Search For K+ Decays To A Muon And Invisible Particles

The NA62 experiment at CERN reports searches for K+ -> mu N+ and K+ -> 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(10(-8)) of the neutrino mixing parameter vertical bar U-mu 4 vertical bar(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(10(-5)) to O(10(-7)). An improved upper limit of 1.0x10(-6) is established at 90% CL on the K+ -> mu(+)nu nu(nu) over bar branching fraction. (C) 2021 The Author(s). Published by Elsevier B.V.