Hard diffractive electroproduction of longitudinally polarized vector mesons is calculated within the leading $\alpha_s\ln{Q^2\over\Lambda_{QCD}^2}$ approximation of QCD using the leading order parton densities within the nucleon. Novel QCD features of the production of excited states and of the restoration of flavor symmetry are analyzed. At the onset of the asymptotic regime, our analysis finds an important role of quark Fermi motion within the diffractively produced vector mesons, and we suggest to use this effect to measure the high momentum tail of the wave function of the vector mesons. We deduce a kinematical boundary for the region of applicability of the decomposition of the hard amplitudes over powers of $Q^2$ and/or a limit on the increase of the cross sections of hard processes at small $x$, and briefly analyze its consequences. We also estimate the nuclear attenuation of the diffractive electroproduction of vector mesons and compare with estimates of the shadowing of the longitudinal structure function.Comment: 64 pages, REVTeX, 16 figures optionally included using epsfig.sty, Revisions in regards to Charmonium photo- and electroproduction, final version to appear in Phys. Rev.

Hard diffractive photo-and electroproduction of heavy vector mesons (J/ψ and Υ) is evaluated within the leading α s ln 1 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 qq light-cone wave function. In that context, conventional non-relativistic 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 φ V (z, b = 0) ∝ z(1 − z). We then design an interpolation to match the quarkonium models at large inter-quark separations with QCD expectations at small distances. We use these results to compare our predictions for the forward differential cross section of J/ψ photo-and electroproduction with recent experimental results from HERA. In addition, our earlier discussion of ρ o electroproduction is updated in light of recent experimental and theoretical enhancements.

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We address the question of whether the nucleon's nonperturbative antiquark sea can be attributed entirely to its virtual meson cloud and, in essence, whether there exists a smooth transition between hadronic and quarkgluon degrees of freedom. We take into account contributions from and K mesons and compare them with the nucleon's antiquark distributions which serve as nonperturbative inputs to the QCD evolution equations. While the agreement of our calculations with data-based parametrizations is satisfactory and scale independent for the flavor-breaking share of the nucleon's antiquark sea, the flavor singlet component is quite poorly described. We find that large meson loop momenta, ͉k͉Ϸ0.8 GeV, dominate in the calculation of the nucleon's sea quark distributions in the meson cloud model. This value is close to the scale where perturbative QCD evolution starts, which further indicates the limitations of the applicability of the physical picture of a meson cloud around the nucleon in high-energy processes.

Relativistic Mean Field (RMF) theory is used to investigate the behavior of the spin orbit potential, V~s, in nuclear states of very large deformation and high angular velocity. As a by-product we present a set of parameters for an approximation of the relativistic scalar-and vector-potentials S and V in the Dirac equation in terms of Saxon-Woods shapes. These reproduce more or less the same single particle specta as a full selfconsistent relativistic mean field calculation.