Equation of state and speed of sound of isospin-asymmetric QCD on the lattice

TL;DR

This study computes the equation of state and speed of sound in isospin-asymmetric QCD on the lattice, revealing that the speed of sound exceeds the ideal gas limit in the pion condensed phase and mapping the phase diagram.

Contribution

First lattice QCD calculation of the equation of state and phase diagram with isospin asymmetry at physical quark masses.

Findings

Speed of sound exceeds ideal gas limit in pion condensed phase

Phase diagram in temperature-isospin density plane computed

All relevant thermodynamic quantities obtained analytically from spline interpolation

Abstract

We determine the QCD equation of state at nonzero temperature in the presence of an isospin asymmetry between the light quark chemical potentials on the lattice. Our simulations employ $N_f=2+1$ flavors of dynamical staggered quarks at physical masses, using three different lattice spacings. The main results are based on a two-dimensional spline interpolation of the isospin density, from which all relevant quantities can be obtained analytically. In particular, we present results for the pressure, the interaction measure, the energy and entropy densities, as well as the speed of sound. Remarkably, the latter is found to exceed its ideal gas limit deep in the pion condensed phase, the first account of the violation of this limit in first principles QCD. Finally, we also compute the phase diagram in the temperature -- isospin density plane for the first time. The data for all observables will be useful for the benchmarking of effective theories and low-energy models of QCD and are provided in ancillary files for simple reuse.