Elliptic flow in 200 A GeV Au+Au collisions and 2.76 A TeV Pb+Pb collisions: insights from viscous hydrodynamics + hadron cascade hybrid model

TL;DR

This study uses a hybrid viscous hydrodynamics and hadron cascade model to analyze elliptic flow in heavy-ion collisions at RHIC and LHC energies, revealing slight increases in QGP viscosity and unresolved questions about its temperature dependence.

Contribution

It introduces the VISHNU hybrid model to study elliptic flow and explores the temperature dependence of QGP viscosity, highlighting limitations in constraining (eta/s)_QGP(T).

Findings

QGP shear viscosity eta/s slightly increases from RHIC to LHC energies.

Fitting spectra and v_2 alone cannot uniquely determine (eta/s)_QGP(T).

The nature of QGP viscosity at LHC energies remains an open question.

Abstract

Using the newly developed hybrid model VISHNU which connects viscous hydrodynamics with a hadron cascade model, we study the differential and integrated elliptic flow v_2 at different centrality bins for 200 A GeV Au+Au collisions and 2.76 A TeV Pb+Pb collisions. We find that the average Quark Gluon Plasma (QGP) specific shear viscosity eta/s slightly increases from Relativistic Heavy Ion Collider (RHIC) to Large Hadron Collider (LHC) energies. However, a further study assuming different temperature dependencies for (eta/s)_QGP shows that one cannot uniquely constrain the form of (eta/s)_QGP(T) by fitting the spectra and v_2 alone. Based on our current understanding, the question on whether the QGP fluid is more viscous or more perfect in the temperature regime reached by LHC energies is still open.