Search for heavy neutrinos in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>K</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mo stretchy="false">→</mml:mo><mml:msup><mml:mi>μ</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:msub><mml:mi>ν</mml:mi><mml:mi>H</mml:mi></mml:msub></mml:math>decays

Evidence of a heavy neutrino, ${\ensuremath{\nu}}_{H}$, in the ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{H}$ decays was sought using the E949 experimental data with an exposure of $1.70\ifmmode\times\else\texttimes\fi{}1{0}^{12}$ stopped kaons. With the major background from the radiative ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{\gamma}$ decay understood and suppressed, upper limits (90% C.L.) on the neutrino mixing matrix element between the muon and heavy neutrinos, $|{U}_{\ensuremath{\mu}H}{|}^{2}$, were set at the level of $1{0}^{\ensuremath{-}7}$ to $1{0}^{\ensuremath{-}9}$ for the heavy neutrino mass region 175 to $300\text{ }\text{ }\mathrm{MeV}/{c}^{2}$.