Sub-threshold J/u photoproduction

Lee, Claire Alexandra
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Jefferson Lab experiment E03-008 aimed to measure the J/ψ photoproduc- tion cross section at energies sub-threshold to a free nucleon. Prior data ex- tended only to just above threshold; the closest-to-threshold measurements come from Cornell and SLAC and provide data in the 9.3 to 21 GeV region. For E03-008, a mixed real and quasi-real bremsstrahlung photon beam with an endpoint energy of 5.76 GeV incident on a solid carbon target was used, allowing measurement for the first time in the sub-threshold region. The sub-threshold energy of the beam required momentum contributions from the nucleon target to bring the energy of the reaction up to threshold, thus providing an opportunity to probe higher order nucleon correlations within the nucleus as well as exotic mechanisms that may enhance the cross section. The aim of the analysis was to study the parametrization of the cross section near threshold and comment on the predictions made by theoretical models of the J/ψ photoproduction reaction mechanism. No J/ψ events were ob- served, a result which is consistent with predictions under the assumption of quasi-free photoproduction, and places restictions on exotic mechanisms that would strongly enhance quasi-free production.