Searches for lepton flavour/number violation in K⁺ and π⁰ decays at the NA62 experiment

Searches for the lepton number violating K+ → π−μ+e+ decay and lepton flavour violating decays K+ → π+μ−e+ and π0 → μ−e+ are reported, using NA62 data collected in 2017 and 2018. In the absence of a significant excess above the background in each channel, upper limits on the branching ratios are established 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 × 10−1