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
\nbias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the 10−7 to 10−6 level
\nare established on the elements of the extended neutrino mixing matrix |Ue4|
\n2 and |Uμ4|
\n2 for heavy
\nneutral lepton mass in the ranges 170–448 MeV/c2 and 250–373 MeV/c2, respectively. This improves on
\nthe previous limits from HNL production searches over the whole mass range considered for |Ue4|2 and
\nabove 300 MeV/c2 for |Uμ4|2.