Search for heavy neutrino in decay at ISTRA+ setup

Heavy neutrino nu h with m h [lap] 300 MeV / c 2 can be effectively searched for in kaon decays. We put upper limits on a mixing matrix element | U mu h | 2 for radiatively decaying nu h from K - arrow right mu - nu h ( nu h arrow right nu gamma ) decay chain in the following parameter region: 30 MeV / c 2 [el] m h [el] 80 MeV / c 2 ; 10 - 11 s [el] tau h [el] 10 - 9 s . For the whole region | U mu h | 2 [lap] 5 times 10 - 5 for Majorana type of nu h and | U mu h | 2 [lap] 8 times 10 - 5 for the Dirac case.