Pion-Induced Radiative Corrections to Neutron β Decay

We compute the electromagnetic corrections to neutron beta decay using a low-energy hadronic effective field theory. We identify new radiative corrections arising from virtual pions that were missed in previous studies. The largest correction is a percent-level shift in the axial charge of the nucleon proportional to the electromagnetic part of the pion-mass splitting. Smaller corrections, comparable to anticipated experimental precision, impact the beta-u angular correlations and the beta asymmetry. We comment on implications of our results for the comparison of the experimentally measured nucleon axial charge with first-principles computations using lattice QCD and on the potential of beta decay experiments to constrain beyond-the-standard-model interactions.