Search for K⁺ decays to a lepton and invisible particles

The NA62 experiment at CERN reports searches for K⁺ → e⁺N, K⁺ → µ⁺N and K⁺ → µ⁺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(10⁻⁹) and O(10⁻⁸) on the neutrino mixing parameter |U_e4|² and |U_µ4|², improving on the earlier searches for heavy neutral lepton production and decays in the kinematically accessible mass range. 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 upper limit of 1.0 · 10⁻⁶ is established at 90% CL on the K⁺ → µ⁺ννν¯ branching fraction.