Nπ -state contamination in lattice calculations of the nucleon pseudoscalar form factor

Abstract

The nucleon-pion-state contribution in the QCD three-point function of the pseudoscalar density is calculated to leading order in chiral perturbation theory. It predicts a nucleon-pion-state contamination in lattice estimates for the pseudoscalar form factor GP(Q2) determined with the plateau method. Depending on the momentum transfer Q2 the contamination varies between -20% and +50% for a source-sink separation of 2 fm. The nucleon-pion-state contamination also causes violations in the generalized Goldberger-Treiman relation among the pseudoscalar and the axial nucleon form factors, the dominant source being the nucleon-pion-state contamination in the induced pseudoscalar form factor GP(Q2). Comparing the chiral perturbation theory predictions with lattice results of the PACS Collaboration we find reasonable agreement even for source-sink separations as small as 1.3 fm.