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

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$.

 

JHEP 06 (2015) 190
HEP Data
e-Print: arXiv:1405.4632 | PDF | inSPIRE
CERN-PH-EP-2014-104

Figure 1

The correlation between the number of standard deviations from the expected signal of the TPC $(\sigma_{\mathrm{TPC}})$ and the TOF ($\sigma_{\mathrm{TOF}})$ detectors using the proton mass hypothesis for three different transverse momentum intervals in the 5$\%$ most central Pb-Pb collisions.

Figure 2

Invariant mass distributions in the 10-20$\%$ centrality interval of Pb-Pb collisions for reconstructed decaying particles: (a) $\mathrm{K}^0_\mathrm{S}$, (b) $\Lambda$+$\overline{\mathrm{\Lambda}}$, (c) $\phi$, (d) $\Xi^-$($\overline{\mathrm{\Xi}}^+$), and (e) $\Omega^-$($\overline{\mathrm{\Omega}}^+$).

Figure 3

The measured value of $v_2^{\mathrm{Tot}}$ in the 10-20$\%$ centrality interval of Pb-Pb collisions as a function of the invariant mass for all decaying particles presented in this article.

Figure 4

The $p_{\rm{T}}$-differential ${v}_2$ for different centralities of Pb-Pb collisions at $\sqrt{s_\mathrm{{NN}}} = 2.76$ TeV grouped by particle species.

Figure 5

The $p_{\rm{T}}$-differential ${v}_2$ for different particle species grouped by centrality class of Pb-Pb collisions at $\sqrt{s_\mathrm{{NN}}} = 2.76$ TeV.

Figure 6

The $p_{\rm{T}}$-differential ${v}_2$ for different particle species in (a), (b), (e), (f), measured with the scalar product method with a pseudo-rapidity gap $|\Delta\eta| > 0.9$ in Pb-Pb collisions at $\sqrt{s_\mathrm{{NN}}} = 2.76$ TeV, compared to theoretical, hydrodynamic calculations coupled to a hadronic cascade model . The panels (c), (d), (g) and (h), show the dependence of the ratio of the experimental points to a fit over the theoretical calculations as a function of $p_{\rm{T}}$. The left and right plots present the comparison for the 10-20$\%$ and 40-50$\%$ centrality intervals, respectively. The low transverse momentum points for p+$\overline{\mathrm{p}}$ are out of scale in panels (c) and (d).

Figure 7

The comparison of the $p_{\rm{T}}$-differential ${v}_2$ for $\pi^{\pm}$, $\mathrm{K}$ and p+$\overline{\mathrm{p}}$ for the 10-20$\%$ centrality class of Pb-Pb and Au-Au collisions at the LHC and RHIC, respectively. The RHIC points are extracted from (STAR) and (PHENIX).

Figure 8

The $p_{\rm{T}}/n_q$ dependence of $v_2/n_q$ for $\pi^{\pm}$, $\mathrm{K}$, p+$\overline{\mathrm{p}}$, $\phi$, $\Lambda$+$\overline{\mathrm{\Lambda}}$, and $\mathrm{\Xi^-}$+$\overline{\mathrm{\Xi}}^+$ for Pb-Pb collisions in various centrality intervals at $\sqrt{s_\mathrm{{NN}}} = 2.76$ TeV.

Figure 9

The $p_{\rm{T}}/n_q$ dependence of the double ratio of $v_2/n_q$ for every particle species relative to a fit to $v_2/n_q$ of p and $\overline{\mathrm{p}}$ (see text for details) for Pb-Pb collisions at $\sqrt{s_\mathrm{{NN}}} = 2.76$ TeV.

Figure 10

The $(m_{\rm{T}} - m_0)/n_q$ dependence of $v_2/n_q$ for $\pi^{\pm}$, $\mathrm{K}$, p+$\overline{\mathrm{p}}$, $\phi$, $\Lambda$+$\overline{\mathrm{\Lambda}}$, and $\mathrm{\Xi^-}$+$\overline{\mathrm{\Xi}}^+$ for Pb-Pb collisions in various centrality intervals at $\sqrt{s_\mathrm{{NN}}} = 2.76$ TeV.

Figure 11

The $(m_{\rm{T}} - m_0)/n_q$ dependence of the double ratio of $v_2/n_q$ for every particle species relative to a fit to $v_2/n_q$ of p and $\overline{\mathrm{p}}$ (see text for details) for Pb-Pb collisions at $\sqrt{s_\mathrm{{NN}}} = 2.76$ TeV.

Figure 12

The $p_{\rm{T}}/n_q$ dependence of the double ratio of $v_2/n_q$ for $\pi^{\pm}$, $\mathrm{K}$ relative to a fit to $v_2/n_q$ of p and $\overline{\mathrm{p}}$ (see text for details) in Pb--Pb collisions at $\sqrt{s_\mathrm{{NN}}} = 2.76$ TeV. The LHC points are compared with the results from Au-Au collisions at $\sqrt{s_\mathrm{{NN}}} = 0.2$ TeV from .

Figure 13

The $(m_{\rm{T}} - m_0)/n_q$ dependence of the double ratio of $v_2/n_q$ for $\pi^{\pm}$, $\mathrm{K}$ relative to a fit to $v_2/n_q$ of p and $\overline{\mathrm{p}}$ (see text for details) in Pb-Pb collisions at $\sqrt{s_\mathrm{{NN}}} = 2.76$ TeV. The LHC points are compared with the results from Au-Au collisions at $\sqrt{s_\mathrm{{NN}}} = 0.2$ TeV from .