F_2^gamma at low Q^2 and sigma_{\gamma \gamma} at high energies

Abstract

The parametrisation of the photon structure function in the low Q^2 region is formulated. It includes the VMD contribution and the QCD improved parton model component suitably extrapolated to the low Q^2 region. The parametrisation describes reasonably well existing experimental data on sigma_{\gamma \gamma} for real photons and the low Q^2 data on sigma_{\gamma^* \gamma}. Predictions for sigma_{\gamma \gamma} and for sigma_{\gamma^* \gamma} for energies which may be accesssible in future linear colliders are also given.