Curvature of the QCD phase transition line in a finite volume

TL;DR

This study investigates how finite-volume effects influence the curvature of the QCD phase transition line at finite temperature and chemical potential, using a phenomenological model and non-perturbative methods to reconcile differing lattice simulation results.

Contribution

It introduces a finite-volume analysis of the QCD phase transition curvature using a functional renormalization group approach, highlighting an intermediate volume region with reduced curvature.

Findings

Finite-volume effects can reduce the curvature of the phase transition line.

The size and position of the volume region depend on the pion mass.

Results may explain discrepancies between lattice simulations and infinite-volume models.

Abstract

The curvature which characterizes the QCD phase transition at finite temperature and small values of the chemical potential is accessible to lattice simulations. The results for this quantity which have been obtained by several different lattice simulation methods differ due to different numbers of flavors, different pion masses and different sizes of the simulation volume. In order to reconcile these results, it is important to investigate finite-volume effects on the curvature. We investigate the curvature of the chiral phase transition line at finite temperature and chemical potential in a finite volume. We use a phenomenological model for chiral symmetry breaking and apply non-perturbative functional renormalization group methods which account for critical long-range fluctuations at the phase transition. We find an intermediate volume region in which the curvature of the phase transition line is actually reduced relative to its infinite-volume value, provided periodic spatial boundary conditions are chosen for the quark fields. Size and location of this region depend on the value of the pion mass. Such an effect could account for differences in the curvature between lattice simulations in differently sized volumes and from functional methods in the infinite volume limit. We discuss implications of our results for the QCD phase diagram.