Transverse momentum spectra and nuclear modification factors of identified charged hadrons in p-Pb and Pb-Pb collisions at √sNN = 5.02 TeV with ALICE

The ALICE experiment has measured the production of identified light-flavour hadrons in p-Pb and Pb-Pb collisions at 5.02 TeV in a wide range of transverse momentum. The newest ALICE results on pion, kaon and proton transverse momentum spectra, yield ratios and nuclear modification factors will be presented and discussed in comparison to lower energy results. In particular, the production of identified hadrons in most central Pb-Pb collisions relative to pp collisions is found to be strongly suppressed at high transverse momenta (pT > 8 GeV/c) whereas in p-Pb collisions the nuclear modification factors are consistent with unity. This indicates that the strong suppression of high-pT hadrons measured in central Pb-Pb collisions is not due to an initial state effect but instead to the energy loss of partons traversing a hot and dense QCD medium.


Introduction
The deconfined state of quarks and gluons, the Quark-Gluon Plasma (QGP) predicted by Quantum Chromodynamics (QCD) [1,2,3,4] can be studied in ultra-relativistic heavy-ion collisions. Hydrodynamic and statistical models have been very successful in describing the bulk matter created in heavy-ion collisions. The initial hot and dense matter expands, cools down and undergoes a phase transition from a partonic to a hadronic phase. During the expansion collective flow builds up. Statistical models succeed to describe the observed particle abundances. Hard partons propagating through the hot and dense medium are predicted to lose energy via multiple scattering and gluon radiation. As a result, the yield of final state hadrons at high transverse momentum will be suppressed compared to the reference value from a simple superposition of incoherent proton-proton collisions [5,6]. This observation is typically quantified in terms of the nuclear modification factor, defined as the ratio of the yield in Pb-Pb to the corresponding yield in pp collisions scaled by the number of binary nucleon-nucleon collisions. The ALICE experiment has measured a strong suppression of hadron production at high-p T in central Pb-Pb collisions at √ NN = 5.02 TeV, indicating that a dense and hot medium is created at the LHC energies.

ALICE detector
The ALICE detector [7] consists of a central barrel with a large solenoid providing a 0.5 T magnetic field for tracking and particle identification, a dimuon spectrometer equipped with a dipole magnet a Corresponding author: mvasili@phys.uoa.gr EPJ Web of Conferences 182, 02126 (2018) https://doi.org/10.1051/epjconf/201818202126 ICNFP 2017 EPJ Web of Conferences with a 0.7 Tm field integral, and other forward detectors for triggering and centrality selection. The ALICE central barrel subdetectors have specific unique capabilities for measuring the production of strange and light-flavour hadrons over a wide range of transverse momentum, from pp and p-Pb interactions up to the highest multiplicity environment of the central Pb-Pb collisions. Tracking and vertexing are performed using the Inner Tracking System (ITS), consisting of six layers of silicon detectors, and the Time Projection Chamber (TPC). The two innermost layers of the ITS and the VZERO detector (scintillation hodoscopes placed on either side of the interaction region) are used for triggering. The VZERO also provides the centrality (multiplicity) class definition in Pb-Pb (pp, p-Pb) collisions, while the ITS and the TPC provide particle identification in the low and intermediate p T region, respectively. The Time Of Flight detector (TOF) and the ring imaging Cherenkov detector (HMPID) are used for particle identification up to 6 GeV/c. Furthermore, the relativistic rise of the energy loss in the TPC gas can be used to extend the PID up to 12 GeV/c in Pb-Pb and up to 20 GeV/c in p-Pb collisions.

Identified particle spectra and ratios
The transverse momentum spectra of primary charged particles are measured in the full azimuthal acceptance and within |η| < 0.8. The p T spectra of identified pions, kaons and (anti)protons in Pb-Pb collisions at √ NN = 5.02 TeV are shown for different centrality classes in Fig. 1. A hardening of the spectra is observed going from peripheral to central events. The effect is mass dependent, being more pronounced for heavier particles, and is characteristic of a radial flow. Similar hints of collectivity have been also observed in p-Pb collisions at √ NN = 5.02 TeV [8]. The p T -differential proton-to-pion ratio is shown in Fig. 2 for different multiplicity/centrality classes in pp collisions at √ = 7 TeV and p-Pb, Pb-Pb collisions at √ NN = 5.02 TeV. Across the three systems the ratios evolve with multiplicity in a quantitavely similar way. A depletion of the ratios at low p T and an enhancement at intermediate p T is observed. The proton-to-pion ratio as a function of p T for different centrality classes in Pb-Pb collisions at √ NN = 5.02 TeV has been also compared with measurements at √ NN = 2.76 TeV [9]. In order to quantify the freeze-out parameters a combined fit of the spectra with a Boltzmann-Gibbs blast-wave model [10] was performed, in the ranges 0.5-1 GeV/c, 0.2-1.5 GeV/c, 0.3-3 GeV/c for pions, kaons and protons, respectively. Figure 3 presents the results of blast-wave fits for pp collisions at √ = 7 TeV, p-Pb collisions at √ NN = 5.02 TeV and Pb-Pb collisions at √ NN = 2.76 and 5.02 TeV. In Pb-Pb collisions at 5.02 TeV the blast-wave parameters follow trends observed at lower energy, i.e. the value of the average transverse radial flow velocity increases with centrality, while the kinetic freeze-out temperature decreases.

Nuclear modification factors
The nuclear modification factors in Pb-Pb (R AA ) and p-Pb (R pPb ) collisions are given by: where N AA (N pPb ) and N pp represent the charged particle yield in Pb-Pb (p-Pb) and pp collisions, respectively, and <N coll > is the average number of binary nucleon-nucleon collisions. The nuclear modification factor in Pb-Pb collisions at √ NN = 5.02 TeV as a function of p T is shown in Fig.5 for six centralities and compared with measurements at √ NN = 2.76 TeV [13]. We observe a similar behavior of the R AA within systematic uncertainties. In central collisions (0-5%) a strong suppression is observed, with a minimum at p T~ 7 GeV/c and a significant rise for p T > 7 GeV/c. For peripheral collisions (60-80%) R AA exhibits a moderate suppression (~0.6-0.7). No significant evolution of the R AA with the collision energy is found. The nuclear modification factor in 0-5% central Pb-Pb collisions is compared in Fig 6 (left) to the predictions from Djordjevic et al. [14] and Majumber et al., [15]. Figure 6 (right) shows R AA for 0-10% central Pb-Pb collisions and a prediction from Vitev et al. [16]. The measurement is in good agreement with all three model predictions within their uncertainties. Figure 7 presents the identified hadron R pPb compared to that for inclusive charged particles in non-single diffractive (NSD) p-Pb events at √ NN = 5.02 TeV [8]. The nuclear modification factors at high-p T are consistent with unity within uncertainties, suggesting that final state effects do not play a role. In the transverse momentum range from 3 to 6 GeV/c, the R pPb for (anti)protons is above unity.This mass dependence of the nuclear modification factor is consistent with radial flow. A similar pattern has been observed at RHIC [17]. In Fig. 8 the nuclear modification factors in Pb-Pb collisions at √ NN = 5.02 TeV are shown for pions, kaons and (anti)protons as a function of p T for different centrality classes. Results indicate that all particle species are equally suppressed for all centralities at p T above 8 GeV/c. So, a light-flavour independent energy loss at highp T is measured, as observed also at 2.76 TeV [18].

Conclusions
Measurements of charged particle transverse momentum spectra, yield ratios and nuclear modification factors in p-Pb and Pb-Pb collisions at √ NN = 5.02 TeV by the ALICE collaboration are presented. A slightly higher expansion velocity is measured in central Pb-Pb collisions at 5.02 TeV than at 2.76 TeV. Thus, a stronger radial flow is observed at the higher energy. In Pb-Pb collisions proton-to-pion and kaon-to-pion relative abundances exhibit no significant energy dependence. A strong suppression of high-p T particles is measured in central Pb-Pb collisions at √ NN = 5.02 TeV. No suppression is observed for high-p T particles in minimum bias p-Pb collisions. This indicates that the measured suppression of high-p T hadrons in central Pb-Pb collisions is due to parton energy loss in the hot and dense QCD medium. The nuclear modification factor R AA shows no significant evolution with the collision energy.