New physics limits from kaon decays

TL;DR

This paper reports on recent experimental searches for lepton flavor and number violation in kaon decays, including precise measurements and improved limits, advancing the understanding of potential new physics beyond the Standard Model.

Contribution

It presents new precise measurements of kaon decay ratios and improved limits on lepton number violating decays, along with plans for future ultra-rare decay measurements.

Findings

Sub-percent precision measurement of $R_K$

Improved upper limit on $K^ ightarrow ext{ extbackslash pi}^ ext{ extbackslash mu extbackslash mu}$ decay

Future 10 extpercent{} precision on $K^+ ightarrow ext{ extbackslash pi}^+ u extbackslash nu$ branching ratio

Abstract

Searches for lepton flavour violation and lepton number violation in kaon decays by the NA62 and NA48/2 experiments at CERN are presented. A new measurement of the ratio of charged kaon leptonic decay rates $R_K=\Gamma(K_{e2})/\Gamma(K_{\mu2})$ to sub-percent relative precision is discussed. An improved upper limit on the lepton number violating $K^\pm\rightarrow\pi^\mp\mu^\pm\mu^\pm$ decay rate is also reported. The future 10% precision measurement of the branching ratio of the ultra-rare kaon decay $K^+\rightarrow\pi^+\nu\bar{\nu}$ with the NA62 experiment is finally reviewed.