Temperature dependence of $\eta/s$: uncertainties from the equation of state

TL;DR

This study compares how different equations of state affect the extraction of the shear viscosity to entropy density ratio in heavy-ion collisions, finding that while the EoS influences the minimum ta/s, it does not strongly constrain its temperature dependence.

Contribution

The paper systematically assesses the impact of various equations of state on ta/s extraction in heavy-ion collision modeling, highlighting uncertainties in temperature dependence.

Findings

EoS choice influences the minimum ta/s value.

Analysis does not strongly constrain the temperature dependence of ta/s.

Different EoSs yield consistent ta/s within confidence limits.

Abstract

We perform a global model-to-data comparison on Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV and Pb+Pb collisions at $2.76$ TeV and $5.02$ TeV, using a 2+1D hydrodynamics model with the EKRT initial state and a shear viscosity over entropy density ratio $(\eta/s)(T)$ with a linear $T$ dependence. To quantify the amount of uncertainty due to the choice of the equation of state (EoS), we compare analysis results based on four different EoSs: the well known $s95p$ parametrisation, an updated parametrisation based on the same list of particles in hadron resonance gas, but using recent lattice results for the partonic part of the EoS, and two new parametrisations based on the Particle Data Group 2016 particle list and the recent lattice results. We find that the choice of the EoS does affect the favoured minimum value of $\eta/s$, although within the confidence limits of the analysis. On the other hand, our analysis hardly constrains the temperature dependence of $\eta/s$, no matter the EoS.