Shear stress tensor and specific shear viscosity of hot hadron gas in nuclear collisions

Abstract

We extract the shear stress tensor of nuclear matter in the hadronic phase in high energy nuclear collisions. We use a blast-wave parametrization of the final state of nuclear collisions, including nonequilibrium deformations of particle distributions due to shear stress in the Navier-Stokes approximation. We fit spectra and elliptic flow of identified hadrons for a variety of collision energies and impact parameters at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). The systems analyzed cover a temperature range from about 110 to 150 MeV and vary in their chemical potentials for stable hadrons. We then attempt to estimate the specific shear viscosity $\ensuremath{\eta}/s$ of nuclear matter for various temperatures and chemical potentials in the hadronic phase. We assign systematic uncertainties to our results. Using a recent systematic study of fluid dynamics pseudodata with the same blast-wave model [Z. Yang and R. J. Fries, arXiv:2007.11777], we apply a correction to the raw fit results which removes some biases typical for blast-wave fits. This work is complementary to efforts using viscous fluid dynamics to extract the specific shear viscosity of quark gluon plasma at higher temperatures. We put our work in context with existing theoretical calculations of the specific shear viscosity.