Search for Lepton Number and Flavour Violation in $K^+$ and $\pi^0$ Decays

The NA62 experiment at CERN collected a large sample of charged kaon decays into final states with multiple charged particles in Run 1 from 2016 to 2018. This sample provides sensitivities to rare decays with branching fractions as low as 10−11. Results of searches for the lepton number and/or flavour violating decays K+→π±μ∓e+ and π0→μ−e+ are reported. No evidence of these decays is found, and upper limits on the branching fractions are obtained at 90% confidence level: B(K+→π−μ+e+)<4.2×10−11, B(K+→π+μ−e+)<6.6×10−11, and B(π0→μ−e+)<3.2×10−10. These results improve by approximately one order of magnitude over previous results for these decay modes.