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

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.