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 sγγ, faster than that for σp̄p, 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 ⪅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 ⪅ 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<sγγ<500 GeV. We further show that the difference in model predictions for σγγ of about a factor 2 at sγγ=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.