Improved calculation of electroweak radiative corrections and the value of Vud.

A new method for computing hadronic effects on electroweak radiative corrections to low-energy weak interaction semileptonic processes is described. It employs high order perturbative QCD results originally derived for the Bjorken sum rule along with a large QCD-motivated interpolating function that matches long- and short-distance loop contributions. Applying this approach to the extraction of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element Vud from superallowed nuclear beta decays reduces the theoretical loop uncertainty by about a factor of 2 and gives Vud=0.97377(11)(15)(19). Implications for CKM unitarity are briefly discussed.