Search for lepton number and flavour violation in $K^{+}$ and $\pi^{0}$ decays

TL;DR

This paper reports on searches for lepton number and flavour violating decays of kaons and pions using NA62 data, setting new upper limits that improve previous constraints by an order of magnitude.

Contribution

First search for these specific lepton violation decays with improved upper limits using NA62 data from CERN.

Findings

No evidence for the decays was observed.

Upper limits on branching ratios were set at 90% confidence level.

Results improve previous limits by about an order of magnitude.

Abstract

Searches for the lepton number violating $K^{+} \rightarrow \pi^{-} \mu^{+} e^{+}$ decay and the lepton flavour violating $K^{+} \rightarrow \pi^{+} \mu^{-} e^{+}$ and $\pi^{0} \rightarrow \mu^{-} e^{+}$ decays are reported using data collected by the NA62 experiment at CERN in $2017$-$2018$. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% confidence level: $\mathcal{B}(K^{+}\rightarrow\pi^{-}\mu^{+}e^{+})<4.2\times 10^{-11}$, $\mathcal{B}(K^{+}\rightarrow\pi^{+}\mu^{-}e^{+})<6.6\times10^{-11}$ and $\mathcal{B}(\pi^{0}\rightarrow\mu^{-}e^{+})<3.2\times 10^{-10}$. These results improve by one order of magnitude over previous results for these decay modes.