Extraction of the specific shear viscosity of quark-gluon plasma from two-particle transverse momentum correlations

Victor Gonzalez,Jinjin Pan,Ana Marin,Sumit Basu,Claude A Pruneau,Pedro Ladron De Guevara

doi:10.1140/epjc/s10052-021-09260-z

Abstract

The specific shear viscosity, eta /s, of the quark-gluon plasma formed in ultrarelativistic heavy-ion collisions at RHIC and LHC is estimated based on the progressive longitudinal broadening of transverse momentum two-particle correlators, G_2, reported as a function of collision centrality by the STAR and ALICE experiments. Estimates are computed as a function of collision centrality using the Gavin ansatz which relates the G_2 longitudinal broadening to the specific shear viscosity. Freeze out times required for the use of the ansatz are computed using a linear fit of freeze out times reported as a function of the cubic root of the charged particle pseudorapidity density ({mathrm{d}}N_{mathrm{ch}}/deta )^{1/3}. Estimates of eta /s based on ALICE data exhibit little to no dependence on collision centrality at LHC energy, while estimates obtained from STAR data hint that eta /s might be a function of collision centrality at top RHIC energy.

Highlights

We presented an evaluation of the collision centrality dependence of the shear viscosity per unit of entropy, η/s of the Quark Gluon Plasma produced in A–A collisions at Relativistic Heavy-Ion Collider (RHIC) and Large Hadron Collider (LHC) based on measurements of the G2 correlator by the STAR and ALICE collaborations using the Gavin ansatz embodied in Eq 1
Freeze-out times required to carry out the calculations were determined as a function of the cubic root of the charged particle multiplicity from two-pion Bose–Einstein measurements
Values of η/s obtained in Pb–Pb collisions, based on ALICE data, indicate the shear viscosity per unit of entropy is of the order of the KSS bound and essentially independent of collision centrality at LHC energy

Summary

Introduction

J. C (2021) 81:465 techniques that might enable measurements of specific shear viscosity that are less susceptible to uncertainties associated with initial conditions. Proposed by Gavin et al already more than a decade ago [14], it involves measurements of the longitudinal broadening of a transverse momentum two particle correlator, dubbed G2, with increasing collision centrality. [14,15], is designed to be proportional to the covariance of momentum currents and is as such sensitive to dissipative viscous forces at play during the transverse and longitudinal expansion of the matter formed in A–A collisions. Fast fluid cells tend to slow down whereas slow fluid cells accelerate This has the effect of dampening the expansion and produces a progressive broadening of the G2 correlator with time. Gavin et al showed the broadening, characterized in terms of the difference of the variance of the correlator observed in most central and most peripheral collisions, should be proportional to η/s and given by the following formula called the Gavin ansatz σc2 − σ02

Objectives

Discussion

Conclusion