γγ and γP events at high energies

A real photon has a complicated nature, whereby it may remain unresolved or fluctuate into a vector meson or a perturbative qq pair. Based on this picture, we previously presented a model for γp events that is based on the presence of three main event classes: direct, VMD and anomalous. In γ γ events, a natural generalization gives three-by-three combinations of the nature of the two incoming photons, and thus six distinct event classes. The properties of these classes are constrained by the choices already made, in the γp model, of cut-off procedures and other aspects. It is therefore possible to predict the energy-dependence of the cross section for each of the six components separately. The total cross section thus obtained is in good agreement with data, and also gives support to the idea that a simple factorized ansatz with a pomeron and a reggeon term can be a good approximation. Event properties undergo a logical evolution from pp to γp to γ γ events, with larger charged multiplicity, more transverse energy flow and a higher jet rate in the latter process. (authors). 14 refs., 14 figs