Pion-induced radiative corrections to neutron beta-decay

We compute the electromagnetic corrections to neutron beta decay using a low-energy hadronic effective field theory. We identify and compute 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 $\beta$-$\nu$ angular correlations and the $\beta$-asymmetry. We comment on implications of our results for the comparison of the experimentally measured axial charge with first-principle computations using lattice QCD and on the potential of $\beta$-decay experiments to constrain beyond-the-Standard-Model interactions.