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 2016-2018. This sample provides sensitivities to rare decays with branching ratios as low as 10 −11 . Searches for the lepton number violating $K^{+} \to \pi ^{-}\mu ^{+}e^{+}$ decay and the lepton flavour violating $K^{+} \to \pi ^{+}\mu ^{-}e^{+}$ and $\pi ^{0} \to \mu ^{-}e^{+}$ decays are reported. No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% confidence level. These results improve by one order of magnitude over previous results for these decay modes.