Search for lepton number and flavour violation in K⁺ and π⁰ 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⁺ → π⁻µ⁺e⁺ decay and the lepton flavour violating K⁺ → π⁺µ⁻e⁺ and π⁰ → µ⁻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.