Structure of Real and Virtual Photons from ZEUS

Measurements sensitive to the structure of both real and virtual photons are presented and compared to theoretical models with various photon parton distribution functions (PDFs). Measurements for real photons show a tendency for the available photon PDFs to be too small to describe the data. For virtual photons, the photon PDF is seen to decrease with increasing photon virtuality. In order to describe the Experimental information on the partonic structure of the photon can be obtained from the data taken at the HERA ep collider experiments. Leading order (LO) QCD predicts that photon interactions have a twocomponent nature. In direct photon processes the entire momentum of the photon takes part in the hard subprocess with a parton from the proton whereas in resolved photon processes the photon acts as a source of