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

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