Phase structure and critical temperature of two-flavor QCD with a renormalization group improved gauge action and clover improved Wilson quark action

TL;DR

The paper addresses the finite-temperature phase structure and the chiral transition in two-flavor QCD from first-principles lattice calculations. It employs an RG-improved gauge action together with a clover-improved Wilson quark action on lattices with $N_t=4$, mapping the phase diagram in the $(\beta, K)$ plane, and testing the universality of the chiral transition by analyzing $O(4)$ scaling of a subtracted chiral condensate. The results indicate a second-order chiral transition in the continuum limit, with the chiral-transition point lying near the cusp of the parity-broken phase, and yield a transition temperature in the chiral limit of $T_c=171(4)$ MeV. These findings support the universality of the chiral transition in two-flavor QCD and demonstrate that improved lattice actions can reliably extract thermodynamic quantities at coarse lattice spacings, enhancing the connection to continuum QCD thermodynamics.

Abstract

We study the finite-temperature phase structure and the transition temperature of QCD with two flavors of dynamical quarks on a lattice with the temporal size $N_t=4$, using a renormalization group improved gauge action and the Wilson quark action improved by the clover term. The region of a parity-broken phase is identified, and the finite-temperature transition line is located on a two-dimensional parameter space of the coupling ($β=6/g^2$) and hopping parameter $K$. Near the chiral transition point, defined as the crossing point of the critical line of the vanishing pion mass and the line of finite-temperature transition, the system exhibits behavior well described by the scaling exponents of the three-dimensional O(4) spin model. This indicates a second-order chiral transition in the continuum limit. The transition temperature in the chiral limit is estimated to be $T_c = 171(4)$ MeV.

Figures (16)

• Figure 1: Chiral extrapolation of vector meson mass as a function of $(m_{\rm PS}a)^2$ obtained on $12^3\times 24$ lattice.
• Figure 2: Phase diagram for the RG-improved gauge action and clover quark action at $N_t=4$.
• Figure 3: Chiral extrapolation of pseudo scalar meson mass as a function of $K$ on $12^3\times 24$ lattice.
• Figure 4: Polyakov line obtained on $16^3\times 4$ lattice.
• Figure 5: Polyakov line susceptibility obtained on $16^3\times 4$ lattice. Several data are omitted for clarity of the plot.