Precision Study of Excited State Effects in Nucleon Matrix Elements

TL;DR

This study investigates how excited states affect the calculation of nucleon matrix elements, finding negligible effects on g_A but a significant downward shift on <x>_{u-d} in lattice QCD simulations.

Contribution

It provides a detailed analysis of excited state effects on nucleon matrix elements using high-statistics lattice QCD data at fixed parameters.

Findings

Excited states have negligible impact on g_A.

Excited states cause about a 10% downward shift in <x>_{u-d}.

Analysis performed with large data sets and non-perturbative renormalization.

Abstract

We present a dedicated analysis of the influence of excited states on the calculation of nucleon matrix elements. This calculation is performed at a fixed value of the lattice spacing, volume and pion mass that are typical of contemporary lattice computations. We focus on the nucleon axial charge, g_A, for which we use about 7,500 measurements, and on the average momentum of the unpolarized isovector parton distribution, <x>_{u-d}, for which we use about 23,000 measurements. All computations are done employing N_f=2+1+1 maximally-twisted-mass Wilson fermions and using non-perturbatively calculated renormalization factors. Excited state effects are shown to be negligible for g_A whereas they lead to an O(10%) downward shift for <x>_{u-d}.