Reduced Hadronic Uncertainty in the Determination of V_(ud)

We analyze the universal radiative correction Δ^V_R to neutron and superallowed nuclear β decay by expressing the hadronic γW-box contribution in terms of a dispersion relation, which we identify as an integral over the first Nachtmann moment of the γW interference structure function F^((0))_3. By connecting the needed input to existing data on neutrino and antineutrino scattering, we obtain an updated value of Δ^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 |V_(ud)| = 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.