Hard exclusive two photon processes in QCD

Abstract

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.