Variational approach to the calculation of gA

A long standing problem in lattice QCD has been the discrepancy between the experimental and calculated values for the axial charge of the nucleon, g(A) = G(A) (Q(2) = 0). Though finite volume effects have been shown to be large, it has also been suggested that excited state effects may also play a significant role in suppressing the value of g(A). In this work, we apply a variational method to generate operators that couple predominantly to the ground state, thus systematically removing excited state contamination from the extraction of g(A). The utility and success of this approach is manifest in the early onset of ground state saturation and the early onset of a clear plateau in the correlation function ratio proportional to g(A). Through a comparison with results obtained via traditional methods, we show how excited state effects can suppress g(A) by as much as 8% if sources are not properly tuned or source-sink separations are insufficiently large. Crown Copyright (c) 2013 Published by Elsevier B.V. All rights reserved.