Soft probes of the Quark-Gluon Plasma measured by ALICE

Abstract In these proceedings an overview of the main ALICE results on soft probes of the Quark-Gluon Plasma (QGP) in Pb–Pb collisions at s NN = 2.76  TeV is presented. It comprises measurements of light flavour hadron production, azimuthal flow, and system size. All of the results are compared to hydrodynamical calculations to extract global properties of the QGP. In addition, particle production is also compared to results from statistical models. In order to show the evolution of soft probes with system size, some of the measurements from pp and p–Pb collisions are compared to those from Pb–Pb collisions.


Introduction
The main goal of the heavy-ion program at the Large Hadron Collider (LHC) is to study properties of the Quark-Gluon Plasma (QGP), a state of deconfined quarks and gluons. The evolution of the system created during ultrarelativistic heavy-ion collisions can be understood as divided into a few phases. In our current understanding, a preequlibrium phase [1], which eventually reaches an approximate local thermalization, exists. This stage is followed by an expansion in a common velocity field [2,3]. During the expansion, the system behaves very much like a perfect liquid and cools down until it forms hadrons as a result of chemical freezeout at the temperature T ch [4]. The formed hadron gas cools down further until it reaches the kinetic freeze-out temperature (T kin ) after which the collective expansion stops [5]. The initial shape and energy density of the system are given by the overlapping region of the two Lorentz contracted colliding nuclei. This results in an initial spatial anisotropy which is transformed during the evolu-Email address: Marek.Chojnacki@cern.ch (Marek Chojnacki for the ALICE Collaboration) tion into an anisotropy in the momentum distribution of the produced particles. This effect is called anisotropic flow and it can be characterized by the coefficients of the Fourier expansion of the dN/dφ distribution of produced particles. The Fourier coefficients v n are sensitive to the properties of the system created in the collisions such as the ratio of shear viscosity to the entropy (η/s). The particle type dependence of anisotropic flow can also probe the system freeze-out conditions and the hadronization mechanism. In summary, particle species dependent studies of soft particle production, transverse momentum (p T ) and correlations between the particles provide methods to characterize the dynamical evolution of the system created in ultrarelativistic heavy-ion collisions. In the highest multiplicity class of p-Pb collisions at the LHC, the pseudorapidity density of produced particles is comparable to peripheral heavy-ion collisions. This opens up the possibility to have collective phenomena developing in p-Pb collisions, which could have similar signatures as in heavy-ion collisions. This hypothesis was also checked using LHC data.

ALICE detector
The combination of multiple particle identification systems along with precise tracking (p T resolution lower than 2% for p T below 5 GeV/c) makes ALICE (A Large Ion Collider Experiment) an effective tool for studying the soft probes of the QGP. The results presented here are obtained using data from the pp collision from 2010 and Pb-Pb collisions from 2010 and 2011, and p-Pb collisions from early 2013. During those periods the Inner Tracking System (ITS) and the Time Projection Chamber (TPC) were the main detectors used for reconstructing of charged particle tracks and measuring their momenta in the pseudorapidity range |η| < 0.9. The ITS was also used for triggering and reconstructing the interaction point. Particle identification was performed using the ITS, TPC and the Time-of-Flight (TOF) detectors. The V0 detector, composed of two scintillator arrays measuring particles in range −3.7 < η < −1.7 (V0-C) and 2.8 < η < 5.1 (V0-A), was used for the centrality determination and triggering. The full performance of ALICE during the first LHC run is reported in [6].

p T -spectra of identified hadrons
The collective behaviour of the system created during ultrarelativistic, heavy-ion collision can be described by hydrodynamical models. The identified hadron production at low p T is a major constraining factor for those models. Figure 1 presents the p T spectra for pions, kaons and protons measured by ALICE for the 5% most central Pb-Pb collisions at √ s NN = 2.76 TeV [7,8].
For comparison RHIC results from Au-Au collisions at √ s NN = 200 GeV [9,10] and calculations from hydrodynamical models are also presented. Since the spectra for positive and negative particles were found to be compatible within uncertainties, the combined measurement is presented. The spectra exhibit higher mean p T than at RHIC, which can be attributed to stronger radial flow at LHC energies. From the comparison with hydrodynamical models, we see that VISH2+1 [11] describes well the pion and kaon spectra when p T < 1.5 GeV/c, but it does not describe the proton spectra. The VISHNU model [12] that couples the hydrodynamical evolution of the system to a hadronic cascade model seems to better describe the spectra. The hadronic phase is also implemented in the HKM Model [13] which agrees better with the data than VISH2+1. The Kraków model [14], which also shows good agreement with data, uses nonequilibrium corrections due to the bulk viscosity which changes the effective T ch . The EPOS [15] model uses an approach including the breakup of flux tubes created during initial hard scatterings to describe the spectral shapes over a wide p T range. Figure 2 shows the p T spectra of pions, kaons, protons, K 0 S and Λ measured by ALICE [16] in high multiplicity p-Pb collisions. The results are compared to hydrodynamical models (Kraków [17], EPOS LHC [18], blastwave [19]) and to the QCD-inspired DPMJET event generator [20]. For p T < 2 GeV/c the hydrodynamical models describe the ALICE measurements reasonably well. In the same p T range, DPMJET fails to describe the spectra but it correctly describes the pseudorapidity distribution of charged particles, as shown in [21]. The fact that p-Pb spectra from high multiplicity collisions can be described by hydrodynamical models might indicate the existence of collective phenomena in the system created during p-Pb collisions at the LHC. The particle spectra for both p-Pb and Pb-Pb were fitted with the blast-wave function [19], which assumes a thermalized medium and collective expansion with a common velocity field ended with a one common freeze-out. The results of the fits (i.e. the radial flow velocity β T and the temperature of kinetic freeze-out T kin ) are summarized in Figure 3. The results for p-Pb collisions follow the same trend as those for Pb-Pb collisions: T kin decreases with increasing β T and multiplicity.

Total particle production
To estimate the chemical freeze-out temperature (T ch ), the production yields for different particle species at mid-rapidity measured by ALICE were fitted (see Figure 4) with three thermal models (GSI [22], THER-MUS [23] and SHARE [24]). During the fitting procedure the baryochemical potential (μ B ) was set to 1 MeV since the measured yields of antiparticles and particles are equal within uncertainties. All three models gave a temperature, T ch , around 155 ∼ 156 MeV. The thermal models reproduce many of the ALICE particle yield results, but fail to reproduce the proton and antiproton production yields. Possible reasons for this are: baryon annihilation after chemical freeze-out [25], a non-equilibrium statistical hadronization [24] (implemented in SHARE but not used in the fit), flavour hierarchy in the QCD phase transition from QGP to hadrons [26], missing higher mass resonance states in the equilibrium thermal model, and the existence of a pion condensate [27]. The inability to reproduce the proton and antiproton production by statistical models is one of the most puzzling observations of the first LHC heavy-ion data. The interpretation of this effect is still not clear and needs further investigation. Figure 5 presents the second Fourier coefficient v 2 as a function of p T for different particles species measured by ALICE [28] in the 10-20% (upper plot) and 40-50% (lower plot) centrality intervals in Pb-Pb collisions For p T below 2 GeV/c, a mass ordering (where higher mass corresponds to lower v 2 for a fixed p T ) is seen. This is an effect of the interplay between elliptic and radial flow. For p T larger than 3 GeV/c, the v 2 values tend to divide into two groups, one for mesons and one for baryons. There is one exception, the v 2 of the φmeson, which follows the baryons for central collisions and moves progressively to the meson band for peripheral collisions. The ALICE v 2 measurements were compared to hydrodynamical calculations coupled to a hadronic cascade model [12] (VISHNU). This comparison for the 10-20% centrality bin is presented in Figure 6. The VISHNU model can describe the main characteristics of v 2 for p T lower than 2 GeV/c qualitatively. The mass ordering observed in the data is not reproduced by the model where the Λ and φ v 2 is larger than the proton   and antiproton v 2 . This suggests that the hadronic cascade phase and the hadronic cross-sections in the model require further tuning. The ALICE v 2 measurements were also used to test the number of constituent quarks (NCQ) scaling. The results of this test for two centrality bins are shown in Figure 7, where v 2 /n q as function of p T /n q is plotted. The NCQ scaling should be present for p T /n q higher than 1 GeV/c where coalescence is claimed to be dominant. In this region, the scaling is only approximate since deviations of ±20% are observed in both centrality bins.

Source size
The size of the particle production region was measured by ALICE based on correlation measurements of identical bosons at low relative momentum. Figure 8 presents  shaded regions in Figure 8 represent the effect of the hydrodynamical expansion. The results in Pb-Pb collisions clearly indicate the existence of a hydrodynamical phase. The results for pp and p-Pb collisions are not conclusive and within the uncertainties, can be described by both scenarios: with and without the hydrodynamical phase.

Conclusions
The ALICE collaboration has characterized the QGP in the new energy regime using the data from the first LHC run. The results on soft probes show that the system created during the heavy-ion collisions undergoes a hydrodynamical evolution. The measured radial flow is larger than that for collisions at lower energies. Particle production is described by statistical thermodynamics although some deviations for the proton yield are observed. The possible explanation for that deviation is still unknown and many scenarios are being investigated. Some indications of hydrodynamical expansion were also seen in p-Pb collisions, its origin is still unclear and require further measurements using the second LHC run.