Diffractive heavy quarkonium photoproduction and electroproduction in QCD

Hard diffractive photoproduction and electroproduction of heavy vector mesons ($J/\psi$ and Υ) is evaluated within the leading ${\alpha }_s\ln {(Q}^2/{\Lambda }_{\mathrm{QCD}}^2)$ approximation of QCD. Different from our earlier work on that subject, also the production of transversely polarized vector mesons is calculated. Special emphasis is placed on the role of the vector meson’s $q\overline{q}$ light-cone wave function. In that context, conventional nonrelativistic quarkonium models and a light-front QCD bound state calculation are critically examined and confronted with QCD expectations. Our numerical analysis finds a significant high momentum tail in the latter wave functions and a deviation from the expected asymptotic behavior of ${\phi }_V\mathrm{}(z,b=0\mathrm{})\mathrm{}\propto z\left(1\mathrm{}-z\right).\mathrm{}$ We then design an interpolation to match the quarkonium models at large interquark separations with QCD expectations at small distances. We use these results to compare our predictions for the forward differential cross section of $J/\psi$ photoproduction and electroproduction with recent experimental results from DESY HERA. In addition, our earlier discussion of ρ° electroproduction is updated in light of recent experimental and theoretical enhancements.