γγ Cross-sections and γγ colliders

Abstract

We summarize the predictions of different models for total γγ cross sections. The experimentaly observed rise of σ_γγ with √s_γγ, faster than that for σ̅pp, σ_γp 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 σ_γγ with an accuracy of ≲ / 8- 9%(6 - 7%) is necessary to distinguish among different Regge-Pomeron type models (among the different parametrisations of the EMM models) and a precision of ≲ / 20% is required to distinguish among the predictions of the EMMs and of those models which treat like 'photon like a proton', for the energy range 300 < √s_γγ < 500 GeV. We further show that the difference in model predictions for σ_γγ of about a factor 2 at √s_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.