Observation of prompt single-muon production by 400-GeV protons

We have observed a prompt single-muon signal in the region 0.8 < p/sub t/ < 2.5 GeV/c produced by the interactions of 400-GeV protons in steel. The prompt 1-..mu../sup +/ rate is comparable in magnitude to the prompt 2-..mu.. rate in the same kinematic region. If interpreted as originating from the production and muon decay of charmed particles, the 1-..mu.. signal corresponds to a charm production cross section in the range of 13 to 60 ..mu..b/nucleon, depending on production parameters.