Improved Calculation of Electroweak Radiative Corrections and the Value   of V_ud
  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 N QCD-motivated interpolating function that matches long and short-distance loop contributions. Applying this approach to the extraction of the CKM matrix element V_ud from superallowed nuclear beta decays reduces the theoretical loop uncertainty by about a factor of 2 and gives V_ud = 0.97377(11)(15)(19). Implications for CKM unitarity are briefly discussed. 
William J. Marciano and Alberto Sirlin