Search for Lepton Number and Flavor Violation in <math><msup><mi>K</mi><mo>+</mo></msup></math> and <math><msup><mi>π</mi><mn>0</mn></msup></math> Decays

Searches for the lepton number violating K+→π−μ+e+ decay and the lepton flavor violating K+→π+μ−e+ and π0→μ−e+ decays are reported using data collected by the NA62 experiment at CERN in 2017–2018. No evidence for these decays is found and upper limits of the branching ratios 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 1 order of magnitude over previous results for these decay modes.