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We present a search for a neutral particle, pair produced in pp̅ collisions at√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 GeVand lifetime of 4 × 10-11 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 dimuon events.