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Register a new userStructure of Real and Virtual Photons from ZEUS
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N. Macdonald
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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 data, resolved photon processes are required up to a photon virtuality of at least 4.5 GeV^2.
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Triple differential dijet cross-sections in e p interactions measured with the H1 and ZEUS detectors at HERA are presented. The data are compared to Monte Carlo simulations which differ in their assumptions about photon structure and parton evolution. Effects of the resolved processes of longitudinally polarized virtual photons at HERA are investigated for the first time.
Recent results from the H1 and ZEUS collaborations on hard QCD processes in ep interactions are reviewed. The topics cover jet shapes, the structure of real and virtual photons, and the production of J/Psi mesons and open charm.
The structure of real and virtual photons has been studied in electron-proton scattering processes producing di-jet events at HERA by the H1 and ZEUS collaborations. Data have been compared to next-to-leading order QCD calculations and to the predictions of Monte Carlo generators based on the DGLAP and CCFM formalisms for describing the parton dynamics
The photoproduction of isolated photons has been measured in diffractive events recorded by the ZEUS detector at HERA. Cross sections are evaluated in the photon-proton transverse-energy and pseudorapidity ranges 5<ET(gamma)<15 GeV and -0.7<eta(gamma)<0.9 inclusively and also with a jet with transverse energy and pseudorapidity in the range 4<ET(jet)<35 GeV and -1.5<eta(jet)<1.8, using a total integrated electron-proton luminosity of 374 pb-1. A number of kinematic variables were studied and compared to predictions from the RAPGAP Monte Carlo model. An excess of data is observed above the RAPGAP predictions for z|P(meas)>0.9, where z|P(meas) is the fraction of the longitudinal momentum of the colourless Pomeron exchange that is transferred to the photon-jet final state, giving evidence for direct Pomeron interactions.
Recent results on light hadron spectroscopy are reported, with special emphasis on the evidence for a narrow baryonic state decaying to Ks p and Ks pbar, compatible with the pentaquark state theta^+ observed by fixed target experiments. The data were collected with the ZEUS detector at HERA using an integrated luminosity of 121 pb^-1. The analyses were performed in the central rapidity region of inclusive deep inelastic scattering at an ep centre-of-mass energy of 300-318 GeV. Evidence for a narrow resonance in the Ks p and Ks pbar invariant mass spectrum is obtained, with mass 1521.5 pm 1.5(stat)^{+2.8}_{-1.7}(syst) and width consistent with the experimental resolution. If the Ks p part of the signal is identified with the strange pentaquark theta^+, the Ks pbar part is the first evidence for its antiparticle, thetabar^-. Supporting results on other light hadron resonances are also discussed.
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