Radiative corrections toβdecay and the possibility of a fourth generation

Leading-logarithmic radiative corrections to {β} decay are summed via the renormalization group and structure-dependent O({α}) effects are estimated by a form-factor analysis. These refinements reduce the Kobayashi-Maskawa quark-mixing-matrix element {}Vud{} by 0.13% to 0.9729±{}0.0012. Combined with Kₑ₃-, hyperon-, and b-decay constraints, this implies {}Vud{{}}²+{V}ᵤₛ²+{}Vub{} 2 =0.9954±{}0.0025. Although consistent with unitarity at the 2{σ} level, our result leaves open the possibility of a fourth fermion generation which at 90% confidence level may have mixing {}Vub'{} as large as 0.088.