Searches for heavy neutral lepton production at the NA 62 experiment

Searches for heavy neutral lepton production in K+ → e+N and K+ → μ+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 10−9 and 10−8, respectively, improving on the earlier searches for heavy neutral lepton production and decays in the kinematically accessible mass range. Searches for the K+ → μ+νν?̄? and K+ → μ+νX decays, where X is a new light invisible particle, are also reported.