Exclusive Photon - Photon Processes

Exclusive yy -+ hadron pairs are among the most fundamental processes in &CD, providing a detailed examination of Compton scattering in the crossed channel. In the high momentum transfer domain (s, t, large, 19,~ for t/s fixed), these processes can be computed from first principles in &CD, yielding important information on the nature of the QCD coupling a, and the form of hadron distribution amplitudes. Similarly, the transition form factors y*y, y*y + 7-r’, q”, q’, qc. . . provide rigorous tests of QCD and definitive determinations of the meson distribution amplitudes ~H(z, Q). We show that the assumption of a frozen coupling at low momentum transfers can explain the observed scaling of two-photon exclusive processes.