Search for heavy neutral lepton production in K+ decays

A search for heavy neutral lepton production in K+ decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the 10−7 to 10−6 level are established on the elements of the extended neutrino mixing matrix |Ue4|2 and |Uμ4|2 for heavy neutral lepton mass in the ranges 170–448 MeV/ c2 and 250–373 MeV/ c2 , respectively. This improves on the previous limits from HNL production searches over the whole mass range considered for |Ue4|2 , and above 300 MeV/ c2 for |Uμ4|2 .