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

The elliptic flow coefficient ( v 2 ) of identified particles in Pb-Pb collisions at s N N = 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 |Δ η | > 0 . 9 between the identified hadron under study and the reference particles. The v 2 is reported for π ± , K ± , K S 0 , 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 S 0 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 .