Search for $K^+$ decays to a muon and invisible particles
Phys.Lett.B 816 (2021) 136259, 2021.
The collaboration

Abstract (data abstract)
CERN-SPS. Upper limits at 90% CL on HNL coupling |Umu4|^2 vs mass from a production search in K+ --> mu+N decays at the NA62 experiment at CERN.