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×${10}^4$ 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 ${6.9}_{-1.4\mathrm{}}^{+1.9\mathrm{}}$ nb where the error includes systematic uncertainties. Extrapolated to $Q^2=0\mathrm{}$ with $\sigma \left(Q^2\right)=\sigma \mathrm{}\left(0\right)\mathrm{}{(1+\frac{Q^2}{{\Lambda }^2})}^{-2\mathrm{}}$, the effective cross section for 178- (100-) GeV photons is ${750}_{-130\mathrm{}}^{+180\mathrm{}}$ (${560}_{120}^{200}$) nb and the parameter $\Lambda$ is 3.3±0.2 (2.9±0.2) GeV/c. The $\nu$ 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 $\psi N$ total cross section of 0.9 mb. A first determination of the structure function $F_2\left(c\overline{c}\right)$ for diffractive charm production indicates that charm accounts for approximately $\frac{1}{3}$ of the scale noninvariance observed in inclusive muon-nucleon scattering at low Bjorken $x$.