Investigation of the color-dipole structure in diffractive t-slopes of charmonia photo- and electroproductions

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The diffractive t-slope, Bᵥ, of elastic charmonia (V=J/psi, psi') photo- and leptoproductions off a nucleon is studied at low |t| (< 1 GeV²) in the leading logarithmic approximation of perturbative QCD, with a special emphasis on the space-time evolution of the cc-dipole. We obey a framework based on QCD factorization, which describes a certain Fermi motion effect due to the c(c)-quarks in the proper manner and includes appropriately kinematical corrections of the momentum transfer Deltaₚerp in the t-channel. Assuming the universal two-gluon form factor of the nucleon, we show that the difference of t-slopes for J/psi and psi' is dominated by the contribution from the dipole-charmonium transition process. The calculated difference is found to be Bⱼ/psi-Bₚsi' 0.53 GeV-2 for the photoproduction, in agreement with HERA data. We also calculate the t-dependence of the total cross sections for several center-of-mass energies W. A good agreement of the results with the available data demands that the mass scale appearing in the gluon form factor should significantly decrease with increasing W.