Chiral phase transition of $N_f$=2+1 and 3 QCD at vanishing baryon chemical potential

TL;DR

This study investigates the chiral phase transition in (2+1)-flavor and 3-flavor QCD using lattice simulations, identifying the nature of the transition and estimating critical pion masses at vanishing baryon chemical potential.

Contribution

The paper provides updated lattice simulation results on the chiral phase structure of QCD with different flavor configurations and estimates the critical pion mass for the onset of first-order phase transition.

Findings

Chiral condensates exhibit universal scaling behavior.

The tri-critical point is likely below the physical strange quark mass.

The critical pion mass for first-order transition is about 50 MeV.

Abstract

We present updated results on chiral phase structure in (2+1)-flavor ($N_f$=2+1) and 3-flavor ($N_f=3$) QCD based on the simulations using Highly Improved Staggered Quarks on lattices with temporal extent $N_\tau$ =6 at vanishing baryon chemical potential. In $N_f$=2+1 QCD we have performed simulations with a strange quark fixed to its physical value and two degenerate light quarks whose values are adjusted to have 5 values of Goldstone pion masses in the region of 160 - 80 MeV in the continuum limit. The universal scaling behavior of chiral condensates as well as chiral susceptibilities is discussed and the tri-critical point is suggested to be located below the physical point, i.e. at smaller than physical strange quark mass. In $N_f$=3 QCD simulations with 6 different masses of 3 degenerate quarks corresponding to the Goldstone pion masses in the region of 230 - 80 MeV have also been performed. Our results suggest that the QCD transition with these values of quark masses is of crossover type and an upper bound of the critical pion mass where the first order phase transition starts is estimated to be about 50 MeV.