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 |Ue4|2 and |Uμ4|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+ννˉν branching fraction.