Speed of sound exceeding the conformal bound in dense 2-color QCD

TL;DR

Recent lattice simulations of dense two-color QCD reveal that the sound velocity can exceed the conformal bound of 1/3, challenging previous assumptions and indicating new physics in dense QCD matter.

Contribution

This paper reviews recent lattice and theoretical studies showing the breaking of the conformal sound velocity bound in dense two-color QCD, a novel finding in the field.

Findings

Sound velocity exceeds the conformal bound in dense QC$_2$D.

Lattice simulations confirm the violation of the bound.

Related studies suggest similar violations in other QCD contexts.

Abstract

We review recent works on the Monte Carlo simulations of dense two-color QCD (QC$_2$D) by focusing on the phase diagram, the equation of state, and the sound velocity at nonzero quark chemical potential. A possible upper bound of the sound velocity is known as the conformal bound, namely, $c_s^2/c^2 \leq 1/3$. The sound velocity is below the bound at least in the case of finite-temperature QCD. However, our recent work~\cite{Iida:2022hyy} shows the breaking of this bound in dense QC$_2$D. This phenomenon was previously unknown from any lattice calculations. We also discuss recent related works including lattice studies on QCD at nonzero isospin chemical potential, some effective model analyses, and an analysis based on recent neutron star observations. These works also suggest the breaking of the conformal bound.