A new |Vus| determination from kaon and tau decays

Abstract

The determination of the CKM matrix element |Vus| provides a critical test of Standard Model’s first-row unitarity. In this work, |Vus| is extracted from leptonic and semileptonic kaon decays (Kℓ2 and Kℓ3), as well as exclusive τ → Kντ and semileptonic τ → Kπντ decays. For the semileptonic channels, phase-space integrals are evaluated using standard form-factor parameterizations. For the decay τ− → K0S π−ντ a fit to the spectrum from Belle is performed by incorporating energy-dependent Breit-Wigner resonance descriptions for the participating K∗(892) and K∗(1410) resonances. The individual extraction methods yield compatible values for |Vus| within uncertainties. Combining all determinations gives a global weighted average of |Vus| = 0.22434(29), consistent with current world averages. The corresponding unitarity test reveals a persistent deviation from unity at the 2.5σ level, indicating a lingering tension with Standard Model predictions