Search for heavy neutral lepton production in K+ decays to positrons

A search for heavy neutral lepton (N) production in K+→e+N decays using the data sample collected by the NA62 experiment at CERN in 2017–2018 is reported. Upper limits ofthe extended neutrino mixing matrix element|Ue4|2are established at the level of 10−9overmost of the accessible heavy neutral lepton mass range 144–462 MeV/c2, with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The|Ue4|2range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/c2.