The nuclear-structure dependence of radiative corrections in superallowed Fermi beta-decay

We give a shell-model calculation of the nuclear-structure-dependent axial-vector contribution of the Born graphs to the radiative correction in superallowed Fermi β-decay. The impact of the correction is to give a modest improvement in the test of the conserved-vector-current (CVC) hypothesis, and a slightly smaller value of the quark-mixing matrix element, V_ud. The test of the unitary of the first row of the Kobayashi-Maskawa matrix gives: V_ud² + ν_us² + V_ub² = 0.9962 ± 0.0016, violating the three-generation standard model. Hints from neutron-only data suggest that nuclear-structure corrections in superallowed Fermi β-decay are still not fully under control.