Search for K + decays to a muon and invisible particles

The NA62 experiment at CERN reports searches for $K^{+}\to {\mu }^{+}N$ and $K^{+}\to {\mu }^{+}\nu X$ decays, where N and X are massive invisible particles, using the 2016–2018 data set. The N particle is assumed to be a heavy neutral lepton, and the results are expressed as upper limits of $O\left({10}^{-8}\right)$ of the neutrino mixing parameter $|U_{\mu 4}{|}^2$ for N masses in the range 200–384 MeV/$c^2$ and lifetime exceeding 50 ns. The X particle is considered a scalar or vector hidden sector mediator decaying to an invisible final state, and upper limits of the decay branching fraction for X masses in the range 10–370 MeV/$c^2$ are reported for the first time, ranging from $O\left({10}^{-5}\right)$ to $O\left({10}^{-7}\right)$. An improved upper limit of $1.0\times {10}^{-6}$ is established at 90% CL on the $K^{+}\to {\mu }^{+}\nu \nu \overline{\nu }$ branching fraction.