range in pT was achieved by combining various analysis techniques and different triggers involving the electromagnetic calorimeter (EMCal). In particular, a new single-cluster, shower-shape based method was developed for the identification of high-pT neutral pions, which exploits that the showers originating from their decay photons overlap in the EMCal. Above 4 GeV/c, the measured cross sections are found to exhibit a similar power-law behavior with an exponent of about 6.3. Next-to-leading-order perturbative QCD calculations differ from the measured cross sections by about 30% for the π0, and between 30–50% for the η meson, while generator-level simulations with PYTHIA 8.2 describe the data to better than 10–30%, except at pT<1 GeV/c. The new data can therefore be used to further improve the theoretical description of π0 and η meson production.
The 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.
In the original version of this article unfortunately the copyright line in the PDF was wrong. The original article has been corrected.
This is an addendum to the article JHEP 11 (2015) 205 [1]. The figures 3 (right), 4 (right) and 5 are updated with published results on non-prompt J/ψ-meson production from the CMS collaboration.
The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum (high pT) hadron trigger in proton−proton and central Pb−Pb collisions at sNN−−−√=5.02 TeV. A data-driven statistical method is used to mitigate the large uncorrelated background in central Pb−Pb collisions. Recoil jet distributions are reported for jet resolution parameter R=0.2, 0.4, and 0.5 in the range 7<pT,jet<140 GeV/c and trigger−recoil jet azimuthal separation π/2<Δφ<π. The measurements exhibit a marked medium-induced jet yield enhancement at low pT and at large azimuthal deviation from Δφ∼π. The enhancement is characterized by its dependence on Δφ, which has a slope that differs from zero by 4.7σ. Comparisons to model calculations incorporating different formulations of jet quenching are reported. These comparisons indicate that the observed yield enhancement arises from the response of the QGP medium to jet propagation.
The ALICE Collaboration reports measurements of the semi-inclusive distribution of charged-particle jets recoiling from a high transverse momentum (high pT) charged hadron, in pp and central Pb−Pb collisions at center of mass energy per nucleon−nucleon collision sNN−−−√=5.02 TeV. The large uncorrelated background in central Pb−Pb collisions is corrected using a data-driven statistical approach, which enables precise measurement of recoil jet distributions over a broad range in pT,chjet and jet resolution parameter R. Recoil jet yields are reported for R=0.2, 0.4, and 0.5 in the range 7<pT,chjet<140 GeV/c and π/2<Δφ<π, where Δφ is the azimuthal angular separation between hadron trigger and recoil jet. The low pT,chjet reach of the measurement explores unique phase space for studying jet quenching, the interaction of jets with the quark-gluonnplasma generated in high-energy nuclear collisions. Comparison of pT,chjet distributions from pp and central Pb−Pb collisions probes medium-induced jet energy loss and intra-jet broadening, while comparison of their acoplanarity distributions explores in-medium jet scattering and medium response. The measurements are compared to theoretical calculations incorporating jet quenching.
The ALICE Collaboration reports measurements of the semi-inclusive distribution of charged-particle jets recoiling from a high transverse momentum (high pT) charged hadron, in pp and central Pb−Pb collisions at center of mass energy per nucleon−nucleon collision sNN−−−√=5.02 TeV. The large uncorrelated background in central Pb−Pb collisions is corrected using a data-driven statistical approach, which enables precise measurement of recoil jet distributions over a broad range in pT,chjet and jet resolution parameter R. Recoil jet yields are reported for R=0.2, 0.4, and 0.5 in the range 7<pT,chjet<140 GeV/c and π/2<Δφ<π, where Δφ is the azimuthal angular separation between hadron trigger and recoil jet. The low pT,chjet reach of the measurement explores unique phase space for studying jet quenching, the interaction of jets with the quark-gluonnplasma generated in high-energy nuclear collisions. Comparison of pT,chjet distributions from pp and central Pb−Pb collisions probes medium-induced jet energy loss and intra-jet broadening, while comparison of their acoplanarity distributions explores in-medium jet scattering and medium response. The measurements are compared to theoretical calculations incorporating jet quenching.
The first measurements of skewness and kurtosis of mean transverse momentum (⟨pT⟩) fluctuations are reported in Pb−Pb collisions at sNN−−−√ = 5.02 TeV, Xe−Xe collisions at sNN−−−√ = 5.44 TeV and pp collisions at s√=5.02 TeV using the ALICE detector. The measurements are carried out as a function of system size ⟨dNch/dη⟩1/3|η|<0.5, using charged particles with transverse momentum (pT) and pseudorapidity (η), in the range 0.2<pT<3.0 GeV/c and |η|<0.8, respectively. In Pb−Pb and Xe−Xe collisions, positive skewness is observed in the fluctuations of ⟨pT⟩ for all centralities, which is significantly larger than what would be expected in the scenario of independent particle emission. This positive skewness is considered a crucial consequence of the hydrodynamic evolution of the hot and dense nuclear matter created in heavy-ion collisions. Furthermore, similar observations of positive skewness for minimum bias pp collisions are also reported here. Kurtosis of ⟨pT⟩ fluctuations is found to be in good agreement with the kurtosis of Gaussian distribution, for most central Pb−Pb collisions. Hydrodynamic model calculations with MUSIC using Monte Carlo Glauber initial conditions are able to explain the measurements of both skewness and kurtosis qualitatively from semicentral to central collisions in Pb--Pb system. Color reconnection mechanism in PYTHIA8 model seems to play a pivotal role in capturing the qualitative behavior of the same measurements in pp collisions.
The ALICE Collaboration reports a search for jet quenching effects in high-multiplicity (HM) proton−proton collisions at s√ = 13 TeV, using the semi-inclusive azimuthal-difference distribution Δφ of charged-particle jets recoiling from a high transverse momentum (high-pT,trig) trigger hadron. Jet quenching may broaden the Δφ distribution measured in HM events compared to that in minimum bias (MB) events. The measurement employs a pT,trig-differential observable for data-driven suppression of the contribution of multiple partonic interactions, which is the dominant background. While azimuthal broadening is indeed observed in HM compared to MB events, similar broadening for HM events is observed for simulations based on the PYTHIA 8 Monte Carlo generator, which does not incorporate jet quenching. We elucidate the origin of the broadening by comparing biases induced by HM selection in the data and simulations, and discuss its implications for the study of jet quenching in small collision systems.
The first measurements of K∗(892)0 resonance production as a function of charged-particle multiplicity in Xe−Xe collisions at sNN−−−√= 5.44 TeV and pp collisions at s√= 5.02 TeV using the ALICE detector are presented. The resonance is reconstructed at midrapidity (|y|<0.5) using the hadronic decay channel K∗0→K±π∓. Measurements of transverse-momentum integrated yield, mean transverse-momentum, nuclear modification factor of K∗0, and yield ratios of resonance to stable hadron (K∗0/K) are compared across different collision systems (pp, p−Pb, Xe−Xe, and Pb−Pb) at similar collision energies to investigate how the production of K∗0 resonances depends on the size of the system formed in these collisions. The hadronic rescattering effect is found to be independent of the size of colliding systems and mainly driven by the produced charged-particle multiplicity, which is a proxy of the volume of produced matter at the chemical freeze-out. In addition, the production yields of K∗0 in Xe−Xe collisions are utilized to constrain the dependence of the kinetic freeze-out temperature on the system size using HRG-PCE model.
Results on the transverse spherocity dependence of light-flavor particle production (π, K, p, ϕ, K∗0, K0S, Λ, Ξ) at midrapidity in high-multiplicity pp collisions at s√=13 TeV were obtained with the ALICE apparatus. The transverse spherocity estimator (SpT=1O) categorizes events by their azimuthal topology. Utilizing narrow selections on SpT=1O, it is possible to contrast particle production in collisions dominated by many soft initial interactions with that observed in collisions dominated by one or more hard scatterings. Results are reported for two multiplicity estimators covering different pseudorapidity regions. The SpT=1O estimator is found to effectively constrain the hardness of the events when the midrapidity (|η|<0.8) estimator is used. The production rates of strange particles are found to be slightly higher for soft isotropic topologies, and severely suppressed in hard jet-like topologies. These effects are more pronounced for hadrons with larger mass and strangeness content, and observed when the topological selection is done within a narrow multiplicity interval. This demonstrates that an important aspect of the universal scaling of strangeness enhancement with final-state multiplicity is that high-multiplicity collisions are dominated by soft, isotropic processes. On the contrary, strangeness production in events with jet-like processes is significantly reduced. The results presented in this article are compared with several QCD-inspired Monte Carlo event generators. Models that incorporate a two-component phenomenology, either through mechanisms accounting for string density, or thermal production, are able to describe the observed strangeness enhancement as a function of SpT=1O.
Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at s√=13 TeV and p−Pb collisions at sNN−−−√=5.02 TeV. The correlation functions are measured as a function of relative azimuthal angle Δφ and pseudorapidity separation Δη for pairs of primary charged particles within the pseudorapidity interval |η|<0.9 and the transverse-momentum interval 1<pT<4 GeV/c. Flow coefficients are extracted for the long-range correlations (1.6<|Δη|<1.8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.
We present results on transverse momentum (pT) and rapidity (y) differential production cross sections, mean transverse momentum and mean transverse momentum square of inclusive J/ψ and ψ(2S) at forward rapidity (2.5<y<4) as well as ψ(2S)-to-J/ψ cross section ratios. These quantities are measured in pp collisions at center of mass energies s√=5.02 and 13 TeV with the ALICE detector. Both charmonium states are reconstructed in the dimuon decay channel, using the muon spectrometer. A comprehensive comparison to inclusive charmonium cross sections measured at s√=2.76, 7 and 8 TeV is performed. A comparison to non-relativistic quantum chromodynamics and fixed-order next-to-leading logarithm calculations, which describe prompt and non-prompt charmonium production respectively, is also presented. A good description of the data is obtained over the full pT range, provided that both contributions are summed. In particular, it is found that for pT>15 GeV/c the non-prompt contribution reaches up to 50% of the total charmonium yield.
We present results on transverse momentum (pT) and rapidity (y) differential production cross sections, mean transverse momentum and mean transverse momentum square of inclusive J/ψ and ψ(2S) at forward rapidity (2.5<y<4) as well as ψ(2S)-to-J/ψ cross section ratios. These quantities are measured in pp collisions at center of mass energies s√=5.02 and 13 TeV with the ALICE detector. Both charmonium states are reconstructed in the dimuon decay channel, using the muon spectrometer. A comprehensive comparison to inclusive charmonium cross sections measured at s√=2.76, 7 and 8 TeV is performed. A comparison to non-relativistic quantum chromodynamics and fixed-order next-to-leading logarithm calculations, which describe prompt and non-prompt charmonium production respectively, is also presented. A good description of the data is obtained over the full pT range, provided that both contributions are summed. In particular, it is found that for pT>15 GeV/c the non-prompt contribution reaches up to 50% of the total charmonium yield.
The 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. 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 and generator-level simulations with PYTHIA 8.2 describe the cross sections to about 30% for the π0, and between 30-50% for the η meson. The new data can therefore be used to further improve the theoretical description of π0 and η meson production.
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. The measured cross sections are found to exhibit a similar power-law behavior with an exponent of about 6.3. Next-to-leading-order perturbative QCD calculations differ from the measured cross sections by about 30% for the π0, and between 30-50% for the η meson, while generator-level simulations with PYTHIA 8.2 describe the data to better than 10-30%, except at pT<1 GeV/c. The new data can therefore be used to further improve the theoretical description of π0 and η meson production.
The production cross sections of the prompt charmed mesons D0, D+, D∗+ and D+s were measured at mid-rapidity in proton-proton collisions at a centre-of-mass energy s√=7 TeV with the ALICE detector at the Large Hadron Collider (LHC). D mesons were reconstructed from their decays D0→K−π+, D+→K−π+π+, D∗+→D0π+, D+s→ϕπ+→K−K+π+, and their charge conjugates. With respect to previous measurements in the same rapidity region, the coverage in transverse momentum (pT) is extended and the uncertainties are reduced by a factor of about two. The accuracy on the estimated total cc¯¯ production cross section is likewise improved. The measured pT-differential cross sections are compared with the results of three perturbative QCD calculations.
The production cross sections of the prompt charmed mesons D0, D+, D∗+ and D+s were measured at mid-rapidity in proton-proton collisions at a centre-of-mass energy s√=7 TeV with the ALICE detector at the Large Hadron Collider (LHC). D mesons were reconstructed from their decays D0→K−π+, D+→K−π+π+, D∗+→D0π+, D+s→ϕπ+→K−K+π+, and their charge conjugates. With respect to previous measurements in the same rapidity region, the coverage in transverse momentum (pT) is extended and the uncertainties are reduced by a factor of about two. The accuracy on the estimated total cc¯¯ production cross section is likewise improved. The measured pT-differential cross sections are compared with the results of three perturbative QCD calculations.
The production cross sections of the prompt charmed mesons D0, D+, D∗+ and D+s were measured at mid-rapidity in proton-proton collisions at a centre-of-mass energy s√=7 TeV with the ALICE detector at the Large Hadron Collider (LHC). D mesons were reconstructed from their decays D0→K−π+, D+→K−π+π+, D∗+→D0π+, D+s→ϕπ+→K−K+π+, and their charge conjugates. With respect to previous measurements in the same rapidity region, the coverage in transverse momentum (pT) is extended and the uncertainties are reduced by a factor of about two. The accuracy on the estimated total cc¯¯ production cross section is likewise improved. The measured pT-differential cross sections are compared with the results of three perturbative QCD calculations.
The production of muons from heavy-flavour hadron decays in p-Pb collisions at 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 center-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.
The production of muons from heavy-flavour hadron decays in p-Pb collisions at 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 center-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.
The measurement of two-particle angular correlations is a powerful tool to study jet quenching in a pT region inaccessible by direct jet identification. In these measurements pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the shape of the near-side peak formed by particles associated to a higher pT trigger particle (1<pT,trig< 8 GeV/c). A combined fit of the near-side peak and long-range correlations is applied to the data allowing the extraction of the centrality evolution of the peak shape in Pb-Pb collisions at sNN−−−√ = 2.76 TeV. A significant broadening of the peak in the Δη direction at low pT is found from peripheral to central collisions, which vanishes above 4 GeV/c, while in the Δφ direction the peak is almost independent of centrality. For the 10% most central collisions and 1<pT,assoc< 2 GeV/c, 1<pT,trig< 3 GeV/c a novel feature is observed: a depletion develops around the centre of the peak. The results are compared to pp collisions at the same centre of mass energy and to AMPT model simulations. The comparison to the investigated models suggests that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.
The measurement of two-particle angular correlations is a powerful tool to study jet quenching in a pT region inaccessible by direct jet identification. In these measurements pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the shape of the near-side peak formed by particles associated to a higher pT trigger particle (1<pT,trig< 8 GeV/c). A combined fit of the near-side peak and long-range correlations is applied to the data allowing the extraction of the centrality evolution of the peak shape in Pb-Pb collisions at sNN−−−√ = 2.76 TeV. A significant broadening of the peak in the Δη direction at low pT is found from peripheral to central collisions, which vanishes above 4 GeV/c, while in the Δφ direction the peak is almost independent of centrality. For the 10% most central collisions and 1<pT,assoc< 2 GeV/c, 1<pT,trig< 3 GeV/c a novel feature is observed: a depletion develops around the centre of the peak. The results are compared to pp collisions at the same centre of mass energy and to AMPT model simulations. The comparison to the investigated models suggests that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.
The measurement of two-particle angular correlations is a powerful tool to study jet quenching in a pT region inaccessible by direct jet identification. In these measurements pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the shape of the near-side peak formed by particles associated to a higher pT trigger particle (1<pT,trig< 8 GeV/c). A combined fit of the near-side peak and long-range correlations is applied to the data allowing the extraction of the centrality evolution of the peak shape in Pb-Pb collisions at sNN−−−√ = 2.76 TeV. A significant broadening of the peak in the Δη direction at low pT is found from peripheral to central collisions, which vanishes above 4 GeV/c, while in the Δφ direction the peak is almost independent of centrality. For the 10% most central collisions and 1<pT,assoc< 2 GeV/c, 1<pT,trig< 3 GeV/c a novel feature is observed: a depletion develops around the centre of the peak. The results are compared to pp collisions at the same centre of mass energy and to AMPT model simulations. The comparison to the investigated models suggests that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.
In two-particle angular correlation measurements, jets give rise to a near-side peak, formed by particles associated to a higher pT trigger particle. Measurements of these correlations as a function of pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the centrality and pT dependence of the shape of the near-side peak in the pT range 1<pT< 8 GeV/c in Pb-Pb and pp collisions at sNN−−−√ = 2.76 TeV. A combined fit of the near-side peak and long-range correlations is applied to the data and the peak shape is quantified by the variance of the distributions. While the width of the peak in the Δφ direction is almost independent of centrality, a significant broadening in the Δη direction is found from peripheral to central collisions. This feature is prominent for the low pT region and vanishes above 4 GeV/c. The widths measured in peripheral collisions are equal to those in pp in the Δφ direction and above 3 GeV/c in the Δη direction. Furthermore, for the 10\% most central collisions and 1<pT,assoc< 2 GeV/c, 1<pT,trig< 3 GeV/c a departure from a Gaussian shape is found: a depletion develops around the centre of the peak. The results are compared to AMPT model simulations as well as other theoretical calculations indicating that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.
In two-particle angular correlation measurements, jets give rise to a near-side peak, formed by particles associated to a higher pT trigger particle. Measurements of these correlations as a function of pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the centrality and pT dependence of the shape of the near-side peak in the pT range 1<pT< 8 GeV/c in Pb-Pb and pp collisions at sNN−−−√ = 2.76 TeV. A combined fit of the near-side peak and long-range correlations is applied to the data and the peak shape is quantified by the variance of the distributions. While the width of the peak in the Δφ direction is almost independent of centrality, a significant broadening in the Δη direction is found from peripheral to central collisions. This feature is prominent for the low pT region and vanishes above 4 GeV/c. The widths measured in peripheral collisions are equal to those in pp in the Δφ direction and above 3 GeV/c in the Δη direction. Furthermore, for the 10\% most central collisions and 1<pT,assoc< 2 GeV/c, 1<pT,trig< 3 GeV/c a departure from a Gaussian shape is found: a depletion develops around the centre of the peak. The results are compared to AMPT model simulations as well as other theoretical calculations indicating that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.
In two-particle angular correlation measurements, jets give rise to a near-side peak, formed by particles associated to a higher pT trigger particle. Measurements of these correlations as a function of pseudorapidity (Δη) and azimuthal (Δφ) differences are used to extract the centrality and pT dependence of the shape of the near-side peak in the pT range 1<pT< 8 GeV/c in Pb-Pb and pp collisions at sNN−−−√ = 2.76 TeV. A combined fit of the near-side peak and long-range correlations is applied to the data and the peak shape is quantified by the variance of the distributions. While the width of the peak in the Δφ direction is almost independent of centrality, a significant broadening in the Δη direction is found from peripheral to central collisions. This feature is prominent for the low pT region and vanishes above 4 GeV/c. The widths measured in peripheral collisions are equal to those in pp in the Δφ direction and above 3 GeV/c in the Δη direction. Furthermore, for the 10\% most central collisions and 1<pT,assoc< 2 GeV/c, 1<pT,trig< 3 GeV/c a departure from a Gaussian shape is found: a depletion develops around the centre of the peak. The results are compared to AMPT model simulations as well as other theoretical calculations indicating that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.
Electrons from heavy-flavour hadron decays (charm and beauty) were measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass of energy sNN−−−√=2.76 TeV. The transverse momentum (pT) differential production yields at mid-rapidity were used to calculate the nuclear modification factor RAA in the interval 3<pT<18 GeV/c. The RAA shows a strong suppression compared to binary scaling of pp collisions at the same energy (up to a factor of 4) in the 10% most central Pb-Pb collisions. There is a centrality trend of suppression, and a weaker suppression (down to a factor of 2) in semi-peripheral (50-80%) collisions is observed. The suppression of electrons in this broad pT interval indicates that both charm and beauty quarks lose energy when they traverse the hot medium formed in Pb-Pb collisions at LHC.
Electrons from heavy-flavour hadron decays (charm and beauty) were measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass of energy sNN−−−√=2.76 TeV. The transverse momentum (pT) differential production yields at mid-rapidity were used to calculate the nuclear modification factor RAA in the interval 3<pT<18 GeV/c. The RAA shows a strong suppression compared to binary scaling of pp collisions at the same energy (up to a factor of 4) in the 10% most central Pb-Pb collisions. There is a centrality trend of suppression, and a weaker suppression (down to a factor of 2) in semi-peripheral (50-80%) collisions is observed. The suppression of electrons in this broad pT interval indicates that both charm and beauty quarks lose energy when they traverse the hot medium formed in Pb-Pb collisions at LHC.
Particle identification is an important feature of the ALICE detector at the LHC. In particular, for particle identification via the time-of-flight technique, the precise determination of the event collision time represents an important ingredient of the quality of the measurement. In this paper, the different methods used for such a measurement in ALICE by means of the T0 and the TOF detectors are reviewed. Efficiencies, resolution and the improvement of the particle identification separation power of the methods used are presented for the different LHC colliding systems (pp , p-Pb and Pb-Pb) during the first period of data taking of LHC (Run 1).
Particle identification is an important feature of the ALICE detector at the LHC. In particular, for particle identification via the time-of-flight technique, the precise determination of the event collision time represents an important ingredient of the quality of the measurement. In this paper, the different methods used for such a measurement in ALICE by means of the T0 and the TOF detectors are reviewed. Efficiencies, resolution and the improvement of the particle identification separation power of the methods used are presented for the different LHC colliding systems (pp , p-Pb and Pb-Pb) during the first period of data taking of LHC (Run 1).
Particle identification is an important feature of the ALICE detector at the LHC. In particular, for particle identification via the time-of-flight technique, the precise determination of the event collision time represents an important ingredient of the quality of the measurement. In this paper, the different methods used for such a measurement in ALICE by means of the T0 and the TOF detectors are reviewed. Efficiencies, resolution and the improvement of the particle identification separation power of the methods used are presented for the different LHC colliding systems (pp , p-Pb and Pb-Pb) during the first period of data taking of LHC (Run 1).
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 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 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.
Two-particle angular correlations were measured in pp collisions at s√=7 TeV for pions, kaons, protons, and lambdas, for all particle/anti-particle combinations in the pair. Data for mesons exhibit an expected peak dominated by effects associated with mini-jets and are well reproduced by general purpose Monte Carlo generators. However, for baryon-baryon and anti-baryon--anti-baryon pairs, where both particles have the same baryon number, a near-side anti-correlation structure is observed instead of a peak. This effect is interpreted in the context of baryon production mechanisms in the fragmentation process. It currently presents a challenge to Monte Carlo models and its origin remains an open question.
Two-particle angular correlations were measured in pp collisions at s√=7 TeV for pions, kaons, protons, and lambdas, for all particle/anti-particle combinations in the pair. Data for mesons exhibit an expected peak dominated by effects associated with mini-jets and are well reproduced by general purpose Monte Carlo generators. However, for baryon-baryon and anti-baryon--anti-baryon pairs, where both particles have the same baryon number, a near-side anti-correlation structure is observed instead of a peak. This effect is interpreted in the context of baryon production mechanisms in the fragmentation process. It currently presents a challenge to Monte Carlo models and its origin remains an open question.
Two-particle angular correlations were measured in pp collisions at s√=7 TeV. The analysis was carried out for pions, kaons, protons, and lambdas, for all particle/anti-particle combinations in the pair. Data for mesons exhibit an expected peak dominated by effects associated with mini-jets and are well reproduced by general purpose Monte Carlo generators. However, for baryon--baryon and anti-baryon--anti-baryon pairs, where both particles have the same baryon number, a near-side anti-correlation structure is observed instead of a peak. This effect is interpreted in the context of baryon production mechanisms in the fragmentation process. It currently presents a challenge to Monte Carlo models and its origin remains an open question.
We present the charged-particle pseudorapidity density in Pb-Pb collisions at sNN−−−√=5.02TeV in centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from −3.5 to 5, which is sufficient for reliable estimates of the total number of charged particles produced in the collisions. For the most central (0-5%) collisions we find 21400±1300 while for the most peripheral (80-90%) we find 230±38. This corresponds to an increase of (27±4)% over the results at sNN−−−√=2.76TeV previously reported by ALICE. The energy dependence of the total number of charged particles produced in heavy-ion collisions is found to obey a modified power-law like behaviour. The charged-particle pseudorapidity density of the most central collisions is compared to model calculations --- none of which fully describes the measured distribution. We also present an estimate of the rapidity density of charged particles. The width of that distribution is found to exhibit a remarkable proportionality to the beam rapidity, independent of the collision energy from the top SPS to LHC energies.
We present the charged-particle pseudorapidity density in Pb-Pb collisions at sNN−−−√=5.02TeV in centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from −3.5 to 5, which is sufficient for reliable estimates of the total number of charged particles produced in the collisions. For the most central (0-5%) collisions we find 21400±1300 while for the most peripheral (80-90%) we find 230±38. This corresponds to an increase of (27±4)% over the results at sNN−−−√=2.76TeV previously reported by ALICE. The energy dependence of the total number of charged particles produced in heavy-ion collisions is found to obey a modified power-law like behaviour. The charged-particle pseudorapidity density of the most central collisions is compared to model calculations --- none of which fully describes the measured distribution. We also present an estimate of the rapidity density of charged particles. The width of that distribution is found to exhibit a remarkable proportionality to the beam rapidity, independent of the collision energy from the top SPS to LHC energies.
We present the charged-particle pseudorapidity density in Pb-Pb collisions at sNN−−−√=5.02TeV in centrality classes measured by ALICE. The measurement covers a wide pseudorapidity range from −3.5 to 5, which is sufficient for reliable estimates of the total number of charged particles produced in the collisions. For the most central (0-5%) collisions we find 21400±1300 while for the most peripheral (80-90%) we find 230±38. This corresponds to an increase of (27±4)% over the results at sNN−−−√=2.76TeV previously reported by ALICE. The energy dependence of the total number of charged particles produced in heavy-ion collisions is found to obey a modified power-law like behaviour. The charged-particle pseudorapidity density of the most central collisions is compared to model calculations --- none of which fully describes the measured distribution. We also present an estimate of the rapidity density of charged particles. The width of that distribution is found to exhibit a remarkable proportionality to the beam rapidity, independent of the collision energy from the top SPS to LHC energies.
The measurement of the mass differences for systems bound by the strong force has reached a very high precision with protons and anti-protons. The extension of such measurement from (anti-)baryons to (anti-)nuclei allows one to probe any difference in the interactions between nucleons and anti-nucleons encoded in the (anti-)nuclei masses. This force is a remnant of the underlying strong interaction among quarks and gluons and can be described by effective theories, but cannot yet be directly derived from quantum chromodynamics. Here we report a measurement of the difference between the ratios of the mass and charge of deuterons and anti-deuterons, and 3He and 3He¯¯¯¯¯¯ nuclei carried out with the ALICE (A Large Ion Collider Experiment) detector in Pb-Pb collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV. Our direct measurement of the mass-over-charge differences confirm CPT invariance to an unprecedented precision in the sector of light nuclei. This fundamental symmetry of nature, which exchanges particles with anti-particles, implies that all physics laws are the same under the simultaneous reversal of charge(s) (charge conjugation C), reflection of spatial coordinates (parity transformation P) and time inversion (T).
The strength of forward-backward (FB) multiplicity correlations is measured by the ALICE detector in proton-proton (pp) collisions at s√=0.9, 2.76 and 7 TeV. The measurement is performed in the central pseudorapidity region (|η|<0.8) for the transverse momentum pT>0.3 GeV/c. Two separate pseudorapidity windows of width (δη) ranging from 0.2 to 0.8 are chosen symmetrically around η=0. The multiplicity correlation strength (bcor) is studied as a function of the pseudorapidity gap (ηgap) between the two windows as well as the width of these windows. The correlation strength is found to decrease with increasing ηgap and shows a non-linear increase with δη. A sizable increase of the correlation strength with the collision energy, which cannot be explained exclusively by the increase of the mean multiplicity inside the windows, is observed. The correlation coefficient is also measured for multiplicities in different configurations of two azimuthal sectors selected within the symmetric FB η-windows. Two different contributions, the short-range (SR) and the long-range (LR), are observed. The energy dependence of bcor is found to be weak for the SR component while it is strong for the LR component. Moreover, the correlation coefficient is studied for particles belonging to various transverse momentum intervals chosen to have the same mean multiplicity. Both SR and LR contributions to bcor are found to increase with pT in this case. Results are compared to PYTHIA and PHOJET event generators and to a string-based phenomenological model. The observed dependencies of bcor add new constraints on phenomenological models.
The strength of forward-backward (FB) multiplicity correlations is measured by the ALICE detector in proton-proton (pp) collisions at s√=0.9, 2.76 and 7 TeV. The measurement is performed in the central pseudorapidity region (|η|<0.8) for the transverse momentum pT>0.3 GeV/c. Two separate pseudorapidity windows of width (δη) ranging from 0.2 to 0.8 are chosen symmetrically around η=0. The multiplicity correlation strength (bcor) is studied as a function of the pseudorapidity gap (ηgap) between the two windows as well as the width of these windows. The correlation strength is found to decrease with increasing ηgap and shows a non-linear increase with δη. A sizable increase of the correlation strength with the collision energy, which cannot be explained exclusively by the increase of the mean multiplicity inside the windows, is observed. The correlation coefficient is also measured for multiplicities in different configurations of two azimuthal sectors selected within the symmetric FB η-windows. Two different contributions, the short-range (SR) and the long-range (LR), are observed. The energy dependence of bcor is found to be weak for the SR component while it is strong for the LR component. Moreover, the correlation coefficient is studied for particles belonging to various transverse momentum intervals chosen to have the same mean multiplicity. Both SR and LR contributions to bcor are found to increase with pT in this case. Results are compared to PYTHIA and PHOJET event generators and to a string-based phenomenological model. The observed dependencies of bcor add new constraints on phenomenological models.
The strength of forward-backward (FB) multiplicity correlations is measured by the ALICE detector in proton-proton (pp) collisions at s√=0.9, 2.76 and 7 TeV. The measurement is performed in the central pseudorapidity region (|η|<0.8) for the transverse momentum pT>0.3 GeV/c. Two separate pseudorapidity windows of width (δη) ranging from 0.2 to 0.8 are chosen symmetrically around η=0. The multiplicity correlation strength (bcor) is studied as a function of the pseudorapidity gap (ηgap) between the two windows as well as the width of these windows. The correlation strength is found to decrease with increasing ηgap and shows a non-linear increase with δη. A sizable increase of the correlation strength with the collision energy, which cannot be explained exclusively by the increase of the mean multiplicity inside the windows, is observed. The correlation coefficient is also measured for multiplicities in different configurations of two azimuthal sectors selected within the symmetric FB η-windows. Two different contributions, the short-range (SR) and the long-range (LR), are observed. The energy dependence of bcor is found to be weak for the SR component while it is strong for the LR component. Moreover, the correlation coefficient is studied for particles belonging to various transverse momentum intervals chosen to have the same mean multiplicity. Both SR and LR contributions to bcor are found to increase with pT in this case. Results are compared to PYTHIA and PHOJET event generators and to a string-based phenomenological model. The observed dependencies of bcor add new constraints on phenomenological models.
We report the results of the femtoscopic analysis of pairs of identical pions measured in p-Pb collisions at sNN−−−√=5.02 TeV. Femtoscopic radii are determined as a function of event multiplicity and pair momentum in three spatial dimensions. As in the pp collision system, the analysis is complicated by the presence of sizable background correlation structures in addition to the femtoscopic signal. The radii increase with event multiplicity and decrease with pair transverse momentum. When taken at comparable multiplicity, the radii measured in p-Pb collisions, at high multiplicity and low pair transverse momentum, are 10-20% higher than those observed in pp collisions but below those observed in A-A collisions. The results are compared to hydrodynamic predictions at large event multiplicity as well as discussed in the context of calculations based on gluon saturation.
We report the results of the femtoscopic analysis of pairs of identical pions measured in p-Pb collisions at sNN−−−√=5.02 TeV. Femtoscopic radii are determined as a function of event multiplicity and pair momentum in three spatial dimensions. As in the pp collision system, the analysis is complicated by the presence of sizable background correlation structures in addition to the femtoscopic signal. The radii increase with event multiplicity and decrease with pair transverse momentum. When taken at comparable multiplicity, the radii measured in p-Pb collisions, at high multiplicity and low pair transverse momentum, are 10-20% higher than those observed in pp collisions but below those observed in A-A collisions. The results are compared to hydrodynamic predictions at large event multiplicity as well as discussed in the context of calculations based on gluon saturation.
We report the results of the femtoscopic analysis of pairs of identical pions measured in p-Pb collisions at sNN−−−√=5.02 TeV. Femtoscopic radii are determined as a function of event multiplicity and pair momentum in three spatial dimensions. As in the pp collision system, the analysis is complicated by the presence of sizable background correlation structures in addition to the femtoscopic signal. The radii increase with event multiplicity and decrease with pair transverse momentum. When taken at comparable multiplicity, the radii measured in p-Pb collisions, at high multiplicity and low pair transverse momentum, are 10-20% higher than those observed in pp collisions but below those observed in A-A collisions. The results are compared to hydrodynamic predictions at large event multiplicity as well as discussed in the context of calculations based on gluon saturation.
The transverse momentum (pT) spectrum and nuclear modification factor (RAA) of reconstructed jets in 0-10% and 10-30% central Pb-Pb collisions at sNN−−−√=2.76 TeV were measured. Jets were reconstructed from charged and neutral particles, utilizing the ALICE tracking detectors and Electromagnetic Calorimeter (EMCal), with the anti-kT jet algorithm with a resolution parameter of R=0.2. The jet pT spectra are reported in the pseudorapidity interval of |ηjet|<0.5 for 40<pT,jet<120 GeV/c in 0-10% and for 30<pT,jet<100 GeV/c in 10-30% collisions. Reconstructed jets were required to contain a leading charged particle with pT>5 GeV/c to suppress jets constructed from the combinatorial background in Pb-Pb collisions. The effect of the leading charged particle requirement has been studied in both pp and Pb-Pb collisions and has been shown to have negligible effects on the RAA within the uncertainties of the measurement. The nuclear modification factor is obtained by dividing the jet spectrum measured in Pb-Pb by that in pp collisions scaled by the number of independent nucleon-nucleon collisions estimated using a Glauber model. RAA is found to be 0.28±0.04 in 0-10% and 0.35±0.04 in 10-30% collisions, independent of pT,jet within the uncertainties of the measurement. The observed suppression is in fair agreement with expectations from two model calculations with different approaches to jet quenching.
The transverse momentum (pT) spectrum and nuclear modification factor (RAA) of reconstructed jets in 0-10% and 10-30% central Pb-Pb collisions at sNN−−−√=2.76 TeV were measured. Jets were reconstructed from charged and neutral particles, utilizing the ALICE tracking detectors and Electromagnetic Calorimeter (EMCal), with the anti-kT jet algorithm with a resolution parameter of R=0.2. The jet pT spectra are reported in the pseudorapidity interval of |ηjet|<0.5 for 40<pT,jet<120 GeV/c in 0-10% and for 30<pT,jet<100 GeV/c in 10-30% collisions. Reconstructed jets were required to contain a leading charged particle with pT>5 GeV/c to suppress jets constructed from the combinatorial background in Pb-Pb collisions. The effect of the leading charged particle requirement has been studied in both pp and Pb-Pb collisions and has been shown to have negligible effects on the RAA within the uncertainties of the measurement. The nuclear modification factor is obtained by dividing the jet spectrum measured in Pb-Pb by that in pp collisions scaled by the number of independent nucleon-nucleon collisions estimated using a Glauber model. RAA is found to be 0.28±0.04 in 0-10% and 0.35±0.04 in 10-30% collisions, independent of pT,jet within the uncertainties of the measurement. The observed suppression is in fair agreement with expectations from two model calculations with different approaches to jet quenching.
The production of (anti-)deuteron and (anti-)3He nuclei in Pb-Pb collisions at sNN−−−√ = 2.76 TeV has been studied using the ALICE detector at the LHC. The spectra exhibit a significant hardening with increasing centrality. Combined blast-wave fits of several particles support the interpretation that this behavior is caused by an increase of radial flow. The integrated particle yields are discussed in the context of coalescence and thermal-statistical model expectations. The particle ratios, 3He/d and 3He/p, in Pb-Pb collisions are found to be in agreement with a common chemical freeze-out temperature of Tchem≈156 MeV. These ratios do not vary with centrality which is in agreement with the thermal-statistical model. In a coalescence approach, it excludes models in which nucleus production is proportional to the particle multiplicity and favors those in which it is proportional to the particle density instead. In addition, the observation of 31 anti-tritons in Pb-Pb collisions is reported. For comparison, the deuteron spectrum in pp collisions at s√=7 TeV is also presented. While the p/π ratio is similar in pp and Pb-Pb collisions, the d/p ratio in pp collisions is found to be lower by a factor of 2.2 than in Pb-Pb collisions.
The production of (anti-)deuteron and (anti-)3He nuclei in Pb-Pb collisions at sNN−−−√ = 2.76 TeV has been studied using the ALICE detector at the LHC. The spectra exhibit a significant hardening with increasing centrality. Combined blast-wave fits of several particles support the interpretation that this behavior is caused by an increase of radial flow. The integrated particle yields are discussed in the context of coalescence and thermal-statistical model expectations. The particle ratios, 3He/d and 3He/p, in Pb-Pb collisions are found to be in agreement with a common chemical freeze-out temperature of Tchem≈156 MeV. These ratios do not vary with centrality which is in agreement with the thermal-statistical model. In a coalescence approach, it excludes models in which nucleus production is proportional to the particle multiplicity and favors those in which it is proportional to the particle density instead. In addition, the observation of 31 anti-tritons in Pb-Pb collisions is reported. For comparison, the deuteron spectrum in pp collisions at s√=7 TeV is also presented. While the p/π ratio is similar in pp and Pb-Pb collisions, the d/p ratio in pp collisions is found to be lower by a factor of 2.2 than in Pb-Pb collisions.
The production of (anti-)deuteron and (anti-)3He nuclei in Pb-Pb collisions at sNN−−−√ = 2.76 TeV has been studied using the ALICE detector at the LHC. The spectra exhibit a significant hardening with increasing centrality. Combined blast-wave fits of several particles support the interpretation that this behavior is caused by an increase of radial flow. The integrated particle yields are discussed in the context of coalescence and thermal-statistical model expectations. The particle ratios, 3He/d and 3He/p, in Pb-Pb collisions are found to be in agreement with a common chemical freeze-out temperature of Tchem≈156 MeV. These ratios do not vary with centrality which is in agreement with the thermal-statistical model. In a coalescence approach, it excludes models in which nucleus production is proportional to the particle multiplicity and favors those in which it is proportional to the particle density instead. In addition, the observation of 31 anti-tritons in Pb-Pb collisions is reported. For comparison, the deuteron spectrum in pp collisions at s√=7 TeV is also presented. While the p/π ratio is similar in pp and Pb-Pb collisions, the d/p ratio in pp collisions is found to be lower by a factor of 2.2 than in Pb-Pb collisions.
The production of (anti-)deuteron and (anti-)3He nuclei in Pb-Pb collisions at sNN−−−√ = 2.76 TeV has been studied using the ALICE detector at the LHC. The spectra exhibit a significant hardening with increasing centrality. Combined blast-wave fits of several particles support the interpretation that this behavior is caused by an increase of radial flow. The integrated particle yields are discussed in the context of coalescence and thermal-statistical model expectations. The particle ratios, 3He/d and 3He/p, in Pb-Pb collisions are found to be in agreement with a common chemical freeze-out temperature of Tchem≈156 MeV. These ratios do not vary with centrality which is in agreement with the thermal-statistical model. In a coalescence approach, it excludes models in which nucleus production is proportional to the particle multiplicity and favors those in which it is proportional to the particle density instead. In addition, the observation of 31 anti-tritons in Pb-Pb collisions is reported. For comparison, the deuteron spectrum in pp collisions at s√=7 TeV is also presented. While the p/π ratio is similar in pp and Pb-Pb collisions, the d/p ratio in pp collisions is found to be lower by a factor of 2.2 than in Pb-Pb collisions.
We have studied the transverse-momentum (pT) dependence of the inclusive J/ψ production in p-Pb collisions at sNN−−−√=5.02 TeV, in three center-of-mass rapidity (ycms) regions, down to zero pT. Results in the forward and backward rapidity ranges (2.03<ycms<3.53 and −4.46<ycms<−2.96) are obtained by studying the J/ψ decay to μ+μ−, while the mid-rapidity region (−1.37<ycms<0.43) is investigated by measuring the e+e− decay channel. The pT dependence of the J/ψ production cross section and nuclear modification factor are presented for each of the rapidity intervals, as well as the J/ψ mean pT values. Forward and mid-rapidity results show a suppression of the J/ψ yield, with respect to pp collisions, which decreases with increasing pT. At backward rapidity no significant J/ψ suppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/ψ production in Pb-Pb collisions are also discussed.
We have studied the transverse-momentum (pT) dependence of the inclusive J/ψ production in p-Pb collisions at sNN−−−√=5.02 TeV, in three center-of-mass rapidity (ycms) regions, down to zero pT. Results in the forward and backward rapidity ranges (2.03<ycms<3.53 and −4.46<ycms<−2.96) are obtained by studying the J/ψ decay to μ+μ−, while the mid-rapidity region (−1.37<ycms<0.43) is investigated by measuring the e+e− decay channel. The pT dependence of the J/ψ production cross section and nuclear modification factor are presented for each of the rapidity intervals, as well as the J/ψ mean pT values. Forward and mid-rapidity results show a suppression of the J/ψ yield, with respect to pp collisions, which decreases with increasing pT. At backward rapidity no significant J/ψ suppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/ψ production in Pb-Pb collisions are also discussed.
We have studied the transverse-momentum (pT) dependence of the inclusive J/ψ production in p-Pb collisions at sNN−−−√=5.02 TeV, in three center-of-mass rapidity (ycms) regions, down to zero pT. Results in the forward and backward rapidity ranges (2.03<ycms<3.53 and −4.46<ycms<−2.96) are obtained by studying the J/ψ decay to μ+μ−, while the mid-rapidity region (−1.37<ycms<0.43) is investigated by measuring the e+e− decay channel. The pT dependence of the J/ψ production cross section and nuclear modification factor are presented for each of the rapidity intervals, as well as the J/ψ mean pT values. Forward and mid-rapidity results show a suppression of the J/ψ yield, with respect to pp collisions, which decreases with increasing pT. At backward rapidity no significant J/ψ suppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/ψ production in Pb-Pb collisions are also discussed.
Charged jet production cross sections in p-Pb collisions at sNN−−−√=5.02 TeV measured with the ALICE detector at the LHC are presented. Using the anti-kT algorithm, jets have been reconstructed in the central rapidity region from charged particles with resolution parameters R=0.2 and R=0.4. The reconstructed jets have been corrected for detector effects and the underlying event background. To calculate the nuclear modification factor, RpPb, of charged jets in p-Pb collisions, a pp reference was constructed by scaling previously measured charged jet spectra at s√=7 TeV. In the transverse momentum range 20≤pT,ch jet≤120 GeV/c, RpPb is found to be consistent with unity, indicating the absence of strong nuclear matter effects on jet production. Major modifications to the radial jet structure are probed via the ratio of jet production cross sections reconstructed with the two different resolution parameters. This ratio is found to be similar to the measurement in pp collisions at s√=7 TeV and to the expectations from PYTHIA pp simulations and NLO pQCD calculations at sNN−−−√=5.02 TeV.
Charged jet production cross sections in p-Pb collisions at sNN−−−√=5.02 TeV measured with the ALICE detector at the LHC are presented. Using the anti-kT algorithm, jets have been reconstructed in the central rapidity region from charged particles with resolution parameters R=0.2 and R=0.4. The reconstructed jets have been corrected for detector effects and the underlying event background. To calculate the nuclear modification factor, RpPb, of charged jets in p-Pb collisions, a pp reference was constructed by scaling previously measured charged jet spectra at s√=7 TeV. In the transverse momentum range 20≤pT,ch jet≤120 GeV/c, RpPb is found to be consistent with unity, indicating the absence of strong nuclear matter effects on jet production. Major modifications to the radial jet structure are probed via the ratio of jet production cross sections reconstructed with the two different resolution parameters. This ratio is found to be similar to the measurement in pp collisions at s√=7 TeV and to the expectations from PYTHIA pp simulations and NLO pQCD calculations at sNN−−−√=5.02 TeV.
A measurement of dijet correlations in p-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector is presented. Jets are reconstructed from charged particles measured in the central tracking detectors and neutral energy deposited in the electromagnetic calorimeter. The transverse momentum of the full jet (clustered from charged and neutral constituents) and charged jet (clustered from charged particles only) is corrected event-by-event for the contribution of the underlying event, while corrections for underlying event fluctuations and finite detector resolution are applied on an inclusive basis. A projection of the dijet transverse momentum, kTy=pch+neT,jetsin(Δφdijet) with Δφdijet the azimuthal angle between a full and charged jet and pch+neT,jet the transverse momentum of the full jet, is used to study nuclear matter effects in p-Pb collisions. This observable is sensitive to the acoplanarity of dijet production and its potential modification in p-Pb collisions with respect to pp collisions. Measurements of the dijet kTy as a function of the transverse momentum of the full and recoil charged jet, and the event multiplicity are presented. No significant modification of kTy due to nuclear matter effects in p-Pb collisions with respect to the event multiplicity or a PYTHIA8 reference is observed.
A measurement of dijet correlations in p-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector is presented. Jets are reconstructed from charged particles measured in the central tracking detectors and neutral energy deposited in the electromagnetic calorimeter. The transverse momentum of the full jet (clustered from charged and neutral constituents) and charged jet (clustered from charged particles only) is corrected event-by-event for the contribution of the underlying event, while corrections for underlying event fluctuations and finite detector resolution are applied on an inclusive basis. A projection of the dijet transverse momentum, kTy=pch+neT,jetsin(Δφdijet) with Δφdijet the azimuthal angle between a full and charged jet and pch+neT,jet the transverse momentum of the full jet, is used to study nuclear matter effects in p-Pb collisions. This observable is sensitive to the acoplanarity of dijet production and its potential modification in p-Pb collisions with respect to pp collisions. Measurements of the dijet kTy as a function of the transverse momentum of the full and recoil charged jet, and the event multiplicity are presented. No significant modification of kTy due to nuclear matter effects in p-Pb collisions with respect to the event multiplicity or a PYTHIA8 reference is observed.
A measurement of dijet correlations in p-Pb collisions at sNN−−−√=5.02 TeV with the ALICE detector is presented. Jets are reconstructed from charged particles measured in the central tracking detectors and neutral energy deposited in the electromagnetic calorimeter. The transverse momentum of the full jet (clustered from charged and neutral constituents) and charged jet (clustered from charged particles only) is corrected event-by-event for the contribution of the underlying event, while corrections for underlying event fluctuations and finite detector resolution are applied on an inclusive basis. A projection of the dijet transverse momentum, kTy=pch+neT,jetsin(Δφdijet) with Δφdijet the azimuthal angle between a full and charged jet and pch+neT,jet the transverse momentum of the full jet, is used to study nuclear matter effects in p-Pb collisions. This observable is sensitive to the acoplanarity of dijet production and its potential modification in p-Pb collisions with respect to pp collisions. Measurements of the dijet kTy as a function of the transverse momentum of the full and recoil charged jet, and the event multiplicity are presented. No significant modification of kTy due to nuclear matter effects in p-Pb collisions with respect to the event multiplicity or a PYTHIA8 reference is observed.
The measurement of primary π±, K±, p and p¯¯¯ production at mid-rapidity (|y|< 0.5) in proton-proton collisions at s√=7 TeV performed with ALICE (A Large Ion Collider Experiment) at the Large Hadron Collider (LHC) is reported. Particle identification is performed using the specific ionization energy loss and time-of-flight information, the ring-imaging Cherenkov technique and the kink-topology identification of weak decays of charged kaons. Transverse momentum spectra are measured from 0.1 up to 3 GeV/c for pions, from 0.2 up to 6 GeV/c for kaons and from 0.3 up to 6 GeV/c for protons. The measured spectra and particle ratios are compared with QCD-inspired models, tuned to reproduce also the earlier measurements performed at the LHC. Furthermore, the integrated particle yields and ratios as well as the average transverse momenta are compared with results at lower collision energies.
The measurement of primary π±, K±, p and p¯¯¯ production at mid-rapidity (|y|< 0.5) in proton-proton collisions at s√=7 TeV performed with ALICE (A Large Ion Collider Experiment) at the Large Hadron Collider (LHC) is reported. Particle identification is performed using the specific ionization energy loss and time-of-flight information, the ring-imaging Cherenkov technique and the kink-topology identification of weak decays of charged kaons. Transverse momentum spectra are measured from 0.1 up to 3 GeV/c for pions, from 0.2 up to 6 GeV/c for kaons and from 0.3 up to 6 GeV/c for protons. The measured spectra and particle ratios are compared with QCD-inspired models, tuned to reproduce also the earlier measurements performed at the LHC. Furthermore, the integrated particle yields and ratios as well as the average transverse momenta are compared with results at lower collision energies.
The measurement of primary π±, K±, p and p¯¯¯ production at mid-rapidity (|y|< 0.5) in proton-proton collisions at s√=7 TeV performed with ALICE (A Large Ion Collider Experiment) at the Large Hadron Collider (LHC) is reported. Particle identification is performed using the specific ionization energy loss and time-of-flight information, the ring-imaging Cherenkov technique and the kink-topology identification of weak decays of charged kaons. Transverse momentum spectra are measured from 0.1 up to 3 GeV/c for pions, from 0.2 up to 6 GeV/c for kaons and from 0.3 up to 6 GeV/c for protons. The measured spectra and particle ratios are compared with QCD-inspired models, tuned to reproduce also the earlier measurements performed at the LHC. Furthermore, the integrated particle yields and ratios as well as the average transverse momenta are compared with results at lower collision energies.
We present results of a search for two hypothetical strange dibaryon states, i.e. the H-dibaryon and the possible Λn¯ bound state. The search is performed with the ALICE detector in central (0-10%) Pb-Pb collisions at sNN−−−√=2.76 TeV, by invariant mass analysis in the decay modes Λn¯→d¯π+ and H-dibaryon →Λpπ−. No evidence for these bound states is observed. Upper limits are determined at 99% confidence level for a wide range of lifetimes and for the full range of branching ratios. The results are compared to thermal, coalescence and hybrid UrQMD model expectations, which describe correctly the production of other loosely bound states, like the deuteron and the hypertriton.
We present results of a search for two hypothetical strange dibaryon states, i.e. the H-dibaryon and the possible Λn¯¯¯¯¯¯ bound state. The search is performed with the ALICE detector in central (0-10%) Pb-Pb collisions at sNN−−−√=2.76 TeV, by invariant mass analysis in the decay modes Λn¯¯¯¯¯¯→d¯¯¯π+ and H-dibaryon →Λpπ−. No evidence for these bound states is observed. Upper limits are determined at 99% confidence level for a wide range of lifetimes and for the full range of branching ratios. The results are compared to thermal, coalescence and hybrid UrQMD model expectations, which describe correctly the production of other loosely bound states, like the deuteron and the hypertriton.
We report the first measurement at the LHC of coherent photoproduction of ρ0 mesons in ultra-peripheral Pb-Pb collisions. The invariant mass and transverse momentum distributions for ρ0 production are studied in the π+π− decay channel at mid-rapidity. The production cross section in the rapidity range |y|<0.5 is found to be dσ/dy=425±10(stat.) +42−50(sys.) mb. Coherent ρ0 production is studied with and without requirement of nuclear breakup, and the fractional yields for various breakup scenarios are presented. The results are compared with those from lower energies and with model predictions.
We report the first measurement at the LHC of coherent photoproduction of ρ0 mesons in ultra-peripheral Pb-Pb collisions. The invariant mass and transverse momentum distributions for ρ0 production are studied in the π+π− decay channel at mid-rapidity. The production cross section in the rapidity range |y|<0.5 is found to be dσ/dy=425±10(stat.) +42−50(sys.) mb. Coherent ρ0 production is studied with and without requirement of nuclear breakup, and the fractional yields for various breakup scenarios are presented. The results are compared with those from lower energies and with model predictions.
We report the first measurement at the LHC of coherent photoproduction of ρ0 mesons in ultra-peripheral Pb-Pb collisions. The invariant mass and transverse momentum distributions for ρ0 production are studied in the π+π− decay channel at mid-rapidity. The production cross section in the rapidity range |y|<0.5 is found to be dσ/dy=425±10(stat.) +42−50(sys.) mb. Coherent ρ0 production is studied with and without requirement of nuclear breakup, and the fractional yields for various breakup scenarios are presented. The results are compared with those from lower energies and with model predictions based on the Glauber model and the color dipole model. The measured cross section is found to be inconsistent with a scaling of the γ-nucleon cross section using the Glauber model.
Prompt D meson and non-prompt J/ψ yields are studied as a function of the multiplicity of charged particles produced in inelastic proton-proton collisions at a centre-of-mass energy of s√=7 TeV. The results are reported as a ratio between yields in a given multiplicity interval normalised to the multiplicity-integrated ones (relative yields). They are shown as a function of the multiplicity of charged particles normalised to the average value for inelastic collisions (relative charged-particle multiplicity). D0, D+ and D∗+ mesons are measured in five pT intervals from 1 to 20 GeV/c and for |y|<0.5 via their hadronic decays. The D-meson relative yield is found to increase with increasing charged-particle multiplicity. For events with multiplicity six times higher than the average multiplicity of inelastic collisions, a yield enhancement of a factor about 15 relative to the multiplicity-integrated yield in inelastic collisions is observed. The yield enhancement is independent of transverse momentum within the uncertainties of the measurement. The D0-meson relative yield is also measured as a function of the relative multiplicity at forward pseudorapidity. The non-prompt J/ψ, i.e. the B hadron, contribution to the inclusive J/ψ production is measured in the di-electron decay channel at central rapidity. It is evaluated for pT>1.3 GeV/c and |y|<0.9, and extrapolated to pT>0. The fraction of non-prompt J/ψ in the inclusive J/ψ yields shows no dependence on the charged-particle multiplicity at central rapidity. Charm and beauty hadron relative yields exhibit a similar increase with increasing charged-particle multiplicity. The measurements are compared to PYTHIA 8, EPOS 3 and percolation calculations.
Prompt D meson and non-prompt J/ yields are studied as a function of the multiplicity of charged particles produced in inelastic proton-proton collisions at a centre-of-mass energy of TeV. The results are reported as a ratio between yields in a given multiplicity interval normalised to the multiplicity-integrated ones (relative yields). They are shown as a function of the multiplicity of charged particles normalised to the average value for inelastic collisions (relative charged-particle multiplicity). D, D and D mesons are measured in five intervals from 1 to 20 GeV/ and for via their hadronic decays. The D-meson relative yield is found to increase with increasing charged-particle multiplicity. For events with multiplicity six times higher than the average multiplicity of inelastic collisions, a yield enhancement of a factor about 15 relative to the multiplicity-integrated yield in inelastic collisions is observed. The yield enhancement is independent of transverse momentum within the uncertainties of the measurement. The D-meson relative yield is also measured as a function of the relative multiplicity at forward pseudorapidity. The non-prompt J/, i.e. the B hadron, contribution to the inclusive J/ production is measured in the di-electron decay channel at central rapidity. It is evaluated for GeV/ and , and extrapolated to . The fraction of non-prompt J/ in the inclusive J/ yields shows no dependence on the charged-particle multiplicity at central rapidity. Charm and beauty hadron relative yields exhibit a similar increase with increasing charged-particle multiplicity. The measurements are compared to PYTHIA 8, EPOS 3 and percolation calculations.
Prompt D meson and non-prompt J/ψ yields are studied as a function of the multiplicity of charged particles produced in inelastic proton-proton collisions at a centre-of-mass energy of s√=7 TeV. The results are reported as a ratio between yields in a given multiplicity interval normalised to the multiplicity-integrated ones (relative yields). They are shown as a function of the multiplicity of charged particles normalised to the average value for inelastic collisions (relative charged-particle multiplicity). D0, D+ and D∗+ mesons are measured in five pT intervals from 1 to 20 GeV/c and for |y|<0.5 via their hadronic decays. The D-meson relative yield is found to increase with increasing charged-particle multiplicity. For events with multiplicity six times higher than the average multiplicity of inelastic collisions, a yield enhancement of a factor about 15 relative to the multiplicity-integrated yield in inelastic collisions is observed. The yield enhancement is independent of transverse momentum within the uncertainties of the measurement. The D0-meson relative yield is also measured as a function of the relative multiplicity at forward pseudorapidity. The non-prompt J/ψ, i.e. the B hadron, contribution to the inclusive J/ψ production is measured in the di-electron decay channel at central rapidity. It is evaluated for pT>1.3 GeV/c and |y|<0.9, and extrapolated to pT>0. The fraction of non-prompt J/ψ in the inclusive J/ψ yields shows no dependence on the charged-particle multiplicity at central rapidity. Charm and beauty hadron relative yields exhibit a similar increase with increasing charged-particle multiplicity. The measurements are compared to PYTHIA 8, EPOS 3 and percolation calculations.
The transverse momentum (pT) dependence of the nuclear modification factor RAA and the centrality dependence of the average transverse momentum ⟨pT⟩ for inclusive J/ψ have been measured with ALICE for Pb-Pb collisions at sNN−−−√ = 2.76 TeV in the e+e− decay channel at mid-rapidity (|y|<0.8). The ⟨pT⟩ is significantly smaller than the one observed for pp collisions at the same centre-of-mass energy. Consistently, an increase of RAA is observed towards low pT. These observations might be indicative of a sizable contribution of charm quark coalescence to the J/ψ production. Additionally, the fraction of non-prompt J/ψ from beauty hadron decays, fB, has been determined in the region 1.5<pT<10 GeV/c in three centrality intervals. No significant centrality dependence of fB is observed. Finally, the RAA of non-prompt J/ψ is discussed and compared with model predictions. The nuclear modification in the region 4.5<pT<10 GeV/c is found to be stronger than predicted by most models.
The transverse momentum (pT) dependence of the nuclear modification factor RAA and the centrality dependence of the average transverse momentum ⟨pT⟩ for inclusive J/ψ have been measured with ALICE for Pb-Pb collisions at sNN−−−√ = 2.76 TeV in the e+e− decay channel at mid-rapidity (|y|<0.8). The ⟨pT⟩ is significantly smaller than the one observed for pp collisions at the same centre-of-mass energy. Consistently, an increase of RAA is observed towards low pT. These observations might be indicative of a sizable contribution of charm quark coalescence to the J/ψ production. Additionally, the fraction of non-prompt J/ψ from beauty hadron decays, fB, has been determined in the region 1.5<pT<10 GeV/c in three centrality intervals. No significant centrality dependence of fB is observed. Finally, the RAA of non-prompt J/ψ is discussed and compared with model predictions. The nuclear modification in the region 4.5<pT<10 GeV/c is found to be stronger than predicted by most models.
The transverse momentum (pT) dependence of the nuclear modification factor RAA and the centrality dependence of the average transverse momentum ⟨pT⟩ for inclusive J/ψ have been measured with ALICE for Pb-Pb collisions at sNN−−−√ = 2.76 TeV in the e+e− decay channel at mid-rapidity (|y|<0.8). The ⟨pT⟩ is significantly smaller than the one observed for pp collisions at the same centre-of-mass energy. Consistently, an increase of RAA is observed towards low pT. These observations might be indicative of a sizable contribution of charm quark coalescence to the J/ψ production. Additionally, the fraction of non-prompt J/ψ from beauty hadron decays, fB, has been determined in the region 1.5<pT<10 GeV/c in three centrality intervals. No significant centrality dependence of fB is observed. Finally, the RAA of non-prompt J/ψ is discussed and compared with model predictions. The nuclear modification in the region 4.5<pT<10 GeV/c is found to be stronger than predicted by most models.
The transverse momentum (pT) dependence of the nuclear modification factor RAA and the centrality dependence of the average transverse momentum ⟨pT⟩ for inclusive J/ψ have been measured with ALICE for Pb-Pb collisions at sNN−−−√ = 2.76 TeV in the e+e− decay channel at mid-rapidity (|y|<0.8). The ⟨pT⟩ is significantly smaller than the one observed for pp collisions at the same centre-of-mass energy. Consistently, an increase of RAA is observed towards low pT. These observations might be indicative of a sizable contribution of charm quark coalescence to the J/ψ production. Additionally, the fraction of non-prompt J/ψ from beauty hadron decays, fB, has been determined in the region 1.5<pT<10 GeV/c in three centrality intervals. No significant centrality dependence of fB is observed. Finally, the RAA of non-prompt J/ψ is discussed and compared with model predictions. The nuclear modification in the region 4.5<pT<10 GeV/c is found to be stronger than predicted by most models.
The production of the hypertriton nuclei 3ΛH and 3Λ¯H¯¯¯¯ has been measured for the first time in Pb-Pb collisions at sNN−−−√ = 2.76 TeV with the ALICE experiment at LHC energies. The total yield, dN/dy ×B.R.(3ΛH→3He,π−)=(3.86±0.77(stat.)±0.68(syst.))×10−5 in the 0-10% most central collisions, is consistent with the predictions from a statistical thermal model using the same temperature as for the light hadrons. The coalescence parameter B3 shows a dependence on the transverse momentum, similar to the B2 of deuterons and the B3 of 3He nuclei. The ratio of yields S3 = 3ΛH/(3He ×Λ/p) was measured to be S3 = 0.60 ± 0.13 (stat.) ± 0.21 (syst.) in 0-10% centrality events; this value is compared to different theoretical models. The measured S3 is fully compatible with thermal model predictions. The measured 3ΛH lifetime, τ=181+54−39(stat.)±33(syst.) ps is compatible within 1σ with the world average value.
The production of the hypertriton nuclei 3ΛH and 3Λ¯H¯¯¯¯ has been measured for the first time in Pb-Pb collisions at sNN−−−√ = 2.76 TeV with the ALICE experiment at LHC energies. The total yield, dN/dy ×B.R.(3ΛH→3He,π−)=(3.86±0.77(stat.)±0.68(syst.))×10−5 in the 0-10% most central collisions, is consistent with the predictions from a statistical thermal model using the same temperature as for the light hadrons. The coalescence parameter B3 shows a dependence on the transverse momentum, similar to the B2 of deuterons and the B3 of 3He nuclei. The ratio of yields S3 = 3ΛH/(3He ×Λ/p) was measured to be S3 = 0.60 ± 0.13 (stat.) ± 0.21 (syst.) in 0-10% centrality events; this value is compared to different theoretical models. The measured S3 is fully compatible with thermal model predictions. The measured 3ΛH lifetime, τ=181+54−39(stat.)±33(syst.) ps is compatible within 1σ with the world average value.
We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb--Pb collisions at sNN−−−√=2.76 TeV as a function of collision centrality and the average transverse momentum of the pair kT. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with kT, following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with ⟨dNch/dη⟩1/3. This behaviour is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller sNN−−−√, a decrease in the Rout/Rside ratio is seen, which is in qualitative agreement with specific predictions from hydrodynamic models. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the LHC.
The size of the particle emission region in high-energy collisions can be deduced using the femtoscopic correlations of particle pairs at low relative momentum. Such correlations arise due to quantum statistics and Coulomb and strong final state interactions. In this paper, results are presented from femtoscopic analyses of π±π±, K±K±, K0SK0S, pp, and p¯¯¯p¯¯¯ correlations from Pb-Pb collisions at sNN−−−√=2.76 TeV by the ALICE experiment at the LHC. One-dimensional radii of the system are extracted from correlation functions in terms of the invariant momentum difference of the pair. The comparison of the measured radii with the predictions from a hydrokinetic model is discussed. The pion and kaon source radii display a monotonic decrease with increasing average pair transverse mass mT which is consistent with hydrodynamic model predictions for central collisions. The kaon and proton source sizes can be reasonably described by approximate mT-scaling.
The size of the particle emission region in high-energy collisions can be deduced using the femtoscopic correlations of particle pairs at low relative momentum. Such correlations arise due to quantum statistics and Coulomb and strong final state interactions. In this paper, results are presented from femtoscopic analyses of π±π±, K±K±, K0SK0S, pp, and p¯¯¯p¯¯¯ correlations from Pb-Pb collisions at sNN−−−√=2.76 TeV by the ALICE experiment at the LHC. One-dimensional radii of the system are extracted from correlation functions in terms of the invariant momentum difference of the pair. The comparison of the measured radii with the predictions from a hydrokinetic model is discussed. The pion and kaon source radii display a monotonic decrease with increasing average pair transverse mass mT which is consistent with hydrodynamic model predictions for central collisions. The kaon and proton source sizes can be reasonably described by approximate mT-scaling.
The size of the particle emission region in high-energy collisions can be deduced using the femtoscopic correlations of particle pairs at low relative momentum. Such correlations arise due to quantum statistics and Coulomb and strong final state interactions. In this paper, results are presented from femtoscopic analyses of π±π±, K±K±, K0SK0S, pp, and p¯¯¯p¯¯¯ correlations from Pb-Pb collisions at sNN−−−√=2.76 TeV by the ALICE experiment at the LHC. One-dimensional radii of the system are extracted from correlation functions in terms of the invariant momentum difference of the pair. The comparison of the measured radii with the predictions from a hydrokinetic model is discussed. The pion and kaon source radii display a monotonic decrease with increasing average pair transverse mass mT which is consistent with hydrodynamic model predictions for central collisions. The kaon and proton source sizes can be reasonably described by approximate mT-scaling.
We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb--Pb collisions at sNN−−−√=2.76 TeV as a function of collision centrality and the average transverse momentum of the pair kT. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with kT, following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with ⟨dNch/dη⟩1/3. This behaviour is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller sNN−−−√, a decrease in the Rout/Rside ratio is seen, which is in qualitative agreement with specific predictions from hydrodynamic models. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the LHC.
We report on the measurement of freeze-out radii for pairs of identical-charge pions measured in Pb--Pb collisions at sNN−−−√=2.76 TeV as a function of collision centrality and the average transverse momentum of the pair kT. Three-dimensional sizes of the system (femtoscopic radii), as well as direction-averaged one-dimensional radii are extracted. The radii decrease with kT, following a power-law behavior. This is qualitatively consistent with expectations from a collectively expanding system, produced in hydrodynamic calculations. The radii also scale linearly with ⟨dNch/dη⟩1/3. This behaviour is compared to world data on femtoscopic radii in heavy-ion collisions. While the dependence is qualitatively similar to results at smaller sNN−−−√, a decrease in the Rout/Rside ratio is seen, which is in qualitative agreement with specific predictions from hydrodynamic models. The results provide further evidence for the production of a collective, strongly coupled system in heavy-ion collisions at the LHC.
The nuclear modification factor, RAA, of the prompt charmed mesons D0, D+ and D∗+, and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass energy sNN−−−√=2.76 TeV in two transverse momentum intervals, 5<pT<8 GeV/c and 8<pT<16 GeV/c, and in six collision centrality classes. The RAA shows a maximum suppression of a factor of 5-6 in the 10% most central collisions. The suppression and its centrality dependence are compatible within uncertainties with those of charged pions. A comparison with the RAA of non-prompt J/ψ from B meson decays, measured by the CMS Collaboration, hints at a larger suppression of D mesons in the most central collisions.
The nuclear modification factor, RAA, of the prompt charmed mesons D0, D+ and D∗+, and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass energy sNN−−−√=2.76 TeV in two transverse momentum intervals, 5<pT<8 GeV/c and 8<pT<16 GeV/c, and in six collision centrality classes. The RAA shows a maximum suppression of a factor of 5-6 in the 10% most central collisions. The suppression and its centrality dependence are compatible within uncertainties with those of charged pions. A comparison with the RAA of non-prompt J/ψ from B meson decays, measured by the CMS Collaboration, hints at a larger suppression of D mesons in the most central collisions.
The nuclear modification factor, RAA, of the prompt charmed mesons D0, D+ and D∗+, and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass energy sNN−−−√=2.76 TeV in two transverse momentum intervals, 5<pT<8 GeV/c and 8<pT<16 GeV/c, and in six collision centrality classes. The RAA shows a maximum suppression of a factor of 5-6 in the 10% most central collisions. The suppression and its centrality dependence are compatible within uncertainties with those of charged pions. A comparison with the RAA of non-prompt J/ψ from B meson decays, measured by the CMS Collaboration, hints at a larger suppression of D mesons in the most central collisions.
The nuclear modification factor, RAA, of the prompt charmed mesons D0, D+ and D∗+, and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass energy sNN−−−√=2.76 TeV in two transverse momentum intervals, 5<pT<8 GeV/c and 8<pT<16 GeV/c, and in six collision centrality classes. The RAA shows a maximum suppression of a factor of 5-6 in the 10% most central collisions. The suppression and its centrality dependence are compatible within uncertainties with those of charged pions. A comparison with the RAA of non-prompt J/ψ from B meson decays, measured by the CMS Collaboration, hints at a larger suppression of D mesons in the most central collisions.
We report on results obtained with the Event Shape Engineering technique applied to Pb-Pb collisions at sNN−−−√=2.76 TeV. By selecting events in the same centrality interval, but with very different average flow, different initial state conditions can be studied. We find the effect of the event-shape selection on the elliptic flow coefficient v2 to be almost independent of transverse momentum pT, as expected if this effect is due to fluctuations in the initial geometry of the system. Charged hadron, pion, kaon, and proton transverse momentum distributions are found to be harder in events with higher-than-average elliptic flow, indicating an interplay between radial and elliptic flow.
We report on results obtained with the Event Shape Engineering technique applied to Pb-Pb collisions at sNN−−−√=2.76 TeV. By selecting events in the same centrality interval, but with very different average flow, different initial state conditions can be studied. We find the effect of the event-shape selection on the elliptic flow coefficient v2 to be almost independent of transverse momentum pT, as expected if this effect is due to fluctuations in the initial geometry of the system. Charged hadron, pion, kaon, and proton transverse momentum distributions are found to be harder in events with higher-than-average elliptic flow, indicating an interplay between radial and elliptic flow.
The transverse momentum distributions of the strange and double-strange hyperon resonances (Σ(1385)±, Ξ(1530)0) produced in p-Pb collisions at sNN−−−√=5.02 TeV were measured in the rapidity range −0.5<yCMS<0 for event classes corresponding to different charged-particle multiplicity densities, ⟨dNch/dηlab⟩. The mean transverse momentum values are presented as a function of ⟨dNch/dηlab⟩, as well as a function of the particle masses and compared with previous results on hyperon production. The integrated yield ratios of excited to ground-state hyperons are constant as a function of ⟨dNch/dηlab⟩. The equivalent ratios to pions exhibit an increase with ⟨dNch/dηlab⟩, depending on their strangeness content.
The transverse momentum distributions of the strange and double-strange hyperon resonances (Σ(1385)±, Ξ(1530)0) produced in p-Pb collisions at sNN−−−√=5.02 TeV were measured in the rapidity range −0.5<yCMS<0 for event classes corresponding to different charged-particle multiplicity densities, ⟨dNch/dηlab⟩. The mean transverse momentum values are presented as a function of ⟨dNch/dηlab⟩, as well as a function of the particle masses and compared with previous results on hyperon production. The integrated yield ratios of excited to ground-state hyperons are constant as a function of ⟨dNch/dηlab⟩. The equivalent ratios to pions exhibit an increase with ⟨dNch/dηlab⟩, depending on their strangeness content.
Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at s√=13 TeV and p−Pb collisions at sNN−−−√=5.02 TeV. The correlation functions are measured as a function of relative azimuthal angle Δφ and pseudorapidity separation Δη for pairs of primary charged particles within the pseudorapidity interval |η|<0.9 and the transverse-momentum interval 1<pT<4 GeV/c. Flow coefficients are extracted for the long-range correlations (1.6<|Δη|<1.8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.
Long- and short-range correlations for pairs of charged particles are studied via two-particle angular correlations in pp collisions at √sNN = 13 TeV and p–Pb collisions at √s = 5.02 TeV. The correlation functions are measured as a function of relative azimuthal angle ∆φ and pseudorapidity separation ∆η for pairs of primary charged particles within the pseudorapidity interval |η| < 0.9 and the transverse-momentum interval 1 < pT < 4 GeV/c. Flow coefficients are extracted for the long-range correlations (1.6 < |∆η| < 1.8) in various high-multiplicity event classes using the low-multiplicity template fit method. The method is used to subtract the enhanced yield of away-side jet fragments in high-multiplicity events. These results show decreasing flow signals toward lower multiplicity events. Furthermore, the flow coefficients for events with hard probes, such as jets or leading particles, do not exhibit any significant changes compared to those obtained from high-multiplicity events without any specific event selection criteria. The results are compared with hydrodynamic-model calculations, and it is found that a better understanding of the initial conditions is necessary to describe the results, particularly for low-multiplicity events.