Elliptic flow of identified hadrons in Pb-Pb collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV

Abstract

Abstract:The elliptic flow coefficient (v 2 ) of identified particles in Pb-Pb collisions at √ s NN = 2.76 TeV was measured with the ALICE detector at the Large Hadron Collider (LHC). The results were obtained with the Scalar Product method, a two-particle correlation technique, using a pseudo-rapidity gap of |∆η| > 0.9 between the identified hadron under study and the reference particles. The v 2 is reported for π ± , K ± , K 0 S , p+p, φ, Λ+Λ, Ξ − +Ξ + and Ω − +Ω + in several collision centralities. In the low transverse momentum (p T ) region, p T < 3 GeV/c, v 2 (p T ) exhibits a particle mass dependence consistent with elliptic flow accompanied by the transverse radial expansion of the system with a common velocity field. The experimental data for π ± and the combined K ± and K 0 S results, are described fairly well by hydrodynamic calculations coupled to a hadronic cascade model (VISHNU) for central collisions. However, the same calculations fail to reproduce the v 2 (p T ) for p+p, φ, Λ+ Λ and Ξ − +Ξ + . For transverse momentum values larger than about 3 GeV/c, particles tend to group according to their type, i.e. mesons and baryons. The present measurements exhibit deviations from the number of constituent quark (NCQ) scaling at the level of ±20% for p T > 3 GeV/c. The ALICE collaboration 34 IntroductionLattice quantum chromodynamics calculations predict a transition from ordinary nuclear matter to the Quark-Gluon Plasma (QGP) [1][2][3][4], in which the constituents, the quarks and the gluons, are deconfined. At low values of the baryochemical potential, a crossover transition is expected to take place at a temperature of about 150 MeV and at an energy density of about 0.5 GeV/fm 3 [5,6]. These conditions are accessible in the laboratory by colliding heavy ions at ultra-relativistic energies. The study of the properties of this deconfined matter is the main goal of the heavy-ion collision program at the Large Hadron Collider (LHC). The existence of the QGP has been stipulated by observations at the Relativistic Heavy Ion Collider (RHIC) [7][8][9][10]. The first experimental results from the heavy-ion program at the LHC [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] have also provided evidence of the existence of the QGP in this new energy regime.-1 - JHEP06(2015)190Anisotropic flow, which characterises the momentum anisotropy of the final state particles, can probe the properties, such as the ratio of shear viscosity to entropy density (η/s), of the system created in heavy-ion interactions. In nuclear collisions, the impact parameter vector and the beam axis define the reaction plane. It was recently realized that the overlap region of the colliding nuclei exhibits an irregular shape driven by the initial density profile of nucleons participating in the collision which is different from one event to the other. The symmetry plane of this irregular shape fluctuates around the reaction plane in every event. This spatial anisotropy of the overlap region is transformed into an anisotropy in momen...

Higher harmonic anisotropic flow of identified particles in Pb–Pb collisions with the ALICE detector

“…For higher values of p T ( p T > 3 GeV/c), particles tend to group according to their type, i.e. mesons and baryons, however this grouping holds at best approximately, similarly to what was observed in [4].…”

“…The charged particle momenta were measured by the ITS and the TPC with a full azimuthal coverage in the pseudo-rapidity range |η| < 0.9. Charged pions and kaons as well as (anti-)protons were identified by combining the signals from the TPC and Time Of Flight (TOF) detectors similarly to the procedure followed in [4]. In this analysis, pions and protons are identified with a purity of > 90% in a transverse momentum range of 0.3 < p T < 6 GeV/c and kaons with a purity of > 85% in 0.3 < p T < 4 GeV/c.…”

“…1 presents the same picture that develops for the 20-30% centrality range. Note that the v 2 values for this centrality range were calculated with the scalar product method with |∆η| > 0.9 and without MC subtraction can be found in [4].…”

(Anti-)deuteron production and anisotropic flow measured with ALICE at the LHC

“…The measured (d+d) v 2 was compared with the v 2 of other identified particles [4]. Results for events with centrality in 30-40% are shown in Figure 2.…”

“…n q = 2 × 3. In these proceedings, the measurements of (anti-)deuterons v 2 are compared to other identified particles [4] and to a model based on hydrodynamics [5,6].…”