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Heavy quark production in photon-nucleon and photon-photon collisions

We discuss several mechanisms of heavy quark production in (real) photon-nucleon and (real) photon - (real) photon collisions. In particular we focuse on application of the Saturation Model. We discuss how to generalize the formula from virtual photon - proton scattering and analyze threshold effects. We discuss a possibility to measure the cross section for $γγ\to 2 c 2 \bar c$. In addition to the main dipole-dipole contribution included in a recent analysis, we propose how to calculate within the same formalism the hadronic single-resolved contribution to heavy quark production. At high energies this yields a sizeable correction of about 30-40 % for inclusive charm production and 15-20 % for bottom production. We consider a subasymptotic component to the dipole-dipole approach. We get a good description of recently measured $σ(γγ\to c \bar c X)$. Adding all possible contributions to $e^+ e^- \to b \bar b X$ together removes a huge deficit observed in earlier works but does not solve the problem totally. Whenever possible, we compare the present approach to the standard collinear one. We propose how to distinguish different mechanisms by measuring heavy quark-antiquark correlations.