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

Heavy neutrino νh with mh≲300MeV/c2 can be effectively searched for in kaon decays. We put upper limits on a mixing matrix element |Uμh|2 for radiatively decaying νh from K−→μ−νh(νh→νγ) decay chain in the following parameter region: 30MeV/c2⩽mh⩽80MeV/c2; 10−11s⩽τh⩽10−9s. For the whole region |Uμh|2≲5⋅10−5 for Majorana type of νh and |Uμh|2≲8⋅10−5 for the Dirac case.