REAL AND VIRTUAL PHOTON CONTRIBUTIONS TO INELASTIC EP SCATTERING

We investigate direct and resolved photon contributions o hard ep scat tering at high energies. Some terms of the direct contribution are alr eady included in the resolved photon one, described in terms of the ph oton structure functions. A suitable subtraction mechanism has to be f ound in order to avoid double counting. Inelastic ep scattering requir es the knowledge of photon structure functions over a wide range in ph oton virtuality, in particular from approximately zero up to almost 10 (5) GeV2 at HERA. We analyse the real and virtual photon structure fun ction and we comment on the structure of the photon in the intermediat e region of low virtuality. We finally describe how ep cross sections can be obtained as functions of the virtuality of the exchanged photon .