Search for heavy neutrino in leptonic decays of K⁺

A high statistics data sample of the K⁺ → μ⁺ν_μ decay was recorded in 2012 by the OKA collaboration. The missing mass analysis was performed to search for a decay channel K⁺ → μ⁺ν_H with a stable heavy neutrino in the final state. The obtained missing mass spectrum did not reveal statistically significant peaks corresponding to stable heavy neutrinos in the mass range (220 < m_νH < 375) MeV/c². Instead, we update upper limits on the branching ratio and on the value of the mixing element |U_μH|².