Search for heavy neutrino in K−→μ−νh(νh→νγ) decay at ISTRA+ setup

Abstract Heavy neutrino ν h with m h ≲ 300 MeV / c 2 can be effectively searched for in kaon decays. We put upper limits on a mixing matrix element | U μ h | 2 for radiatively decaying ν h from K − → μ − ν h ( ν h → ν γ ) decay chain in the following parameter region: 30 MeV / c 2 ⩽ m h ⩽ 80 MeV / c 2 ; 10 − 11 s ⩽ τ h ⩽ 10 − 9 s . For the whole region | U μ h | 2 ≲ 5 ⋅ 10 − 5 for Majorana type of ν h and | U μ h | 2 ≲ 8 ⋅ 10 − 5 for the Dirac case.