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

Research paper by ** ALICE Collaboration**

Indexed on: **14 Jul '15**Published on: **14 Jul '15**Published in: **Nuclear Experiment**

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#### Abstract

The elliptic flow coefficient ($v_{2}$) of identified particles in Pb-Pb
collisions at $\sqrt{s_\mathrm{{NN}}} = 2.76$ TeV was measured with the ALICE
detector at the LHC. The results were obtained with the Scalar Product method,
a two-particle correlation technique, using a pseudo-rapidity gap of
$|\Delta\eta| > 0.9$ between the identified hadron under study and the
reference particles. The $v_2$ is reported for $\pi^{\pm}$, $\mathrm{K}^{\pm}$,
$\mathrm{K}^0_\mathrm{S}$, p+$\overline{\mathrm{p}}$, $\mathrm{\phi}$,
$\Lambda$+$\overline{\mathrm{\Lambda}}$, $\Xi^-$+$\overline{\Xi}^+$ and
$\Omega^-$+$\overline{\Omega}^+$ in several collision centralities. In the low
transverse momentum ($p_{\mathrm{T}}$) region, $p_{\mathrm{T}} < 2 $GeV/$c$,
$v_2(p_\mathrm{T})$ 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 $\pi^{\pm}$ and $\mathrm{K}$
are described fairly well by hydrodynamical calculations coupled to a hadronic
cascade model (VISHNU) for central collisions. However, the same calculations
fail to reproduce the $v_2(p_\mathrm{T})$ for p+$\overline{\mathrm{p}}$,
$\mathrm{\phi}$, $\Lambda$+$\overline{\mathrm{\Lambda}}$ and
$\Xi^-$+$\overline{\Xi}^+$. For transverse momentum values larger than about 3
GeV/$c$, particles tend to group according to their type, i.e. mesons and
baryons. However, the experimental data at the LHC exhibit deviations from the
number of constituent quark (NCQ) scaling at the level of $\pm$20$\%$ for
$p_{\mathrm{T}} > 3 $GeV/$c$.