Lattice simulations with G-parity Boundary Conditions

TL;DR

This paper introduces G-parity boundary conditions in lattice QCD to accurately simulate pions with momentum, aiding precise calculations of kaon decay matrix elements while maintaining isospin symmetry.

Contribution

It develops a formalism and numerical methods for implementing G-parity boundary conditions in lattice QCD simulations, enabling better control of pion momenta in calculations.

Findings

Successfully implemented G-parity boundary conditions on dynamical ensembles.

Demonstrated the method's effectiveness with pion mass of 420 MeV.

Provided a framework for future precision kaon decay studies.

Abstract

We discuss G-parity lattice boundary conditions as a means to impose momentum on the pion ground state without breaking isospin symmetry. This technique is expected to be critical for the precision measurement of $K\rightarrow(\pi\pi)_{I=0}$ matrix elements where physical kinematics demands moving pions in the final state and the statistical noise caused by disconnected contributions will make it difficult to use multi-exponential fits to isolate this as an excited state. We present a formalism for computing hadronic Green's functions with G-parity boundary conditions, derive the discretized action and its symmetries, discuss how the strange quark can be introduced and detail techniques for the numerical implementation of these boundary conditions. We demonstrate and test these methods using several $16^3\times 32$ dynamical domain wall ensembles with a $420$ MeV pion mass and G-parity boundary conditions in one and two spatial directions.