Finite nuclear size effects and the precise extraction of Vud

The precise extraction of Vud from superallowed nuclear beta decays provides one of the best avenues for testing the Standard Model at low energies. To probe new physics at the multi-TeV scale, all Standard Model effects at the sub-permille level must be properly taken into account. In this talk, I will focus on effects related to the finite nuclear size, which impact the precise determination of the nuclear weak decay form factor and the isospin-symmetry-breaking correction to the Fermi matrix element. Using the 26mAl→26Mg decay as an example, I will show that a proper treatment of nuclear size effects leads to a reduced tension in first-row Cabibbo–Kobayashi–Maskawa (CKM) matrix unitarity.