Large-angle two-photon exclusive channels in quantum chromodynamics

Detailed leading-order quantum-chromodynamics (QCD) predictions are given for the scaling, angular, and helicity dependence of the reactions $\gamma \gamma \to M\overline{M}$ ($M=\pi ,K,\rho$, etc.) at large momentum transfer. In addition to providing a basic test of QCD at short distances, measurements can be used to determine the process-independent meson distribution amplitudes ${\phi }_M\mathrm{}(x,Q)\mathrm{}$. Other related two-photon channels such as $\gamma \gamma \to \gamma \rho$, ${\gamma }^{*}\gamma \to {\pi }^0,{\eta }^0,{\eta }^{\prime }$, and ${\eta }_c\to \gamma \gamma$ are also discussed. We also prove the existence of a fixed Regge singularity at $J=0\mathrm{}$ which couples to $\gamma \rho \to \gamma \rho$ in the $t$ channel but not to $\gamma \pi \to \gamma \pi$.