Recent results from the ALICE Experiment at LHC

. ALICE (A Large Ion Collider Experiment) at the LHC performed high statistics measurements in Pb-Pb collisions at the top LHC energy, complemented with large recent reference datasets in elementary proton-proton collisions at the same energy. Elementary pp collisions are serving as baseline for testing QCD properties and allow the study of the changes induced by the hot and dense medium produced in heavy ion collisions. Key observables like nuclear modiﬁcation factors, jet production, ﬂow phenomena and spectra for identiﬁed particles, related to the di ﬀ erent stages of collision evolution, are presented and compared with the most recent results from p-Pb and Xe-Xe collisions, thus allowing to probe both initial cold nuclear matter and ﬁnal state e ﬀ ects, combined with the system size dependence of the measurements.


Introduction
Nuclear collisions at relativistic energies produce a strongly interacting matter under extreme conditions of temperature and energy density. In these collisions, a deconfined state of quarks and gluons (Quark Gluon Plasma -QGP) [1,2] is expected to be formed. ALICE (A Large Ion Collider Experiment) is one of the four large experiments at the LHC (Large Hadron Collider) and is dedicated to the study of heavy-ion collisions. The ALICE detector system and performance are described in detail in [3]. The central barrel tracking detectors (ITS -Inner Tracking System and TPC -Time Projection Chamber) are located inside a soleinoid magnet, which provide an uniform magnetic field of 0.5 T parallel to the beam axis for momentum measurements. Two V0 scintillator counters, covering 2.8 < η < 5.1 and -3.7 < η < -1.7, provide information for trigering, event selection and collision centrality determination. ALICE detector provide excellent particle identification (PID) for hadrons, leptons and photons over a wide momentum range. Hadron PID is made using TPC, ITS, TOF (Time of Flight detector) and HMPID (High Momentum Particle Identification Detector) detectors. Electron identification is achieved in a Transition Radiation Detector (TRD), while the high momentum neutral particles at midrapidity are identified with Electro-Magnetic Calorimeter (EMCal). of charged particles, dN ch /dη , on colliding system, energy and centrality can provide information about relative contributions to particle production from hard scatterings and soft processes and about the energy density achieved in these collisions.
The mid-rapidity charged-particle multiplicity normalised by the number of nucleon pairs participating in the collision as a function of the number of participants is presented in the left part of Figure 1 [4]. The data are from pp, p-Pb, Xe-Xe, Pb-Pb collisions at LHC energies ( √ s NN = 2.76 and 5.02 TeV) and Cu-Cu, Au-Au collisions at RHIC top energy ( √ s NN = 0.2 TeV). The average multiplicity densities scaled by the number of participant pairs show a slight increase with N part . For more central collisions, Xe-Xe results are higher than Pb-Pb results at similar number of participants. The same behavior was observed also at RHIC, where Cu-Cu multiplicities are larger than in Au-Au for most central collisions [5].  [4]. Right: p T -integrated yield ratios to pions as a function of charged particle multiplicity density measured in | y |<0.5 [7].
Strangeness enhancement was originally proposed as a signature of QGP formation [6] and has been observed in heavy-ion collisions at relativistic energies [16]. Figure 1 (right panel) presents the ratios of the yields of K 0 S , Λ, φ, Ξ and Ω to charged pion yield (π + +π − ) as a function of dN ch /dη [7]. Data from pp and p-Pb collisions show an enhancement of strange and multi-strange compared to non-strange particle production, while in AA collisions no significant change with multiplicity was observed. For similar multiplicity, the results do not show any dependence of the system or the collision energy.
The kinetic freeze-out parameters (the kinetic freeze-out temperature, T kin and the transverse collective flow velocity, β T ) are obtained by blast-wave fits to p T spectra. The blastwave model [8] assumes that the system is in local thermal equilibrium and the produced particles are moving with a common collective transverse flow velocity and freeze-out (stop interacting) at a temperature T kin . The 1-σ contours in the T kin -β T plane, as well as the blastwave fit parameters, T kin and β T , are presented in Figure 2 (left panel). In AA collisions, β T increases from peripheral to central collisions, indicating a stronger expansion in more central collisions. In pp and p-Pb we observe similar evolution towards high multiplicity. The freeze-out temperature decreases in more central collisions suggesting that the fireball expands and decouples later in time. Another observable sensitive to the collective expansion is the average transverse momentum p T of the emitted particles. It is found that p T for all studied particle species increases from peripheral to most central collisions. For both Pb-Pb and Xe-Xe collisions, an increase in p T with the particle mass is observed and this dependence reflects the radial collective expansion of the produced systems ( Figure 2). The difference in p T values between p and π/K is larger for higher multiplicities suggesting that the average radial collective flow is stronger in more central collisions. For the φ meson that has similar mass as the proton, the behavior is the same in central collisions.

Anisotropic flow
In a non-central nuclear collision, the overlap region between the two colliding nuclei is not spherically symmetric. The almond shaped interaction volume produce an asymmetric pressure gradients, which converts the initial-state geometric anisotropy to a final state momentum-space anisotropy. Anisotropic flow is characterized by the coefficients, v n , in the Fourier expansion of the particle azimuthal distribution. The second harmonic coefficient, v 2 , is called elliptic flow.
The v 2 (p T ) of π ± , K ± , p +p, φ-meson, K 0 S and Λ +Λ produced in Pb-Pb collisions at √ s NN =5.02 TeV for various centrality classes are shown in Figure 3. All the v 2 values increase from central to peripheral collisions up to the 40-50% centrality interval for all particle species. In the low-p T region (p T <3 GeV/c), the elliptic flow coefficient of identified particles exhibits mass-ordering behavior due to the interplay between radial and anisotropic flow (at the same p T , heavier species have a lower v 2 than lighter ones). In the intermediate p T range (3< p T <8-10 GeV/c), particles group according to their type, mesons or baryons, indicating quark coalescence as a possible hadron production mechanism in this region. At high p T (p T >10 GeV/c), the magnitude of v 2 is similar for pions and protons within uncertainties.
To test particle type scaling and mass ordering, the φ-meson has an important role as its mass is close to the proton mass. In a collective hydrodynamic expansion of the system, the p T spectra of particles are determined by thermal freeze-out temperature, flow velocity and particle mass, therefore we expect similar spectra for proton and φ-meson due to similar masses. At low p T , the v 2 of the φ-meson follows the p +p v 2 , while at intermediate p T follows the π ± v 2 .

High-pT suppresion
Energetic high-p T partons originating from the hard scatterings of the incoming partons propagate through the hot and dense medium produced in the collision and lose energy via collisional and radiative processes. The magnitude of energy loss is sensitive to the medium energy density and the parton path length.
An observable for quantifying medium effects in heavy-ion collsions is the nuclear modification factor, R AA : where d 2 N/dp T dη is the differential yield per event in the A+A collision, T AA = N bin /σ pp inel is the average nuclear overlap function which describes the nuclear geometry, N bin is the average number of binary nucleon-nucleon collisions, σ pp inel is the inelastic nucleon-nucleon inelastic cross-section and d 2 σ pp /dp T dη is the production cross section in pp collisions at the same energy. In the absence of nuclear effects, hard processes are expected to scale with the number of binary collisions and R AA equals unity. If R AA < 1, the AA yields at a given p T is suppressed compared with expected value from the scaled pp yields.
Comparison of nuclear modification factors in Xe-Xe collisions at √ s NN = 5.44 GeV and Pb-Pb collisions at √ s NN = 5.02 GeV is presented in Figure 4. In the left panel, the R AA (p T ) comparison is made at similar charged particle multiplicity density (similar medium density). For most central Xe-Xe collisions (0-5% centrality), the nuclear modification factor is comparable with R AA obtained in 10-20% Pb-Pb collisions. For the semi-central collisions (30-40% Xe-Xe vs. 40-50% Pb-Pb), the results are in agreement within uncertainties. In the right panel, the R AA dependence on average charged-particle multiplicity density is compared for three p T intervals (low, medium and high). Again, a similarity in R AA is seen at the same dN ch /dη for more central collisions ( dN ch /dη > 400). These results could provide new input to constrain path length dependence of the energy loss. Heavy quarks (charm and beauty), because of their large mass, are produced mainly in the early stages of nuclear collisions and therefore experience the entire evolution of the created medium. As all other energetic partons, the c and b quarks interact with the medium constituents via collisional (elastic) and radiative (inelastic) processes. The radiative energy loss is smaller for heavy than for light quarks due to the so-called dead-cone effect [11].
Quarkonia are bound states of heavy quark-antiquark pairs (charmonia, cc and bottomonia, bb). The color potential in quarkonia is screened in the QGP leading to a suppression of their production. The screening depends on the radius of the quarkonium state and therefore on its binding energy, which results in a prediction of sequential suppression with increasing medium temperature [13,14]. At the LHC energies, there is an increased contribution due to recombination processes from the larger charm quark density, leading to a smaller suppression. In the case of open heavy flavour, the presence of the medium creates a competition between fragmentation (one quark producing a spray of hadrons -jet) and recombination (the c quark picks up a light antiquark from the medium).
The p T -differential R AA of D + s and average R AA of prompt D 0 , D + , D * + measured in central 0-10% Pb-Pb collisions at √ s NN = 5.02 TeV [12] is shown in the left panel of Figure 5.
The average non-strange D-meson R AA shows a suppression in most central events up to a factor about 5 for p T > 5 GeV/c. The prompt D + s R AA is larger, although compatible within uncertainties, than non-strange D-meson R AA in the full measured p T range. These results provide a hint of charm hadronisation via coalescence (the recombination of the c quark in the QGP where s quarks are abundant).
In the right panel of Figure 5, J/Ψ nuclear modification factor in Au-Au and Pb-Pb collisions as a function of collision centrality is presented . At RHIC top energy, the J/Ψ production is strongly suppressed and the suppression increases with the number of participants. At LHC energies, the data show a smaller suppression for central collisions compared to RHIC results and a weaker centrality dependence. This reduced suppression at LHC energies is an evidence for increasing recombination in Pb-Pb collisions. Furthermore, the R AA has constant value over a broad range in centrality and seems both recombination and suppression are decreasing similarly with decreasing centrality.

Jets
High transverse momentum partonic interactions in early stages of nuclear collisions lead to a production of two high-p T back-to-back partons which subsequently evolve as parton showers, hadronize, and are experimentally observed as jets events in the detector. Jets play an important role in studying QGP properties produced in these collisions because during their propagation through the hot and dense medium, the interaction with the colored medium will lead to parton energy loss (jet quenching) [15]. Figure 6 shows the jet nuclear modification factor, defined as the ratio of the measured jet spectra in Pb-Pb collisions at √ s NN =5.02 TeV to the pp reference scaled by the number of binary collisions. The measured jet R AA is significantly smaller than unity, indicating a strong suppression of jets in most central 0-10% Pb-Pb collisions. The suppression is strongest at lower jet transverse momentum, while at high p T the R AA increases slowly. With increasing p T , more partons loose more energy and build-up a constant R AA .

Summary/Conclusions
ALICE provides a plethora of precision measurements for many hadron types allowing better understanding and validation of QCD for LHC collisions. Data showed a strangeness enhancement in smaller systems (pp and pA collisions). The largest radial flow velocities in Pb-Pb collisions, while Xe-Xe results are consistent with Pb-Pb. J/Ψ nuclear modification factor at LHC energies showed a smaller suppression than the RHIC results suggesting the increase of recombination processes due to abundance of c andc quarks at these energies. In most central collisions, the R AA of jets are strongly suppressed due to interaction with the hot and dense medium, therefore these results will place stronger constrains on quark-medium interactions.