Reduced Hadronic Uncertainty in the Determination of Vud

We analyze the universal radiative correction Delta(V)(R) to neutron and superallowed nuclear beta decay by expressing the hadronic gamma W-box contribution in terms of a dispersion relation, which we identify as an integral over the first Nachtmann moment of the gamma W interference structure function F-3((0)). By connecting the needed input to existing data on neutrino and antineutrino scattering, we obtain an updated value of Delta(V)(R) = 0.02467(22), wherein the hadronic uncertainty is reduced. Assuming other standard model theoretical calculations and experimental measurements remain unchanged, we obtain an updated value of vertical bar V-ud vertical bar = 0.97370(14), raising tension with the first row Cabibbo-Kobayashi-Maskawa unitarity constraint. We comment on ways current and future experiments can provide input to our dispersive analysis.