Exclusive processes in QCD:: Two-photon physics involving baryons

Exclusive two-photon processes involving baryons have been calculated within the framework of perturbative QCD. In particular, the angular dependence of the process $\text{γγ →}\text{p}\text{p}$ is predicted as a function of the universal baryonic distribution amplitudes φ(x_i). The overall normalization is then found by a comparison with the process $\text{Ψ →}\text{p}\text{p}$, so that we can present a unique, absolutely normalized QCD prediction for the two-photon processes. We argue that these predictions, based on the $\text{Ψ →}\text{p}\text{p}$ decay rate, should be more reliable than those based on baryonic form factors. Presently not enough data exist to give a meaningful differential cross section for the process $\text{γγ →}\text{p}\text{p}$. Nevertheless, a total cross section has been obtained. Excluding the region close to threshold, quite good agreement between our predictions and experimental data is found. The analysis in this paper is trivially generalized to other baryons as well.