On charmonia and charmed mesons photoproduction at high energy

In this report we compare the predictions of the collinear parton model and the kT -factorization approach in J/Ψ and D ⋆ meson photoproduction at HERA energies. It is shown that obtained D⋆ meson spectra over pT and η are very similar in the parton model and kT -factorization approach and they underestimate the experimental data. Opposite, the predictions of the both approaches for pT and z-spectra in the J/Ψ photoproduction are very different as well as the prediction obtained for the spin parameter α(pT ). 1 Hard processes in the parton model and kT -factorization approach. Nowadays, there are two approaches which are used in a study of the charmonia and charmed mesons photoproduction at high energies. In the conventional collinear parton model [1] it is suggested that hadronic cross section, for example σ(γp → cc̄X, s), and the relevant partonic cross section σ̂(γg → cc̄, ŝ) are connected as follows σ(γp → cc̄X, s) = ∫ dxG(x, μ)σ̂(γg → cc̄, ŝ), (1) where ŝ = xs, G(x, μ) is the collinear gluon distribution function in a proton, x is the fraction of a proton momentum, μ is the typical scale of a hard process. The μ evolution of the gluon distribution G(x, μ) is described by DGLAP evolution equation [2]. In the so-called kT -factorization approach hadronic and partonic cross sections are related by the following condition [3, 4, 5]: σ(γp → cX, s) = ∫ dx x ∫ d~k T ∫ dφ 2π Φ(x,~k T , μ )σ̂(γg → cc̄, ŝ, ~k T ), (2) where σ̂(γg → cc̄, ŝ, ~k2 T ) is the cc̄-pair photoproduction cross section on off mass-shell gluon, k = kT 2 = −~k2 T is the gluon virtuality, ŝ = xs − ~k2 T , φ is the azimuthal angle in the transverse XOY plane between vector ~kT and the fixed OX axis. The unintegrated gluon distribution function Φ(x,~k2 T , μ ) can be related to the conventional gluon distribution by xG(x, μ) = μ