Dimuon production is studied in 400-GeV proton-nucleus collisions. A strong enhancement is observed at 9.5 GeV mass in a sample of 9000 dimuon events with a mass $m_{\mu^+\mu^-} \to$ 5 GeV.
A precise measurement of the atomic-mass dependence of dimuon production induced by 800-GeV protons is reported. Over 450 000 muon pairs with dimuon mass M≥4 GeV were recorded from targets of H2, C, Ca, Fe, and W. The ratio of dimuon yield per nucleon for nuclei versus H2, R=YA/Y2H, is sensitive to modifications of the antiquark sea in nuclei. No nuclear dependence of this ratio is observed over the range of target-quark momentum fraction 0.1<xt<0.3. For xt<0.1 the ratio is slightly less than unity for the heavy nuclei. These results are compared with predictions of models of the European Muon Collaboration effect.
The differential cross sections dσ/dxF for J/ψ produced inclusively in 800 GeV/c p-Cu and p-Be collisions have been measured in the kinematic range 0.30≤xF≤0.95 through the decay mode J/ψ→μ+μ−. They are compared with the predictions of the semilocal duality model for several sets of parton density functions. No evidence for a suggested intrinsic charm contribution to the cross section is observed. The ratio of the differential cross sections for Cu and Be targets confirms the suppression of J/ψ production in heavy nuclei at large xF.
We present total and differential cross sections for charm mesons produced in 600 GeV/ c π - emulsion interactions. Fits to d 2 σ / dx F dp T 2 ∞ (1−| x F |) n exp (- bp T 2 ) for 676 electronically reconstructed D mesons with x F >0 give n =4.25±0.24 ( stat .)±0.23 ( syst .) and b =0.76±0.03±0.03 ( GeV / c ) -2 . The total inclusive D + and D 0 cross sections are σ ( π - N → D ± ; x F >0) = 8.66±0.46±1.96 μb nucleon and σ(π - N→D 0 D 0 ; x F >0)=22.05±1.37±4.82μb nucleonk, where a linear dependence on the mean atomic weight of the target is assumed. These results are compared to next-to-leading order QCD predictions.
We present results on charm pair correlations measured in proton-emulsion interactions at s =38.7 GeV. The predictions of leading order QCD for the distributions in invariant mass, rapidity gap, x F , and polar angle in the charm pair CMS are qualitatively consistent with our measurements. The mean p T of the pairs is equal within errors to that measured in dilepton production at the same energy and mass range.
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