Searches for lepton flavour and lepton number violation in $K^{+}$ decays at NA62

TL;DR

This paper reports a search for rare kaon decays violating lepton flavor and number, setting new upper limits on their branching ratios with no significant signals observed, thus constraining new physics models.

Contribution

First search for these specific lepton flavor and number violating kaon decays using NA62 data, establishing significantly improved upper limits on their branching ratios.

Findings

No significant excess observed above background.

Established upper limits on branching ratios: <4.2×10⁻¹¹ and <6.6×10⁻¹¹.

Limits improved by factors of 12 and 8 over previous results.

Abstract

A search for the lepton number violating $K^{+}\rightarrow\pi^{-}\mu^{+}e^{+}$ and lepton flavour violating $K^{+}\rightarrow\pi^{+}\mu^{-}e^{+}$ decays is reported using NA62 data collected in 2017 and 2018. No significant excess is observed above the background expectation and therefore upper limits are established on the branching ratios at $90\%$ confidence level: $\mathcal{B}(K^{+}\rightarrow\pi^{-}\mu^{+}e^{+})<4.2\times10^{-11}$ and $\mathcal{B}(K^{+}\rightarrow\pi^{+}\mu^{-}e^{+})<6.6\times10^{-11}$. These results improve over the previous limits by factors of $12$ and $8$ respectively.