Charm production by muons and its role in scale noninvariance

Interactions of 209-GeV muons in the multimuon spectrometer at Fermilab have yielded more than 8×10⁴ events with two muons in the final state. After reconstruction and cuts, the data contain 20 072 events with (81±10)% attributed to the diffractive production of charmed states decaying to muons. The cross section for diffractive charm muoproduction is 6.9_-1.4^+1.9 nb where the error includes systematic uncertainties. Extrapolated to Q²=0 with σ(Q²)=σ(0)(1+Q² / Λ²)^-2, the effective cross section for 178- (100-) GeV photons is 750_-130⁺¹⁸⁰ (560₁₂₀²⁰⁰) nb and the parameter Λ is 3.3±0.2 (2.9±0.2) GeV/c. The ν dependence of the cross section is similar to that of the photongluon-fusion model. Okubo-Zweig-Iizuka selection rules and unitarity allow the muon data to set a 90%-confidence lower limit on the ψN total cross section of 0.9 mb. A first determination of the structure function F₂(cc̅ ) for diffractive charm production indicates that charm accounts for approximately 1 / 3 of the scale noninvariance observed in inclusive muon-nucleon scattering at low Bjorken x.