Up, down, and strange nucleon axial form factors from lattice QCD

We report a calculation of the nucleon axial form factors GAq(Q²) and GPq(Q²) for all three light quark flavors q∈u,d,s in the range 0≤Q²1.2 GeV² using lattice QCD. This work was done using a single ensemble with pion mass 317 MeV and made use of the hierarchical probing technique to efficiently evaluate the required disconnected loops. We perform nonperturbative renormalization of the axial current, including a nonperturbative treatment of the mixing between light and strange currents due to the singlet-nonsinglet difference caused by the axial anomaly. The form factor shapes are fit using the model-independent z expansion. From GAq(Q²), we determine the quark contributions to the nucleon spin and axial radii. By extrapolating the isovector GP^u-d(Q²), we obtain the induced pseudoscalar coupling relevant for ordinary muon capture and the pion-nucleon coupling constant. We find that the disconnected contributions to GP form factors are large, and give an interpretation based on the dominant influence of the pseudoscalar poles in these form factors.