Is first-row CKM unitarity violated (Cabibbo angle anomaly)?
In plain words
The probabilities for an up quark to turn into each kind of down-type quark should add to exactly one. Current data fall short by about a tenth of a percent, roughly 3 standard deviations.
Precise statement
Test $\left|V_{ud}\right|^2 + \left|V_{us}\right|^2 + \left|V_{ub}\right|^2 = 1$, with $V_{ud}$ from superallowed $0+$ to $0+$ nuclear beta decays and neutron decay, and $V_{us}$ from K_l3, K_mu2/pi_mu2 and $\tau$ decays. The current deficit is of order 1e-3 at about $3\sigma$, together with an internal K_l3 versus K_mu2 tension; an answer is confirmation above $5\sigma$ or resolution to below $2\sigma$.
What would settle it
Improved electroweak radiative corrections and nuclear-structure corrections for superallowed decays, new lattice QCD form factors for K_l3, and new K_l3/K_mu2 ratio measurements (NA62).
Status in the literature
Unverified note
A 2026 lattice-QCD analysis (arXiv:2604.04715) and related work still find a first-row deficit near $3\sigma$.