Transverse Momentum Distributions and Nuclear Modification Factors in Heavy-Ion Collisions with ALICE at the Large Hadron Collider

In this work, Pb-Pb collisions measured in 2010 during the first data taking period (Run 1) at a centre-of-mass energy of √snn = 2.76 TeV and data taken in 2015 during Run 2 at √snn = 5.02 TeV are analysed. In November 2017, the LHC brought xenon ions to collision for the first time; this data set that was taken at √snn = 5.44 TeV is also analysed. Transverse momentum pt distributions at high pt of charged particles have shown that particle yields in heavy-ion (AA) collisions are suppressed compared to a superposition of independent nucleon-nucleon collisions (binary collision scaling). This observation is related to parton energy loss in the Quark Gluon Plasma (QGP). To obtain the charged-particle yield as a function of pt, corrections are made for tracking efficiency and acceptance, for contamination by secondary particles from weak decays or secondary interactions and for the pt resolution. To circumvent differences in the particle composition of event generators and data, the charged-particle reconstruction efficiency is calculated from the particle-dependent efficiencies weighted by the relative abundances of each particle measured during Run 1. The correction for contamination with secondary particles is usually obtained from Monte-Carlo (MC) simulations. The abundances of secondary particles in data and MC is estimated by analysing the distance of closest approach of tracks to the event vertex. It is found that the contamination correction from MC has to be scaled up by ~50% to match the data. The improvement of the analysis methods resulted in a reduction of the total relative systematic uncertainties by about 50% compared to previous analyses due to an improved reconstruction and calibration procedure in Run 2, as well as to improved track selection methods. The transverse momentum distribution of charged particles from Pb-Pb and Xe-Xe collisions were measured for nine classes of centrality. The measurement was performed for particles within -0.8 to 0.8 in pseudo-rapidity and for a transverse-momentum range of 0.15<pt<50 GeV. The nuclear modification factor (RAA) is defined as the pt-differential yield in a AA collision divided by the cross section in pp collisions, scaled by the nuclear overlap function calculated in a Monte-Carlo Glauber approach. Any suppression of particle yields in AA compared to a superposition of individual pp collisions results in a nuclear modification factor below unity. All measurements exhibit a moderate suppression for peripheral collisions. With increasing collision centrality, a pronounced suppression with RAA ~ 0.13 at intermediate pt develops. At higher pt, a significant rise of the nuclear modification factor is observed. The comparison of RAA as a function of the charged particle density per unit of rapidity dN_ch/dη shows a remarkable agreement of the observed suppression at high pt in Xe-Xe and Pb-Pb collisions at both energies scales for dN_ch/dη>400. This scaling does not hold for collisions with lower particle multiplicities. This observation is consistent with a dependence of the partonic energy loss on the square of the path length in the medium. At lower transverse momenta a dependence of RAA on the collision energy is observed, which might be due to the collision energy dependence of the bulk particle production

Azimuthal anisotropy of charged jet production in root s(NN)=2.76 TeV Pb-Pb collisions

We present measurements of the azimuthal dependence of charged jet production in central and semi-central root s(NN) = 2.76 TeV Pb-Pb collisions with respect to the second harmonic event plane, quantified as nu(ch)(2) (jet). Jet finding is performed employing the anti-k(T) algorithm with a resolution parameter R = 0.2 using charged tracks from the ALICE tracking system. The contribution of the azimuthal anisotropy of the underlying event is taken into account event-by-event. The remaining (statistical) region-to-region fluctuations are removed on an ensemble basis by unfolding the jet spectra for different event plane orientations independently. Significant non-zero nu(ch)(2) (jet) is observed in semi-central collisions (30-50% centrality) for 20 <p(T)(ch) (jet) <90 GeV/c. The azimuthal dependence of the charged jet production is similar to the dependence observed for jets comprising both charged and neutral fragments, and compatible with measurements of the nu(2) of single charged particles at high p(T). Good agreement between the data and predictions from JEWEL, an event generator simulating parton shower evolution in the presence of a dense QCD medium, is found in semi-central collisions. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Pseudorapidity and transverse-momentum distributions of charged particles in proton-proton collisions at root s=13 TeV

The pseudorapidity (eta) and transverse-momentum (p(T)) distributions of charged particles produced in proton-proton collisions are measured at the centre-of-mass energy root s = 13 TeV. The pseudorapidity distribution in vertical bar eta vertical bar <1.8 is reported for inelastic events and for events with at least one charged particle in vertical bar eta vertical bar <1. The pseudorapidity density of charged particles produced in the pseudorapidity region vertical bar eta vertical bar <0.5 is 5.31 +/- 0.18 and 6.46 +/- 0.19 for the two event classes, respectively. The transverse-momentum distribution of charged particles is measured in the range 0.15 <p(T) <20 GeV/c and vertical bar eta vertical bar <0.8 for events with at least one charged particle in vertical bar eta vertical bar <1. The evolution of the transverse momentum spectra of charged particles is also investigated as a function of event multiplicity. The results are compared with calculations from PYTHIA and EPOS Monte Carlo generators. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Centrality evolution of the charged-particle pseudorapidity density over a broad pseudorapidity range in Pb-Pb collisions at root s(NN)=2.76TeV

Peer reviewe

生産增加と貨幣需要

We report on two-particle charge-dependent cor- relations in pp, p\u2013Pb, and Pb\u2013Pb collisions as a function of the pseudorapidity and azimuthal angle difference, \u3b7 and \u3c6 respectively. These correlations are studied using the balance function that probes the charge creation time and the develop- ment of collectivity in the produced system. The dependence of the balance function on the event multiplicity as well as on the trigger and associated particle transverse momentum ( pT ) in pp, p\u2013Pb, and Pb\u2013Pb collisions at 1asNN = 7, 5.02, and 2.76 TeV, respectively, are presented. In the low transverse momentum region, for 0.2 < pT < 2.0 GeV/c, the balance function becomes narrower in both \u3b7 and \u3c6 directions in all three systems for events with higher multiplicity. The experimental findings favor models that either incorporate some collective behavior (e.g. AMPT) or different mecha- nisms that lead to effects that resemble collective behavior (e.g. PYTHIA8 with color reconnection). For higher values of transverse momenta the balance function becomes even narrower but exhibits no multiplicity dependence, indicating that the observed narrowing with increasing multiplicity at low pT is a feature of bulk particle production

Multiplicity dependence of light (anti-)nuclei production in p–Pb collisions at sNN=5.02 TeV

The measurement of the deuteron and anti-deuteron production in the rapidity range −1 < y < 0 as a function of transverse momentum and event multiplicity in p–Pb collisions at √sNN = 5.02 TeV is presented. (Anti-)deuterons are identified via their specific energy loss dE/dx and via their time-of- flight. Their production in p–Pb collisions is compared to pp and Pb–Pb collisions and is discussed within the context of thermal and coalescence models. The ratio of integrated yields of deuterons to protons (d/p) shows a significant increase as a function of the charged-particle multiplicity of the event starting from values similar to those observed in pp collisions at low multiplicities and approaching those observed in Pb–Pb collisions at high multiplicities. The mean transverse particle momenta are extracted from the deuteron spectra and the values are similar to those obtained for p and particles. Thus, deuteron spectra do not follow mass ordering. This behaviour is in contrast to the trend observed for non-composite particles in p–Pb collisions. In addition, the production of the rare 3He and 3He nuclei has been studied. The spectrum corresponding to all non-single diffractive p-Pb collisions is obtained in the rapidity window −1 < y < 0 and the pT-integrated yield dN/dy is extracted. It is found that the yields of protons, deuterons, and 3He, normalised by the spin degeneracy factor, follow an exponential decrease with mass number

Measurement of electrons from semileptonic heavy-flavour hadron decays at midrapidity in pp and Pb–Pb collisions at √sNN = 5.02 TeV

The differential invariant yield as a function of transverse momentum (pT) of electrons from semileptonic heavy-flavour hadron decays was measured at midrapidity in central (0–10%), semi-central (30–50%) and peripheral (60–80%) lead–lead (Pb–Pb) collisions at √sNN = 5.02 TeV in the pT intervals 0.5–26 GeV/c (0–10% and 30–50%) and 0.5–10 GeV/c (60–80%). The production cross section in proton–proton (pp) collisions at √s = 5.02 TeV was measured as well in 0.5 < pT < 10 GeV/c and it lies close to the upper band of perturbative QCD calculation uncertainties up to pT = 5 GeV/c and close to the mean value for larger pT. The modification of the electron yield with respect to what is expected for an incoherent superposition of nucleon–nucleon collisions is evaluated by measuring the nuclear modification factor RAA. The measurement of the RAA in different centrality classes allows in-medium energy loss of charm and beauty quarks to be investigated. The RAA shows a suppression with respect to unity at intermediate pT, which increases while moving towards more central collisions. Moreover, the measured RAA is sensitive to the modification of the parton distribution functions (PDF) in nuclei, like nuclear shadowing, which causes a suppression of the heavy-quark production at low pT in heavy-ion collisions at LHC

Transverse momentum dependence of inclusive primary charged-particle production in p-Pb collisions at sNN=5.02 TeV\sqrt{s_\mathrm{{NN}}}=5.02~\text {TeV}

The transverse momentum ($p_T$) distribution of primary charged particles is measured at midrapidity in minimum-bias p-Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV with the ALICE detector at the LHC in the range 0.15 < $p_T$ < 50 GeV/c. The spectra are compared to the expectation based on binary collision scaling of particle production in pp collisions, leading to a nuclear modification factor consistent with unity for $p_T$ larger than 2 GeV/c. The measurement is compared to theoretical calculations and to data in Pb-Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeVThe transverse momentum ( $p_{\mathrm T}$ ) distribution of primary charged particles is measured at midrapidity in minimum-bias p–Pb collisions at $\sqrt{s_{\mathrm {NN}}}=5.02$  TeV with the ALICE detector at the LHC in the range $0.15<p_{\mathrm T}<50$  GeV/ $c$ . The spectra are compared to the expectation based on binary collision scaling of particle production in pp collisions, leading to a nuclear modification factor consistent with unity for $p_{\mathrm T}$ larger than 2 GeV/ $c$ , with a weak indication of a Cronin-like enhancement for $p_\mathrm {T}$ around 4  $\mathrm {GeV}\!/c$ . The measurement is compared to theoretical calculations and to data in Pb–Pb collisions at $\sqrt{s_{\mathrm {NN}}}=2.76$  TeV.The transverse momentum ($p_{\mathrm T}$) distribution of primary charged particles is measured at midrapidity in minimum-bias p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ALICE detector at the LHC in the range $0.15<p_{\mathrm T}<50$ GeV/$c$. The spectra are compared to the expectation based on binary collision scaling of particle production in pp collisions, leading to a nuclear modification factor consistent with unity for $p_{\mathrm T}$ larger than 2 GeV/$c$, with a weak indication of a Cronin-like enhancement for $p_{\rm T}$ around 4 GeV/$c$. The measurement is compared to theoretical calculations and to data in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV

HΛ3 and H‾Λ‾3 lifetime measurement in Pb–Pb collisions at √sNN=5.02 TeV via two-body decay

An improved value for the lifetime of the (anti-)hypertriton has been obtained using the data sample of Pb–Pb collisions at √sNN = 5.02 TeV collected by the ALICE experiment at the LHC. The (anti-)hypertriton has been reconstructed via its charged two-body mesonic decay channel and the lifetime has been determined from an exponential fit to the dN/d(ct) spectrum. The measured value, τ = 242+34 −38 (stat.) ± 17 (syst.) ps, is compatible with representative theoretical predictions, thus contributing to the solution of the longstanding hypertriton lifetime puzzle