Scrutinizing CKM unitarity with a new measurement of the Kμ3/Kμ2 branching fraction

Precision tests of first-row unitarity of the Cabibbo-Kobayashi-Maskawa matrix currently display two intriguing tensions, both at the 3 sigma level. First, combining determinations of V-ud from superallowed beta decays with V-us from kaon decays suggests a deficit in the unitarity relation. At the same time, a tension of similar significance has emerged between K-l2 and K-l3 decays. In this Letter, we point out that a measurement of the K-mu 3/K-mu 2 branching fraction at the level of 0.2% would have considerable impact on clarifying the experimental situation in the kaon sector, especially in view of tensions in the global fit to kaon data as well as the fact that the K-mu 2 channel is currently dominated by a single experiment. Such a measurement, as possible for example at NA62, would further provide important constraints on physics beyond the Standard Model, most notably on the role of right-handed vector currents. (c) 2023 The Author(s). Published by Elsevier B.V.

Automated summary

Precision tests of the Cabibbo-Kobayashi-Maskawa matrix indicate tensions in the unitarity relation, both in V-ud and V-us determinations, as well as between K-l2 and K-l3 decays. The authors suggest that a measurement of K-mu 3/K-mu 2 branching fraction at 0.2% precision would provide crucial insights into the experimental situation and constraints on physics beyond the Standard Model. Such a measurement, possible at NA62, would also shed light on the role of right-handed vector currents.