The $N_f=2$ QCD chiral phase transition with Wilson fermions at zero and imaginary chemical potential

TL;DR

This study investigates the nature of the chiral phase transition in two-flavor QCD with Wilson fermions, finding a large first order region at imaginary chemical potential and a critical pion mass significantly higher than previous staggered fermion results.

Contribution

It provides the first direct determination of the first order transition region for two-flavor QCD with Wilson fermions at zero and imaginary chemical potential, highlighting a much larger critical pion mass.

Findings

Critical pion mass approximately 560 MeV.

First order transition region is large, no extrapolation needed.

Results align with previous studies using improved Wilson fermions.

Abstract

The order of the thermal phase transition in the chiral limit of Quantum Chromodynamics (QCD) with two dynamical flavors of quarks is a long-standing issue and still not known in the continuum limit. Whether the transition is first or second order has important implications for the QCD phase diagram and the existence of a critical endpoint at finite densities. We follow a recently proposed approach to explicitly determine the region of first order chiral transitions at imaginary chemical potential, where it is large enough to be simulated, and extrapolate it to zero chemical potential with known critical exponents. Using unimproved Wilson fermions on coarse $N_t=4$ lattices, the first order region turns out to be so large that no extrapolation is necessary. The critical pion mass $m_\pi^c\approx 560$ MeV is by nearly a factor 10 larger than the corresponding one using staggered fermions. Our results are in line with investigations of three-flavour QCD using improved Wilson fermions and indicate that the systematic error on the two-flavour chiral transition is still of order 100\%.