The QCD phase transition with physical-mass, chiral quarks

Abstract

We report on the first lattice calculation of the QCD phase transition using chiral fermions at physical values of the quark masses. This calculation uses 2+1 quark flavors, spatial volumes between (4 fm${)}^3$ and (11 fm${)}^3$ and temperatures between 139 and 196 MeV . Each temperature was calculated using a single lattice spacing corresponding to a temporal Euclidean extent of $N_t=8$. The disconnected chiral susceptibility, ${\chi }_{\mathrm{disc}}$ shows a pronounced peak whose position and height depend sensitively on the quark mass. We find no metastability in the region of the peak and a peak height which does not change when a 5 fm spatial extent is increased to 10 fm. Each result is strong evidence that the QCD ``phase transition'' is not first order but a continuous cross-over for $m_{\pi }=135$ MeV. The peak location determines a pseudo-critical temperature $T_c=155(1)(8)$ MeV. Chiral $SU(2{)}_L\times SU(2{)}_R$ symmetry is fully restored above 164 MeV, but anomalous $U(1{)}_A$ symmetry breaking is non-zero above $T_c$ and vanishes as $T$ is increased to 196 MeV.

Comments: 6 pages and 4 figures