Bulk first-order phase transition in three-flavor lattice QCD with $O(a)$-improved Wilson fermion action at zero temperature

TL;DR

This study discovers a bulk first-order phase transition in three-flavor lattice QCD with Wilson fermions at strong coupling, highlighting the importance of improved gauge actions for realistic simulations at certain lattice spacings.

Contribution

The paper identifies a bulk phase transition in three-flavor lattice QCD with Wilson fermions and shows it can be avoided using improved gauge actions, informing future simulation strategies.

Findings

First-order phase transition observed at strong coupling with standard gauge action.

Transition disappears with improved gauge actions, indicating their importance.

Transition occurs at lattice spacings around 0.1 fm, relevant for realistic QCD simulations.

Abstract

Three-flavor QCD simulation with the $O(a)$-improved Wilson fermion action is made employing an exact fermion algorithm developed for odd number of quark flavors. For the plaquette gauge action, an unexpected first-order phase transition is found in the strong coupling regime ($\beta\lesssim$ 5.0) at relatively heavy quark masses ($m_{\mathrm{PS}}/m_{\mathrm{V}}\sim$ 0.74--0.87). Strong metastability persists on a large lattice of size $12^3\times 32$, which indicates that the transition has a bulk nature. The phase gap becomes smaller toward weaker couplings and vanishes at $\beta\simeq 5.0$, which corresponds to a lattice spacing $a\simeq$ 0.1 fm. The phase transition is not found if the improved gauge actions are employed. Our results imply that realistic simulations of QCD with three flavors of dynamical Wilson-type fermions at lattice spacings in the range $a=$ 0.1--0.2 fm require use of improved gauge actions. Possible origins of the phase transition is discussed.