Search for K⁺ decays to a muon and invisible particles

The NA62 experiment at CERN reports searches for K⁺ → μ⁺N and K⁺ → μ⁺νX 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(10⁻⁸) of the neutrino mixing parameter |U_μ4|² for N masses in the range 200–384 MeV/c² 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/c² are reported for the first time, ranging from O(10⁻⁵) to O(10⁻⁷). An improved upper limit of 1.0 × 10⁻⁶ is established at 90% CL on the K⁺ → μ⁺ννν̅ branching fraction.