Nuclear-structure corrections in superallowed beta decays
In plain words
Precise decay rates of certain nuclei (superallowed 0+ to 0+ decays) give the strength of the weak coupling between up and down quarks, but only after small nuclear corrections are subtracted. A small deficit that may signal new physics depends on these corrections.
Precise statement
Compute the isospin-symmetry-breaking correction $\delta_{C}$ and the nuclear-structure-dependent radiative correction $\delta_{\mathrm{NS}}$ for the best-measured superallowed 0+ -> 0+ transitions (10C to 74Rb) with absolute uncertainty below $1e-4$, using ab initio methods with electroweak currents. Answer: corrected Ft values and $V_{ud}$, deciding whether the first-row CKM sum $\mid V_{ud}\mid^{2}+\mid V_{us}\mid^{2}+\mid V_{ub}\mid^{2}$ deviates from 1 (reported deficit of about 2 to 3 standard deviations).
What would settle it
Ab initio calculations of $\delta_C$ and $\delta_{\mathrm{NS}}$ for light emitters (10C, 14O) checked against measured isospin-breaking observables, then extended to heavier emitters.
Status in the literature
Unverified note
$V_{ud} = 0.97373(31)$ from the 2020 Hardy-Towner survey is limited mainly by these nuclear corrections, whose model dependence was under ab initio study as of 2026.