Pion beta decay and Cabibbo-Kobayashi-Maskawa unitarity

Pion beta decay, pi(+)-> pi(0)e(+)nu(gamma), provides a theoretically clean +/- 0.3% determination of the Cabibbo-Kobayashi-Maskawa matrix element V-ud. Although impressive, that result falls short of super-allowed nuclear beta decays where an order of magnitude better precision already exists. Here, we advocate a new strategy for utilizing pion beta decay, based on its utility in determining Vus/Vud via the ratio R-v = Gamma (K ->pi/nu/(gamma))/Gamma(pi(+)-> pi(0)e(+)nu(gamma)) which provides a measure of f(+)(K)(0)vertical bar V-us vertical bar /f(+)(pi)(0)vertical bar V-ud vertical bar independent of the Fermi constant and short-distance radiative corrections. Its dependence on the ratio of two hadronic vector current form factors provides an interesting computational goal for lattice gauge theory studies. Employing a recent lattice based value f(+)(K)(0)/f(+)(pi)(0) = 0.970(2), we find V-us /V-ud = 0.22910(91) compared to V-us/ V-ud = 0.23131(45) obtained from R-A = Gamma(K -> mu nu(gamma))/Gamma(pi -> mu nu(gamma)). Those independent V-us/V-ud determinations exhibit a 2.2 sigma discrepancy. That tension suggests a needed shift in the lattice f(+)(K)(0)/f(+)(pi)(0) value towards the consistency range 0.961(4), experimental/theory input changes or new physics effects. Other features and implications of R-v and R-A are also discussed.