  We present a search for a neutral particle, pair-produced in p-pbar
collisions at sqrt(s)=1.96 TeV, which decays into two muons and lives long
enough to travel at least 5 cm before decaying. The analysis uses ~380 pb^-1 of
data recorded with the D0 detector. The background is estimated to be about one
event. No candidates are observed, and limits are set on the pair production
cross section times branching fraction into dimuons + X for such particles. For
a mass of 10 GeV and lifetime of 4x10^-11 s, we exclude values greater than
0.14 pb (95% C.L.). These results are used to limit the interpretation of
NuTeV's excess of di-muon events.
