Precision tests with Kl3 and Kl2 decays

TL;DR

Recent experimental and theoretical advancements, especially in lattice QCD, have refined the tests of CKM unitarity using Kl3 and Kl2 decays, confirming the relation within 1% and opening avenues for new physics searches.

Contribution

This paper reviews recent improvements in measurements and lattice QCD calculations that have enhanced the precision of CKM unitarity tests using Kl3 and Kl2 decays.

Findings

Vus now consistent with unitarity within 1%.

Progress in lattice QCD reduces systematic uncertainties.

Kl3 and Kl2 decays can probe physics beyond the Standard Model.

Abstract

The analysis made in 2000 indicated that the unitarity relation Vud^2 + Vus^2 + Vub^2 = 1 might be broken at the 2.3 sigma level. At that time, however, Vus was inferred from old experimental data. Since then, a great experimental and theoretical effort has been invested to understand the source of that discrepancy. Thanks to the new and improved measurements by BNL-E865, KLOE, KTeV, ISTRA+ and NA48, the old Kl3 decay rate got shifted so that the new Vus is now consistent with unitarity. On the theory side, much progress in the lattice QCD has been made in order to tame the systematic uncertainties related to the computation of the Kl3 form factors. This joint progress allowed to assess the validity of the CKM unitarity relation at the level of less than 1%. The key challenge of the future lattice studies will be to simulate lighter pions in the region in which ChPT predictions apply. Also interesting is the recent progress in accurately computing the kaon and pion decay constants on the lattice, which then give us access to Vus and Vud from the corresponding leptonic decays. In addition, we discuss that the Kl3 and Kl2 decays offer the possibility to test various scenarios of physics beyond Standard Model.