Nucleon axial and pseudoscalar isovector form factors from lattice QCD

Abstract

The axial structure of the nucleon, in particular the iso-vector axial and (induced) pseudoscalar form factors, can be analyzed from first principles using Monte Carlo techniques in lattice QCD. Effective field theory predicts that pseudoscalar form factors are dominated by a pion pole, which originates from an exchange of a virtual pion. While experiments show agreement with this expectation, previous lattice results have violated it. We show that this discrepency can be traced back to excited state contaminations in the three-point correlation function. To this end we use effective field theory to gain insight into the structure of nucleon pion excited state contributions, which enables us to disentangle the ground state contribution reliably. The extracted form factors then satisfy the pion pole dominance assumption as well as the constraints due to the partial conservation of the axial current up to expected discretization effects. Using a large landscape of ensembles, mostly generated within the CLS-effort, we examine the parametrization dependance and take all relevant limits. We find good agreement both with experiment and with predictions from chiral perturbation theory.