The NA62 experiment at CERN collected the world’s largest dataset of charged kaon decays to di-lepton final states in 2016-2018, using dedicated trigger lines. Upper limits on the rates of several K $^{+}$ decays violating lepton flavour and lepton number conservation, obtained by analysing this dataset, are reported.
Jerhot, J., & NA62. (2023). Searches for lepton flavour and lepton number violating K $^{+}$ decays at the NA62 experiment. Journal of Physics: Conference Series, 2446. https://doi.org/10.1088/1742-6596/2446/1/012022 (Original work published 2023)