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Precision tests of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix set stringent constraints on the possible Beyond Standard Model (BSM) physics. Among others, the matrix element V_ud that is most precisely extracted from superallowed nuclear beta decays plays a dominant role in the unitarity test; however, a precise determination of its value requires an accurate understanding of all higher-order Standard Model effects that enter the decay process. I will discuss a recent progress in the determination of the radiative correction (RC) in neutron and superallowed beta decay based on dispersion relation, which results in a reduction of the size of V_ud and leads to an apparent tension with the first-row CKM unitarity. I will also discuss possible future steps to resolve such controversy. |
