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We report on the first observation of neutrino induced production of muon pairs in the electromagnetic field of a nucleus. The data has been obtained using the CHARM II detector exposed to the CERN wideband neutrino and antineutrino beams. A clear signal of 55 +/- 16 events is seen in a sample of dimuons of opposite charge without visible recoil at the vertex. The cross section is determined and found to be in agreement with the standard model prediction

The hard contributions to the heavy quarkonium-nucleon cross sections are calculated based on the QCD factorization theorem and the nonrelativistic quarkonium model. We evaluate the nonperturbative cross sections which dominates at p s NN ഠ 20 GeV at the CERN Super Proton Synchrotron (SPS) andbecomes a correction at p s NN ഠ 6 TeV at the CERN Large Hadron Collider. J͞C production at the CERN SPS is well described by hard QCD, when the larger absorption cross sections of the x states predicted by QCD are taken into account. We predict an A-dependent polarization of the x states. The expansion of small wave packets is discussed. [S0031-9007(98)06627-7] PACS numbers: 24.85. + p, 12.39.Jh, 14.40.Lb, 25.75.DwThe aim of the present paper is to investigate color coherent phenomena in the propagation of hadronic states with hidden charm in pA and AB collisions. We restrict the consideration to the central rapidity region of nuclear beams at p s ഠ 20A GeV at the CERN Super Proton Synchrotron (SPS) where formation time effects in J͞C production are corrections. Qualitatively new effects for p s ഠ 200A GeV at the BNL Relativistic Heavy Ion Collider (RHIC) and p s ഠ 6A TeV at the CERN Large Hadron Collider (LHC) energies will be discussed also. We assume that hadronic states with hidden flavor are produced predominantly in hard parton collisions and calculate the absorption factor S of the total yield of heavy quarkonia which is not distorted by the initial state interaction; cf. [1].Under these assumptions S can be directly measured by the ratio of the yields of heavy quarkonium to the dimuon yield [2]. We want to emphasize that the agreement of our calculations of the J͞C production in pA and in AB collisions with the data strongly supports the idea that hidden charm states are perturbatively produced in NN collisions.The QCD factorization theorem is used to evaluate the perturbative QCD (PQCD) cross sections of heavy quarkonium interactions with ordinary hadrons. However, the charmonium states (here denoted X) are not sufficiently small to ignore nonperturbative QCD physics. Thus, we evaluate the nonperturbative QCD contribution to the cross sections of charmonium-nucleon interaction by using an interpolation between known cross sections. The J͞C-N cross section evaluated in this paper is in reasonable agreement with the Stanford Linear Accelerator Center (SLAC) data [3]. We find that the suppression of the J͞C in pA and AB collisions is reasonably well described within the present approach.We predict a new QCD effect-the polarization of produced x c states: cc states with nonzero orbital momentum will be polarized due to the longitudinally directed nuclear (color filter) target. Their polarization is evaluated quantitatively. Random interactions with comovers will depolarize the x states. Thus, measuring the x polarization may turn out to be an effective method to investigate their interactions with comovers.The production of charmonium states in AB collisions has been proposed (via J͞C suppression) as signal for Debye s...

Charmonium production and absorption in heavy ion collisions is studied with the Ultrarelativisitic Quantum Molecular Dynamics model. We compare the scenario of universal and time independent color-octet dissociation cross sections with one of distinct color-singlet J/ψ, ψ ′ and χ c states, evolving from small, color transparent configurations to their asymptotic sizes. The measured J/ψ production cross sections in pA and AB collisions at SPS energies are consistent with both -purely hadronic -scenarios. The predicted rapidity dependence of J/ψ suppression can be used to discriminate between the two experimentally. The importance of interactions with secondary hadrons and the applicability of thermal reaction kinetics to J/ψ absorption are investigated. We discuss the effect of nuclear stopping and the role of leading hadrons. The dependence of the ψ ′ /J/ψ ratio on the model assumptions and the possible influence of refeeding processes is also studied. * Supported by the Alexander v. Humboldt Foundation †