Heavy-Neutrino Search Using<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>K</mml:mi></mml:mrow><mml:mrow><mml:mi>μ</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Decay

The muon momentum spectrum in ${K}_{\ensuremath{\mu}2}$ decay has been measured by using a high-resolution magnetic spectrograph to look for a discrete muon peak associated with heavy-neutrino emission. The spectrum revealed no distinct peak, and the upper bound of the mixing ratio between the muon neutrino and a massive neutrino has been determined to be ${10}^{\ensuremath{-}4}$-${10}^{\ensuremath{-}6}$ in the mass range of 70-300 MeV/${\mathit{c}}^{2}$.