Exclusive Photon-Photon Processes

Exclusive $\gamma\gamma \to$ hadron pairs are among the most fundamental processes in QCD, providing a detailed examination of Compton scattering in the crossed channel. In the high momentum transfer domain $(s,t, {large},\theta_{cm}$ for $t/s$ fixed), these processes can be computed from first principles in QCD, yielding important information on the nature of the QCD coupling $\alpha_s$ and the form of hadron distribution amplitudes. Similarly, the transition form factors $\gamma^*\gamma$, $\gamma^*\gamma \to \pi^0, \eta^0,\eta^\prime,\eta_c ...$ provide rigorous tests of QCD and definitive determinations of the meson distribution amplitudes $\phi_H(x,Q)$. We show that the assumption of a frozen coupling at low momentum transfers can explain the observed scaling of two-photon exclusive processes.