Search for heavy neutrino in $K^{+} to mu^{+} u_{H}$ decay

A high statistics data sample of the $K^{+}tomu^{+} u_{mu}$ decay was accumulated by the OKA experiment in 2012. The missing mass analysis was performed to search for the decay channel $K^{+}tomu^{+} u_{H}$ with a hypothetic stable heavy neutrino in the final state. The obtained missing mass spectrum does not show peaks which could be attributed to existence of stable heavy neutrinos in the mass range (220 $< m_{ u_{H}} <$ 375) MeV$/c^{2}$. As a result, we obtain upper limits on the branching ratio and on the value of the mixing element $|U_{mu H}|^{2}$.