Radiative corrections to neutron and nuclear beta decays revisited

Abstract

The universal radiative corrections common to neutron and superallowed nuclear beta decays (also known as ``inner'' corrections) are revisited in light of a recent dispersion relation study that found $+2.467(22)%$, i.e., about $2.4\ensuremath{\sigma}$ larger than the previous evaluation. For comparison, we consider several alternative computational methods. All employ an updated perturbative QCD four-loop Bjorken sum rule defined QCD coupling supplemented with a nucleon form factor based Born amplitude to estimate axial-vector induced hadronic contributions. In addition, we now include hadronic contributions from low ${Q}^{2}$ loop effects based on duality considerations and vector meson resonance interpolators. Our primary result, 2.426(32)%, corresponds to an average of a light-front holographic QCD approach and a three-resonance interpolator fit. It reduces the dispersion relation discrepancy to approximately $1.1\ensuremath{\sigma}$ and thereby provides a consistency check. Consequences of our new radiative correction estimate, along with that of the dispersion relation result, for Cabibbo-Kobayashi-Maskawa unitarity are discussed. The neutron lifetime-${g}_{A}$ connection is updated and shown to suggest a shorter neutron lifetime less than 879 s. We also find an improved bound on exotic, non--Standard Model, neutron decays or oscillations of the type conjectured as solutions to the neutron lifetime problem, $\mathrm{BR}(n\ensuremath{\rightarrow}\text{exotics})<0.16%$.