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 $|U_{ \ell 4}|^2$ ($\ell=e,\mu$) for heavy neutral lepton mass in the range $170-448~{ \rm MeV}/c^2$. This improves on the results from previous production searches in $K^+$ decays, setting more stringent limits and extending the mass range.
Categories: hep-ex