Radiative corrections to neutron and nuclear beta decays revisited

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 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 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, BR(n -> exotics) < 0.16%.