Search for Heavy Neutral Lepton Production in NA62#

A search for heavy neutral lepton (N) production in K+ -> mu N+ and K+ -> e(+)N decays is carried out by the NA62 experiment using the data collected in 2016-2018. The K+ -> mu N+ results are expressed as upper limits of O(10(-8)) of the neutrino mixing parameter vertical bar U-mu(4)vertical bar(2) for N masses in the range 200-384 MeV/c(2) and lifetime exceeding 50 ns. From the K+ -> e(+)N analysis, upper limits for neutrino mixing matrix element vertical bar U-e(4)vertical bar(2) are established near the level of 10(-9) over most of the accessible heavy neutral lepton mass range 144-462 MeV/c(2), with the same assumption about the heavy lepton lifetime.