The NA62 experiment at CERN took data in 2016-2018 with the main goal of measuring the $K^+ \to \pi^+ \nu \bar{\nu}$ decay. The high-intensity setup and detector performance make NA62 particularly suited for searching for new-physics effects from different scenarios involving feebly-interacting particles in the MeV-GeV mass range.
A wide spectrum of exotic $K^+$ decays were investigated, resulting in stringent upper limits ($\mathcal{O}(10^{-10} - 10^{-11})$) on the rate of several lepton number and lepton flavor violating processes.
Additionally, searches for $K^+ \to \ell^+ N$ were performed, where $N$ is assumed to be a heavy neutral lepton decaying to an invisible final state.
The results are expressed as upper limits of $\mathcal{O}(10^{-8})$ of the neutrino mixing parameter $|U_{\ell 4}|^2$.
NA62 can also be run as a beam-dump experiment, when the kaon production target is removed and the upstream collimators are moved into a closed position. Analysis of the data collected in beam-dump mode were performed to search for visible decays of exotic mediators, with particular emphasis on dark-photon models.