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

Abstract

A search for the lepton number violating $K^{+}\to {\pi }^{-}{\mu }^{+}{e}^{+}$ and lepton flavour violating $K^{+}\to {\pi }^{+}{\mu }^{-}{e}^{+}$ 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: $\mathcal{B}(K^{+}\to {\pi }^{-}{\mu }^{+}{e}^{+})<4.2\times 10^{-11}$ and $\mathcal{B}(K^{+}\to {\pi }^{+}{\mu }^{-}{e}^{+})<6.6\times 10^{-11}$. These results improve over the previous limits by factors of $12$ and $8$ respectively.

6 pages, 3 figures, Prepared for the proceedings of the 40th International Conference on High Energy physics (ICHEP2020), July 28 - August 6, 2020