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

TL;DR

This study measures the directed flow of charged particles in Pb-Pb collisions at 2.76 TeV, revealing smaller longitudinal tilt and initial density fluctuations, providing new insights into the collision dynamics and initial conditions.

Contribution

It presents the first measurement of the rapidity-even directed flow component relative to spectators at the LHC, highlighting initial density fluctuations and weak participant-spectator plane correlations.

Findings

Negative slope of directed flow is smaller than at RHIC energies.

Rapidity-even directed flow is independent of pseudorapidity with a sign change at certain transverse momenta.

Evidence for dipole-like initial density fluctuations in the overlap zone.

Abstract

The directed flow of charged particles at mid-rapidity is measured in Pb-Pb collisions at $\sqrt{s_\rm{NN}}=2.76$ TeV relative to the collision symmetry plane defined by the spectator nucleons. A negative slope of the rapidity-odd directed flow component with approximately 3 times smaller magnitude than found at the highest RHIC energy is observed. This suggests a smaller longitudinal tilt of the initial system and disfavors the strong fireball rotation predicted for the LHC energies. The rapidity-even directed flow component is measured for the first time with spectators and found to be independent of pseudorapidity with a sign change at transverse momenta $p_{\rm T}$ between $1.2$ and $1.7$ GeV/$c$. Combined with the observation of a vanishing rapidity-even $p_{\rm T}$ shift along the spectator deflection this is strong evidence for dipole-like initial density fluctuations in the overlap zone of the nuclei. Similar trends in the rapidity-even directed flow and the estimate from two-particle correlations at mid-rapidity, which is larger by about a factor of 40, indicate a weak correlation between fluctuating participant and spectator symmetry planes. These observations open new possibilities for investigation of the initial conditions in heavy-ion collisions with spectator nucleons.