Lattice Constraints on the QCD Chiral Phase Transition at Finite Temperature and Baryon Density

TL;DR

This paper reviews recent lattice QCD studies on the nature of the chiral phase transition at finite temperature and baryon density, focusing on constraints for the critical endpoint and the transition's characteristics.

Contribution

It summarizes progress in understanding the chiral transition in the massless limit and bounds the critical point location using lattice QCD data.

Findings

Transition proceeds by an analytic crossover at zero density

Critical endpoint likely occurs at baryon chemical potential > 3 times temperature

Current data constrains the phase diagram and critical temperature

Abstract

The thermal restoration of chiral symmetry in QCD is known to proceed by an analytic crossover, which is widely expected to turn into a phase transition with a critical endpoint as the baryon density is increased. In the absence of a genuine solution to the sign problem of lattice QCD, simulations at zero and imaginary baryon chemical potential in a parameter space enlarged by a variable number of quark flavours and quark masses constitute a viable way to constrain the location of a possible non-analytic phase transition and its critical endpoint. In this article I review recent progress towards an understanding of the nature of the transition in the massless limit, and its critical temperature at zero density. Combined with increasingly detailed studies of the physical crossover region, current data bound a possible critical point to $\mu_B > 3T$.