Hard exclusive two photon processes in QCD

TL;DR

This paper reviews hard two-photon processes in QCD, discussing experimental data and various theoretical models including QCD-based and phenomenological approaches, and addresses related theoretical questions.

Contribution

It provides a comprehensive overview of experimental results and compares different theoretical models for two-photon processes in QCD, highlighting areas for future research.

Findings

Experimental data on two-photon processes are summarized.

Different theoretical approaches, including QCD and phenomenological models, are discussed.

Theoretical questions like QCD sum rules and meson wave functions are examined.

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.