Directed flow at midrapidity in sqrt(s_NN)=2.76 TeV Pb+Pb collisions

TL;DR

This paper presents the first extraction of the rapidity-even directed flow v1 in Pb+Pb collisions at 2.76 TeV, using hydrodynamic models to connect initial state asymmetries with observed flow patterns.

Contribution

It introduces the first viscous hydrodynamic calculation of directed flow and demonstrates its reduced sensitivity to viscosity, enabling better constraints on initial state asymmetries.

Findings

Restoration of factorization in flow harmonics after accounting for momentum conservation.

Directed flow v1 is less sensitive to viscosity compared to higher harmonics.

Predicted v1 in Au-Au collisions at RHIC energies.

Abstract

We analyze published data from the ALICE Collaboration in order to obtain the first extraction of the recently-proposed rapidity-even directed flow observable v1. An accounting of the correlation due to the conservation of transverse momentum restores the factorization seen by ALICE in all other Fourier harmonics and thus indicates that the remaining correlation gives a reliable measurement of directed flow. We then carry out the first viscous hydrodynamic calculation of directed flow, and show that it is less sensitive to viscosity than higher harmonics. This allows for a direct extraction of the dipole asymmetry of the initial state, providing a strict constraint on the non-equilibrium dynamics of the early-time system. A prediction is then made for v1 in Au-Au collisions at RHIC.