Charm Photoproduction at Large Transverse Momentum and the Charm Content of the Photon

The charm quark photoproduction at large transverse momentum in the resolved photon interaction via partonic subprocess c(γ)g → cg is studied. It is shown that at HERA energies the contribution of the charm quark excitation in the photon in the inclusive charm production at p⊥ > 5 GeV/c dominates over the other resolved photon contribution via the gluon content of the photon and it is about 30% of the contribution of the cc̄-pair production via the photon-gluon fusion mechanism. The study of the charm quark photoproduction on protons at high energies is a clean test of QCD as well as our knowledge of the gluon distribution in the proton [1]-[6]. Recent experiment at HERA ep-collider have shown that the resolved photon interaction, which probes the photon structure, also can be very important in the charm quark photoproduction process [7]. It is well known that the pure perturbative QCD calculations are not adequate to describe the observed charm photoproduction data for p⊥ and xF distributions. There are strong dependence on the value of the c-quark mass, k⊥-smearing and other nonperturbative effects. In the region of large transverse momentum p⊥ >> mc the results based on the perturbative QCD must be more reliable. Previous calculations of the c-quark inclusive photoproduction at large p⊥ have included LO and NLO direct charm production via photon-gluon fusion [1]-[6], including the semihard approach calculations [2, 8, 9, 10], the resolved photon contribution via subprocess g(γ)g → cc̄ [5, 11] and the diffractive-like photoproduction in the vector-meson-dominance (VMD) model [12]. Recently we have shown the important role of the c-quark photon structure function (PSF) in the J/ψ [13] and Bc-meson [14] photoproduction at large transverse momentum via partonic subprocesses c(γ)g → J/ψc and c(γ)γ → Bcb. It is obviously that the similar mechanism also plays a crucial role in the open charm photoproduction at large p⊥ via subprocess c(γ)g → cg. Note, that early the charm quark hadroproduction via subprocess cg → cg [15] as well as J/ψ hadroproduction via subprocess cg → J/ψc [16] have been studied, based on charm quark excitation model. The conception of the intrinsic charm quark in the photon or the proton is theoretically justified in the processes, where the charm quark in the initial parton state receives sufficient transfer momentum t̂, which is necessary to excite cc̄ pair. This condition takes place for charm quark photoproduction at the large p⊥, where the relevant scale is Q 2 ∼ M ⊥ = p2⊥ +m 2 c >> m 2 c . In the parton model the measurable cross section for the charm quark photoproduction is defined by the parton level QCD cross section σ̂(cg → cg) and the respective parton distribution functions: