Predicitions for high-energy real and virtual photon-photon scattering from color dipole BFKL-Regge factorization

Abstract

High-energy virtual photon-virtual photon scattering can be viewed as interaction of small size color dipoles from the beam and target photons, which makes $γ^{*}γ^{*}, γ^{*}γ$ scattering at high energies (LEP, LEP200 & NLC) an indispensable probe of short distance properties of the QCD pomeron exchange. Based on the color dipole representation, we investigate consequences for the $γ^{*}γ^{*},γ^{*}γ$ scattering of the incorporation of asymptotic freedom into the BFKL equation which makes the QCD pomeron a series of isolated poles in the angular momentum plane. The emerging color dipole BFKL-Regge factorization allows us to relate in a model-independent way the contributions of each BFKL pole to $γ^{*}γ^{*},γ^{*} γ$ scattering and DIS off protons. Numerical predictions based on our early works on color dipole BFKL phenomenology of DIS on protons are in a good agreement with the experimental data on the photon structure function $F_{2γ}$ and most recent data on the $γ^*γ^*$ cross section $σ^{γ^*γ^*}(Y)$ from OPAL and L3 experiments at LEP200. We discuss the role of non-perturbative dynamics and predict pronounced effect of the Regge-factorization breaking due to large unfactorizable non-perturbative corrections to the perturbative vacuum exchange. We comment on the salient features of the BFKL-Regge expansion for $γ^{*}γ^{*},γ^{*}γ$ scattering including the issue of decoupling of subleading BFKL poles and the soft plus rightmost hard BFKL pole dominance .