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-1 - Real and virtual photon beams are able to elucidate the process of charm production in hadron reactions because they substitute charge for color coupling at one vertex, Charm-and forward ~-photoproduction rates limit the ~N total cross section without assuming vector-meson dominance (VMD), and within VMD yield the ratio of elastic to inelastic ~N scattering 1 , Charm muoproduction data directly test the photon-gluon-fusion (yGF) model 2 , which uses elements of quantum chromodynamics. This Letter presents muon-and photon-nucleon charm production cross sections which impose significant model constraints. We base a second Letter 3 upon differential charm-production and decays are not explicitly simulated. The diffractive and shadowing parameters which are used to describe incoherent and coherent charm production are the same as those adopted in our ~ analysis 7 • If instead we ignore nuclear coherence and shadowing, the reported free nucleon cross sections increase by 9,4%.Decay in flight of muoproduced n and K mesons is the major background.-4 -± + It is simulated using inelastic structure functions 11 and Tf , JC production data 12 taken from another muon experiment. Bubble chamber data 13 are used to parameterize secondary meson-nucleon interactions" This use of experimental input makes the simulated background independent of hadron production models.Comparison of the quanti ties in Table I rules out any possibility that Tf and K decay explain the data. On average, the differences between means of data and Monte Carlo distributions are 3. 7 times smaller for charm than for the background.

Interactions of 209 GeV muons in the Multimuon Spectrometer at Fermi lab have yielded more than 8xl0 4 e'vents 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 +1•9 muoproduction is 6.9_ 1 • 4 nb where the error includes systematic uncertainties. Extrapolated to Q 2 =0 with o(Q 2)=o(O)(l+Q2JA2)-2, the effective cross +180 +200 section for 178 (100) GeV photons is 750_130 (560-120) nb and the parameter A is 3.3±0.2 (2.9±0.2) GeV/c. The \! dependence of the cross section is similar to that of the photon-gluon-fusion model. Okubo-Zweig-Iizuka selection rules and unitarity allow the muon data to set a 90% confidence lower limit on the ~N total cross section of 0.9 mb. A first determination of the structure function F 2 (cc) 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.