Sea quark QED effects and twisted mass fermions

TL;DR

This paper demonstrates how maximally twisted mass fermions can be used to incorporate full unquenched QCD+QED effects on the lattice, enabling precise calculations of isospin breaking corrections with controlled discretization errors.

Contribution

It introduces a method to regularize unquenched QCD+QED with twisted mass fermions and extends the RM123 approach beyond electroquenching for accurate isospin breaking computations.

Findings

Regularization of unquenched QCD+QED on the lattice using twisted mass fermions.

A procedure to determine critical masses for up and down quarks beyond electroquenching.

Extension of the RM123 method to include full QED effects with controlled lattice artifacts.

Abstract

We show that maximally twisted mass fermions can be employed to regularize on the lattice the fully unquenched QCD+QED theory with vanishing $\theta$-term. We discuss how the critical mass of the up and down quarks can be conveniently determined beyond the electroquenched approximation by imposing that certain symmetries of continuum QCD+QED, which are broken by Wilson terms, get restored in the continuum limit. A mixed action setup is outlined that allows to extend beyond the electroquenched approximation the computation (with only O($a^2$) artifacts) of the leading isospin breaking corrections to physical observables using the RM123 method and (pure QCD) ETMC gauge ensembles with $N_f=2+1+1$ dynamical quark flavours.