/ nucl-ex CERN-PH-EP-2013-100

Directed flow of charged particles at mid-rapidity relative to the spectator plane in Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76 TeV

Pages: 17

Abstract: The directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76 TeV relative to the collision plane defined by the spectator nucleons. Both, the rapidity odd ($v_1^{odd}$) and even ($v_1^{even}$) directed flow components are reported. The $v_1^{odd}$ component has a negative slope as a function of pseudorapidity similar to that observed at the highest RHIC energy, but with about a three times smaller magnitude. The $v_1^{even}$ component is found to be non-zero and independent of pseudorapidity. Both components show little dependence on the collision centrality and change sign at transverse momenta around 1.2-1.7 GeV/c for midcentral collisions. The shape of $v_1^{even}$ as a function of transverse momentum and a vanishing transverse momentum shift along the spectator deflection for $v_1^{even}$ are consistent with dipole-like initial density fluctuations in the overlap zone of the nuclei.

Note: *Brief entry*; 17 pages, 4 captioned figures, author from page 12 to 17

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