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