Speed of sound exceeding the conformal bound in dense QCD-like theories

TL;DR

This study uses lattice Monte Carlo simulations to explore dense two-color QCD, revealing a rich phase structure and a superfluid phase where the speed of sound surpasses the conformal bound, indicating novel dense matter properties.

Contribution

The paper demonstrates, through lattice simulations, that the speed of sound in dense QCD-like matter can exceed the conformal limit, a novel finding in the study of dense QCD phases.

Findings

Identified a rich phase structure below the pseudo-critical temperature.

Discovered a superfluid phase with non-zero diquark condensate.

Found that the speed of sound exceeds the conformal bound in high-density regimes.

Abstract

We investigated the phase structure and the equation of state (EoS) for dense two-color QCD at low temperatures using the lattice Monte Carlo simulations. A rich phase structure below the pseudo-critical temperature $T_c$ as a function of quark chemical potential has been revealed. In a high-density regime, we can see a superfluid phase, where the diquark condensate takes a non-zero expectation value. We have newly found that the speed of sound exceeds the conformal bound, which is the value of the relativistic free theory. This talk is based on Refs.~\cite{Iida:2022hyy, Iida:2024irv, Itou:2025vcy}.