Lower Limit On Neutral-Heavy-Muon Mass

Analysis of 122 965 dimuon final states produced by 209-GeV muons in a magnetized iron calorimeter has set a lower limit on the mass of a neutral heavy muon (${\overline{M}}^{0}$). If the ${\overline{M}}^{0}$ is coupled with Fermi strength to a right-handed charged current and decays to $\ensuremath{\mu}\ensuremath{\mu}\ensuremath{\nu}$ with a 10% branching ratio, its mass exceeds 9 ${\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c}}^{2}$.