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 $|U_{\ell 4}|^2$($\ell=e,\mu$)forheavyneutralleptonmassintherange$170-448~{\rm MeV}/c^2$\mathrm{.}Thisimprovesontheresultsfrompreviousproductionsearchesin$K^+$ decays, setting more stringent limits and extending the mass range.