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

Abstract 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 of the extended neutrino mixing matrix element | U e 4 | 2 are established at the level of 10 − 9 over most of the accessible heavy neutral lepton mass range 144–462 MeV/ c 2 , with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The | U e 4 | 2 range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/ c 2 .