Precision study of excited state effects in nucleon matrix elements

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, gA, for which we use about 7500 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 Nf=2+1+1 maximally-twisted-mass Wilson fermions and using non-perturbatively calculated renormalization factors. Excited state effects are shown to be negligible for gA, whereas they lead to an O(10%) downward shift for 〈x〉u-d. © 2011 Elsevier B.V.