Nucleon Axial Charge and Pion Decay Constant from Two-Flavor Lattice QCD

The axial charge of the nucleon g(A) and the pion decay constant f(pi) are computed in two-flavor lattice QCD. The simulations are carried out on lattices of various volumes and lattice spacings. Results are reported for pion masses as low as m(pi) = 130 MeV. Both quantities, g(A) and f(pi), suffer from large finite size effects, which to leading order ChEFT and ChPT turn out to be identical. By considering the naturally renormalized ratio g(A)/f(pi), we observe a universal behavior as a function of decreasing quark mass. From extrapolating the ratio to the physical point, we find g(A)(R) = 1.29(5)(3), using the physical value of f(pi) as input and r(0) = 0.50(1) to set the scale. In a subsequent calculation we attempt to extrapolate g(A) and f(pi) separately to the infinite volume. Both volume and quark mass dependencies of g(A) and f(pi) are found to be well described by ChEFT and ChPT. We find at the physical point g(A)(R) = 1.24(4) and f(pi)(R) = 89.6(1.1)(1.8) MeV. Both sets of results are in good agreement with experiment. As a by-product we obtain the low-energy constant (l) over bar (4) = 4.2(1). C) 2014 The Authors. Published by Elsevier B.V.