The A-Dependence of J/ψ Photoproduction Near Threshold

The primary goal of this experiment in Hall C is the measurement of the Adependence of quasi-free J/ψ photoproduction with an average photon energy near 11 GeV. Analysis of these data will permit a greatly improved determination of the fundamental J/ψ-nucleon total cross section. In order to reduce modeling uncertainties, we will also make precision measurements of the (s, t) dependence of the elementary J/ψ photoproduction cross section on the proton, improving the existing data and extending them to the threshold area. Comparisons to models will allow improved determination of the reaction mechanism. The experiment consists of passing a 50 μA, 11 GeV electron beam through 10% r.l.-thick nuclear targets ranging from Be to Au, for the A-dependence measurements. J/ψ particles produced by real photons (using the targets themselves as radiators) and quasi-real photons from electron scattering near 0 degrees will have typical momenta of 10 GeV and angles of only a few degrees with respect to the beam axis. The J/ψ particles will be identified through their decays to e+e− or μ+μ− pairs (6% branching ratio each). The major part of the data will be taken with the positively-charged leptons detected in the SHMS with P = 5.8 GeV and θ = 15◦, while the negatively-charged leptons will be detected in the HMS with P = 4.2 GeV and θ = 21◦. Due to the very high transverse momenta of the leptons and the good spectrometer resolutions, backgrounds are small and very clean identification of J/ψ particles will be possible. Additional measurements with a liquid hydrogen target (preceded by a 10% r.l. radiator) will be made at several spectrometer settings with 11 GeV, 10.2 GeV and 8.8 GeV electron beam energies to measure the (s, t) dependence of the cross section in the kinematic region accessible with reasonable count rates. The resulting improvement in our knowledge of the elementary cross section is necessary to minimize systematic errors in the A-dependence measurements, due to the substantial Fermi-broadening effects. The cross section near threshold is also of theoretical interest, as the production mechanism is expected to be dominated by multiple gluon exchange. The principal requirements beyond the planned standard Hall C configuration are for a set of nuclear targets, and an external radiator for the liquid hydrogen target. A total of 21 day is needed for data taking, with an additional 3 days needed for checkout, energy change, and configuration changes. ∗contactperson, email [email protected]