γγ Cross-sections and γγ colliders

We summarize the predictions of different models for total γγ cross-sections. The experimentally observed rise of σγγ with s radical γγ, faster than that for σp-barp, σγp is in agreement with the predictions of the Eikonalized Minijet Models as opposed to those of the Regge-Pomeron models. We then show that a measurement of σγγ with an accuracy of < or approx. 8-9% (6-7%) is necessary to distinguish among different Regge-Pomeron type models (the different parameterisations of the EMM models) and a precision of < or approx. 20% is required to distinguish the predictions of the EMMs and of those models which treat 'photon like a proton', for the energy range 300< s radicalγγ <500 GeV. We further show that the difference in model predictions for σγγ of about a factor 2 at s radical γγ = 700 GeV reduces to ∼ 30% when folded with bremsstrahlung γ spectra to calculate σ(e+e-→e+e-γγ→e+e-X). We point out then the special role that γγ colliders can play in shedding light on this all-important issue of calculation of total hadronic cross-sections