Hard exclusive two photon processes in QCD

Understand

This is a short review of some hard two-photon processes: $\\ a) \,\,\gamma\gamma\to {\overline P}_1 P_2,\,\, {\overline P}_1 P_2= \{\pi^+\pi^-, K^+ K^-, K_S K_S, \pi^o\pi^o, \pi^o\eta\}\,, \\ b) \,\,\gamma\gamma\to V_1 V_2,\,\, V_1 V_2=\{\rho^o\rho^o, \phi\phi, \omega\phi, \omega\omega \},\\ c) \,\,\gamma\gamma\to {\rm baryon-antibaryon},\\ d) \,\,\gamma^*\gamma\to P^o,\,\, P^o=\{\pi^o, \eta, \eta^\prime, \eta_c\}$.

• The available experimental data are presented.
• A number of theoretical approaches to calculation of these processes is described, both those based mainly on QCD and more phenomenological (the handbag model, the diquark model, etc).
• Some theoretical questions tightly connected with this subject are discussed, in particular: the applications of various types of QCD sum rules, the endpoint behavior of the leading twist meson wave functions, etc.