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Physics Letters B
The NA62 experiment at CERN reports searches for K + → μ + N and K + → μ + ν 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 |U μ4 | 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 finaldoi:10.1016/j.physletb.2021.136259 fatcat:6at2r53k6bcddfgkpbaztlrz7a