Search for heavy neutral lepton production at the NA62 experiment

Searches for heavy neutral lepton production in K$^{\mathrm{+}}\to $e$^{\mathrm{+}}$N and K$^{\mathrm{+}}\to \mu^{\mathrm{+}}$N decays using the data set collected by the NA62 experiment at CERN in 2016-18 are presented. Upper limits on the elements of the extended neutrino mixing matrix [U$_{\mathrm{e4}}$]$^{\mathrm{2}}$ and [U$_{\mathrm{\mu 4}}$]$^{\mathrm{2\thinspace }}$are established at the levels of 10$^{\mathrm{-9}}$ and 10$^{\mathrm{-8}}$, respectively, improving on the earlier searches for heavy neutral lepton production and decays in the kinematically accessible mass range.