Nucleon Isovector Axial Form Factors

Abstract

We present results for the isovector axial vector form factors obtained using thirteen 2+1+1-flavor highly improved staggered quark (HISQ) ensembles generated by the MILC collaboration. The calculation of nucleon two- and three-point correlation functions has been done using Wilson-clover fermions. In the analysis of these data, we quantify the sensitivity of the results to strategies used for removing excited state contamination and invoke the partially conserved axial current relation between the form factors to choose between them. Our data driven analysis includes removing contributions from multihadron $N \pi$ states that make significant contributions. Our final results are: $g_A = 1.292 (53)_\text{stat}\,(24)_\text{sys}$ for the axial charge; $g_S = 1.085 (50)_\text{stat}\, (103)_\text{sys}$ and $g_T = 0.991 (21)_\text{stat}\, (10)_\text{sys}$ for the scalar and tensor charges; $\langle r_A^2 \rangle = 0.439 (56)_\text{stat} (34)_\text{sys}$ fm${}^2$ for the mean squared axial charge radius, $g_P^\ast = 9.03(47)_\text{stat}(42)_\text{sys} $ for the induced pseudoscalar charge; and $g_{\pi NN} = 14.14(81)_\text{stat}(85)_\text{sys}$ for the pion-nucleon coupling. We also provide a parameterization of the axial form factor $G_A(Q^2)$ over the range $0 \le Q^2 \le 1$ GeV${}^2$ for use in phenomenology and a comparison with other lattice determinations. We find that the various lattice data agree within 10\% but are significantly different from the extraction of $G_A(Q^2)$ from the $\nu$-deuterium scattering data.