Exclusive Production of Quarkonia and Generalized Parton Distributions
The understanding of the nucleon is as of yet not complete. In particular, the contribution of the gluon content is not well understood. Utilizing the framework of Generalized Parton Distributions enables predictions to be made if some information on them is known. We investigated exclusive photo and electroproduction of heavy vector mesons (the quarkonia J/ψ and ϒ), which can give access to the currently poorly constrained gluon distribution Eg. For this reason, we implemented a model for it
... ed a model for it with several variants in order to represent a spread of plausible distributions. We used current experimental results for exclusive φ and ρ0 production to test our variants for Eg. For quarkonium production, the analytic calculation of the Leading Order production amplitudes was performed, verifying results published previously, in particular confirming that in the non-relativistic collinear approximation there is no access to the polarized or gluon helicity flip distributions, i.e. Hg and HgT. Numerical results for both the Leading Order and, in the case of photoproduction, also Next-to-Leading Order amplitudes were calculated, based on our Leading Order amplitudes and already existing Next-to-Leading Order expressions. The observables we looked at are the unpolarized cross section, spin density matrix elements, and two spin-asymmetries --- the transverse single-spin asymmetry An, and a newly discussed double-spin asymmetry ALS, which we identified as a very promising observable for measuring Eg. We find that in the case of J/ψ photoproduction higher order corrections seem not well under control, while for ϒ production the numerical results become much more stable.