Resonance production in γγ collisions

The processes γγ → hadrons can be depicted as follows. One photon creates a q anti q pair which starts to evolve; the other photon can either (A) make its own q anti q pair and the (q anti q q anti q) system continue to evolve or (B) interact with the quarks of the first pair and lead to a modified (q anti q) system in interaction with C = +1 quantum numbers. A review of the recent theoretical activity concerning resonance production and related problems is given under the following headings: hadronic C = +1 spectroscopy (q anti q, qq anti q anti q, q anti q g, gg, ggg bound states and mixing effects); exclusive γγ processes (generalities, unitarized Born method, VDM and QCD); total cross section (soft and hard contributions); q2 dependence of soft processes (soft/hard separation, 1/sup +- +/ resonances); and polarization effects