Speaker
Description
Tests of CKM unitarity are a rigorous tool for constraining possible extensions of the Standard Model. The top-row CKM unitarity test--at the current precision level, a simpler two-flavor Cabibbo unitarity--relies on a combination of kaon, neutron and superallowed nuclear decays. The latter presently give the most precise value of $V_{ud}$ and lead to an apparent 2-3$\sigma$ unitarity deficit. Recent developments in the theory of superallowed nuclear $\beta$ decays regarded the interplay of precise nuclear radii with $V_{ud}$. This connection involves Coulomb corrections, the isospin-breaking correction $\delta_C$ and nuclear structure corrections: $\delta_{NS}$ to $\beta$ decay rates and nuclear polarization correction to charge radii in muonic atoms.
