A further extension to the group of Ginsparg-Wilson (overlap) chiral symmetries

TL;DR

This paper explores the ambiguity in lattice chiral symmetry for overlap fermions, proposing a method involving a second fermion field to resolve issues related to eigenvector mappings and symmetries.

Contribution

It introduces a new approach adding a second fermion field with high mass to clarify the lattice chiral symmetry structure via renormalisation group methods.

Findings

Additional Ginsparg-Wilson relations are derived.

More non-commuting lattice chiral symmetries are identified.

Potential resolution of CP symmetry issues on the lattice.

Abstract

As shown by Mandula, the Ginsparg-Wilson lattice realisation of chiral symmetry has a possible ambiguity: there is no unique lattice chiral symmetry, but an infinite group of symmetries with non-commuting generators. The physical implications of this abundance of symmetry remain unclear. In recent work, it has been shown how these chiral symmetries for overlap fermions can be derived from a renormalisation group blocking in the continuum, transforming the action from the standard continuum action to an equivalent to the lattice overlap action. There is no unique blocking, and different blockings lead to different chiral symmetries. The group of symmetries found by Mandula immediately follows. In this way, the excess chiral symmetry on the lattice can be explained in terms of different renormalisation schemes. The previous work suffered from one technical challenge: there is no continuum analogue of the lattice chiral eigenvectors at eigenvalue $2/a$. As the construction of the overlap operator required a mapping between lattice and continuum eigenvalues, the lack of a continuum counterpart to the doublers of the zero modes creates an ambiguity in the construction. Although the lattice chiral symmetry can still be defined, this leads to difficulties when considering $\mathcal{CP}$ symmetry on the lattice. In this work, we investigate the possibility of resolving this ambiguity by adding a second fermion field to the original continuum action used as a basis of the renormalisation group blockings. This second fermion field has a mass of the order of the momentum cut-off, to simulate the effects of the fermion doublers. Working through the same renormalisation group procedure to map this action to the lattice overlap action yields additional Ginsparg-Wilson relations satisfied by the overlap operator, and more (non-commuting) lattice chiral symmetries.