Elliptic flow of identified hadrons in Pb-Pb collisions at 2.76 TeV

The elliptic flow coefficient (v2) of identified particles in Pb-Pb collisions at √sNN = 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 corre- lation technique, using a pseudo-rapidity gap of |∆η| > 0.9 between the identified hadron under study and the reference particles. The v2 is reported for π±, K±, K0S, p+p, φ, Λ+Λ, Ξ−+Ξ+ and Ω−+Ω+ in several collision centralities. In the low transverse momentum (pT) region, pT < 3GeV/c, v2(pT) 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 K0S 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 v2(pT) for p+p, φ, Λ+ Λ and Ξ−+Ξ+. For transverse momentum values larger than about 3GeV/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 pT > 3 GeV/c.