Evidence for prompt single muon production in 400 GeV proton interactions

We have investigated the origin of prompt muon production in the interaction of 400 GeV protons in a steel target. Using a variable density target and a large acceptance muon identifier, we have observed a substantial signal of prompt single muon for the kinematic region XF?0 and 0.8≲pt≲1.5 GeV. Assuming that the major source is charmed D? production, a model dependent calculation yields σ (pp→charm) of about 40μb.