Varieties and properties of central-branch Wilson fermions

TL;DR

This paper explores the properties of central-branch Wilson fermions, introduces new varieties, and analyzes their symmetry, mass renormalization, and potential for lattice QCD simulations, including two-flavor and staggered-Wilson variants.

Contribution

The paper constructs and studies new varieties of central-branch Wilson fermions, revealing their symmetry properties and mass renormalization behavior, and proposes their usefulness for many-flavor QCD simulations.

Findings

Central-branch Wilson fermions possess an extra $U(1)_{ar V}$ symmetry.

The two-flavor version has seven Dirac spectrum branches and retains key symmetries.

The staggered-Wilson fermion variant is free from additive mass renormalization and minimally doubled.

Abstract

We focus on the four-dimensional central-branch Wilson fermion, which makes good use of six species at the central branch of the Wilson Dirac spectrum and possesses the extra $U(1)_{\overline V}$ symmetry. With introducing new insights we discuss the prohibition of additive mass renormalization for all the six species, SSB of $U(1)_{\overline V}$ in strong-coupling QCD, the absence of the sign problem, and the usefulness for many-flavor QCD simulation. We then construct several varieties of the central-branch fermions and study their properties. In particular, we investigate the two-flavor version, where the Dirac spectrum has seven branches and two species live at the central branch. Although the hypercubic symmetry is broken, the other symmetries are the same as those of the original one. We study this setup in terms of lattice perturbation theory, strong-coupling QCD, the absence of sign problem, and the parameter tuning for Lorentz symmetry restoration. By comparing the properties of the original and two-flavor version, we find that the existence of hypercubic symmetry as well as $U(1)_{\overline V}$ is essential for the absence of additive mass renormalization of all the central-branch species. As the other two-flavor version, we investigate the central-branch staggered-Wilson fermion, which is obtained from the eight-flavor central-branch Wilson fermion via spin diagonalization. We argue that it is free from any additive mass renormalization and is regarded as a minimally doubled fermion with less symmetry breaking.