Krzewicki, M.

Onderwaater, J.

Selyuzhenkov, I.

GSI Repository

Azimuthal correlation measurements with ALICE at the LHC ∗M. Krzewicki1,2, J. Onderwaater1,2, and I. Selyuzhenkov1,2 for the ALICE Collaboration1ExtreMe Matter Institute EMMI and GSI Helmhotzzentrum fu¨r Schwerionenforschung, Darmstadt; 2Frankfurt Institutefor Advanced Studies (FIAS), GermanyAnisotropic flow in heavy-ion collisions is a key ob-servable related to the early time evolution of the nucleus-nucleus interaction. We report on recent results of theALICE Collaboration at the LHC on the directed flow ofcharged particles relative to the spectator plane in Pb-Pbcollisions at √sNN = 2.76 TeV. Also recent findings fromthe search for effects of local parity violation in the stronginteraction are presented.Figure 1: Directed flow versus transverse momentum inPb–Pb collisions at √sNN = 2.76 TeV. (a) v1 for 5-80%centrality range. (b) vodd1 in Pb-Pb collisions compared toAu-Au collisions at RHIC at √sNN = 200 GeV downscaledwith a factor 0.37.Directed flow, v1, is a probe of the spatial orientationand fluctuations of the shape of the fireball created in aheavy-ion collision. The observed negative slope [1] ofthe rapidity-odd v1 component is about three times smallerthan observed at the highest RHIC energy, suggestinga smaller longitudinal tilt of the initial system and dis-favours the picture of strong fireball rotation predicted atthe LHC energies. Measured for the first time with respectto the spectator nucleons, the rapidity-even v1 componentis found to be independent of pseudorapidity and changessign at transverse momenta around 1.2 − 1.7 GeV/c (fig-ure 1). This is expected for dipole-like energy fluctuationswhen momentum of the low pT particles is balanced by thatof the high pT particles. The magnitude of the rapidity-even v1 w.r.t. the participant plane has a similar shape,but is a factor of forty larger, which indicates that fluctuat-ing participant and spectator planes are weakly correlated.Compared to the measurements at the highest RHIC en-ergy, in figure 1(b), vodd1 shows a similar trend includingthe sign change around pT ∼ 1.5 GeV/c in central colli-sions and negative value at all pT for peripheral collisions.∗Supported by GSI, BMBF, HGS-HIRe, and Helmholtz AllianceHA216/EMMIFigure 2: Centrality dependence of the correlationΔ〈cos[ϕα − (m + 1)ϕβ + mΨ2]〉 in comparison withthe Blast Wave model incorporating effects of local chargeconservation. Δ denotes the difference between same andopposite charge correlations.The parity symmetry violation in strong interactions re-mains one of the open fundamental questions about QCD.It is argued that parity symmetry can be locally violatedin a heavy-ion collision which will result in an experimen-tally observable separation of charges along the extrememagnetic field generated by the moving ions, the so calledchiral magnetic effect (CME). An experimental probe ofCME are the azimuthal correlations with respect to the col-lision reaction plane which is perpendicular to that mag-netic field.ALICE observed a clear charge dependence of the two-particle correlation with respect to the reaction plane [2].This can be due to a number of other charge-dependenteffects which preserve parity, such as local charge con-servation (LCC). Higher order charge-dependent correla-tions help to disentangle parity conserving contributionsfrom CME. Recent ALICE results, in figure 2 , showthat models which combine LCC and the radial expansionof the medium, while failing to describe the correlationΔ〈cosn(φα−φβ)〉 in figure 3 [3]), give a good descriptionof the correlations shown in figure 2 up to higher orders.This suggests that a significant fraction of the observedtwo-particle correlation may be due to LCC. A study ofcharge-dependent correlations with identified particles wasalso recently started to further disentangle the CME andLCC contributions.References[1] B. Abelev et al. [ALICE Collaboration], Phys. Rev. Lett. 111(2013) 232302[2] B. Abelev et al. [ALICE Collaboration], Phys.Rev.Lett. 110(2013) 012301[3] Y. Hori [ALICE Collaboration], Nucl. Phys. A904-905 2013(2013) 475cNQM-ALICE-08 GSI SCIENTIFIC REPORT 201372 doi:10.15120/GR-2014-1-NQM-ALICE-08

Azimuthal correlation measurements with ALICE at the LHC

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Azimuthal correlation measurements with ALICE at the LHC

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