Search for heavy neutral lepton production at the NA62 experiment

Searches for heavy neutral lepton production in $K^{+} \to e^{+}N$ and $K^{+} \to \mu ^{+}N$ decays using the data set collected by the NA62 experiment at CERN in 2016-18 are presented. Upper limits on the elements of the extended neutrino mixing matrix and are established at the levels of and , respectively, improving on the earlier searches for heavy neutral lepton production and decays in the kinematically accessible mass range. Searches for the $K^{+} \to \mu ^{+}\nu \nu \bar{\nu }$ and $K^{+} \to \mu ^{-}\nu X$ decays, where $X$ is an new light invisible particle, are also reported.