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Current data fall short by about a tenth of a percent, roughly 3 standard deviations.","posed_since":"2018","precise":"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 $10^{-3}$ 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 $2\\sigma$.","problem_ref":"33185324bb2c8ecbc05f6ecebae2d8261b242b9d27a25491c22e90093046363c","references":"","settled_by":"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_note":"A 2026 lattice-QCD analysis (arXiv:2604.04715) and related work still find a first-row deficit near $3\\sigma$.","title":"Is first-row CKM unitarity violated (Cabibbo angle 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