Search for Lepton Number and Flavor Violation in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>K</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>π</mml:mi><mml:mn>0</mml:mn></mml:msup></mml:math> Decays

Searches for the lepton number violating K+→π−μ+e+ decay and flavor K+→π+μ−e+ π0→μ−e+ decays are reported using data collected by NA62 experiment at CERN in 2017–2018. No evidence these is found upper limits of branching ratios obtained 90% confidence level: B(K+→π−μ+e+)<4.2×10−11, B(K+→π+μ−e+)<6.6×10−11 B(π0→μ−e+)<3.2×10−10. These results improve 1 order magnitude over previous modes.Received 17 May 2021Accepted 22 July 2021DOI:https://doi.org/10.1103/PhysRevLett.127.131802Published American Physical Society under terms Creative Commons Attribution 4.0 International license. Further distribution this work must maintain attribution to author(s) published article’s title, journal citation, DOI. Funded SCOAP3.Published SocietyPhysics Subject Headings (PhySH)Research AreasParticle decaysPhysical SystemsKaonsPionsPropertiesFlavor symmetriesParticles & Fields