QCD Saturation and $\gamma^*-\gamma^*$ Scattering

Two photon collisions at high energy have an important theoretical advantage: the simplicity of the initial state, which gives us a unique opportunity to calculate these processes for large virtualities of both photons in perturbative QCD approach. In this paper we study QCD saturation in two photon collisions in the framework of the Glauber-Mueller approach. The Glauber-Mueller formula is derived emphasizing the impact parameter dependence ($b_t$) of the dipole-dipole amplitude. It is shown that non-perturbative QCD contributions are needed to describe large $b_t$-behaviour, and the way how to deal with them is suggested. Our approach can be viewed as the model for the saturation in which the entire impact parameter dependence is determined by the initial conditions. The unitarity bound for the total cross section, its energy dependence as well aspredictions for future experiments are discussed.It is argued that the total cross section increases faster than any power of $\log(1/x)$ in a wide range of energy or $x$, namely $\sigma(\gamma^*-\gamma^*) \propto (1/Q^2) exp(a \sqrt{\ln(1/x)}) \leq 1/m^2_{\pi}$ where $exp(a \sqrt{\ln(1/x)})$ reflects the $x$ dependence of the gluon density $xG \propto exp(2 a \sqrt{\ln(1/x)})$ and $m_{\pi}$ is the pion mass.