The QCD equation of state to $\mathcal{O}(\mu_B^4)$

TL;DR

This paper calculates the QCD equation of state at finite baryon chemical potential using Taylor expansion methods, providing results up to sixth order to model dense matter in heavy ion collisions.

Contribution

It presents the first sixth-order Taylor expansion coefficients for the QCD equation of state at finite chemical potential using HISQ fermions.

Findings

Cutoff effects are under control at two lattice spacings.

Fourth-order equation of state is valid for modeling dense matter at energies down to 20 GeV.

Results enable better modeling of heavy ion collision conditions.

Abstract

We present results from an ongoing calculation of the QCD equation of state at finite baryon chemical potential $\mu_B$. We use the method of Taylor expansions to circumvent the sign problem and calculate the expansion coefficients to sixth order using HISQ fermions. We work at two lattice spacings, namely $N_\tau=6$ and 8 and, though we do not take the continuum limit, demonstrate that cutoff effects remain under control. We also use our results to construct an equation of state along the freeze-out curve. Using our sixth-order results as a cross-check, we demonstrate that our fourth-order equation of state is suitable for the modeling of dense matter created in heavy ion collisions with center-of-mass energies down to $s_{NN}^{1/2}\sim20$ GeV.