Study of the Structure of the Photon in e-p Collision

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We studied the structure of photon in electron-proton collisions at HERA by using a multi-purpose detector built in an international collaboration (ZEUS) at DESY in Germany The study is performed mainly in photoproduction processes where an electron emits an almost real photon that collides with a proton. The photon behaves like a hadron in the photoproduction process. In particular, the high-E_Tprocesses (hard photoproduction) are expected to be described by perturbative Quantum Chromodynamics (pQCD) as in the hadron-hadron collisions. The measurements of the jet and the heavy quark production provide a testing ground of pQCD.The structure of the photon can be investigated using the resolved processes where the photon is resolved into more than one parton. Photon-proton collisions also include diffractive processes where the photon dissociates into hadrons preserving its quantum numbers. The study of the diffractive process enables us to study the nature of the diffraction and the str … More ucture of the Pomeron, which is introduced in order to explain the cross section behavior of the hadron-hadron collisions.The measured cross sections of the jet production show good agreements with the prediction of pQCD.Those measurements imposed experimental restrictions on certain models based on pQCD, in particular on the structure of the photon. The measurement of the transverse momentum distribution of the photon remnant is consistent with the existence of the point-like coupling of the photon to a quark-pair, which is predicted by pQCD.The cross section of the charm quark production supports the model that allows a charm pair to be produced virtually inside the photon. The measurement also disclosed that the gluon density in the proton rapidly increases as it becomes soft.The large rapidity gap (LRG) in the produced hadrons is the characteristic property of the diffractive process. The behavior of LRG cross section showed that the diffractive process could be understood by the exchange of Pomeron. We measured the quark distribution of the Pomeron, and proved the existence of the gluon content in the Pomeron. Further, the evidence of the exchange of the Pomeron is found in the hard parton-parton collisions in photoproduction. The cross section of the vector meson production is explained by pQCD for the processes that involve a hard scale. Less