Observation of Prompt Single-Muon Production by 400-GeV Protons

We have observed a prompt single-muon signal in the region $0.8<\mathrm{}{p}_t<2.5\mathrm{}$ GeV/c produced by the interactions of 400-GeV protons in steel. The prompt $1-{\mu }^{+}$ 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 μb/nucleon, depending on production parameters.