Excited-state contamination in nucleon correlators from chiral perturbation theory

TL;DR

This paper uses chiral perturbation theory to analyze excited-state contamination in nucleon correlators from lattice QCD, finding small effects in two-point functions but potential overestimation issues in axial charge measurements.

Contribution

It provides a theoretical calculation of excited-state couplings in nucleon correlators, clarifying their impact on lattice QCD results.

Findings

Excited-state contributions are small in two-point functions.

Pion-nucleon excited states may cause overestimation of the nucleon axial charge.

Contamination from these states is unlikely to explain current lattice QCD trends.

Abstract

Techniques to compute hadron properties from lattice QCD rely upon the limit of long time separation. For baryons, the signal-to-noise problem often restricts one to time separations that are not ideally long, and for which couplings to excited states can obstruct the isolation of ground-state baryon properties. We consider excited-state contamination in nucleon two- and three-point functions. Using chiral perturbation theory, we determine couplings to pion-nucleon and pion-delta excited states. In two-point functions, these contributions are small, in accordance with general properties of the spectral weights on a torus. For the axial-current correlation function in the nucleon, the sign of excited-state contributions suggests overestimation of the nucleon axial charge. Thus contamination from pion-nucleon excited states will not likely explain the trend in lattice QCD data.