Heavy Neutral Lepton Search at NA62

In this paper we present the results of the search for heavy neutral lepton (HNL) production in charged kaon decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015. Upper limits at the $10^{-7}$–$10^{-6}$ level are established on the elements $|U_{e4}|$ and $|U_{\mu 4}|$ of the extended neutrino mixing matrix for HNL masses in the range 170–448 $\text {MeV/c}^2$ and 250–376 $\text {MeV/c}^2$, respectively.