High-ET dijet photoproduction at HERA

The cross section for high-ET dijet production in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of 81.8 pb. The events were required to have a virtuality of the incoming photon, Q, of less than 1GeV and a photon-proton centre-of-mass energy in the range 142 < Wγp < 293GeV. Events were selected if at least two jets satisfied the transverse-energy requirements of E T > 20GeV and E jet2 T > 15GeV and pseudorapidity requirements of −1 < η < 3, with at least one of the jets satisfying −1 < η < 2.5. The measurements show sensitivity to the parton distributions in the photon and proton and effects beyond next-to-leading order in QCD. Hence these data can be used to constrain further the parton densities in the proton and photon. The ZEUS Collaboration S. Chekanov, M. Derrick, S. Magill, B. Musgrave, D. Nicholass, J. Repond, R. Yoshida Argonne National Laboratory, Argonne, Illinois 60439-4815, USA n M.C.K. Mattingly Andrews University, Berrien Springs, Michigan 49104-0380, USA M. Jechow, N. Pavel , A.G. Yagües Molina Institut für Physik der Humboldt-Universität zu Berlin, Berlin, Germany S. Antonelli, P. Antonioli, G. Bari, M. Basile, L. Bellagamba, M. Bindi, D. Boscherini, A. Bruni, G. Bruni, L. Cifarelli, F. Cindolo, A. Contin, M. Corradi, S. De Pasquale, G. Iacobucci, A. Margotti, R. Nania, A. Polini, G. Sartorelli, A. Zichichi University and INFN Bologna, Bologna, Italy e D. Bartsch, I. Brock, S. Goers, H. Hartmann, E. Hilger, H.-P. Jakob, M. Jüngst, O.M. Kind, A.E. Nuncio-Quiroz, E. Paul, R. Renner, U. Samson, V. Schönberg, R. Shehzadi, M. Wlasenko Physikalisches Institut der Universität Bonn, Bonn, Germany b N.H. Brook, G.P. Heath, J.D. Morris H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom m M. Capua, S. Fazio, A. Mastroberardino, M. Schioppa, G. Susinno, E. Tassi Calabria University, Physics Department and INFN, Cosenza, Italy e J.Y. Kim, K.J. Ma Chonnam National University, Kwangju, South Korea g Z.A. Ibrahim, B. Kamaluddin, W.A.T. Wan Abdullah Jabatan Fizik, Universiti Malaya, 50603 Kuala Lumpur, Malaysia r Y. Ning, Z. Ren, F. Sciulli Nevis Laboratories, Columbia University, Irvington on Hudson, New York 10027 o J. Chwastowski, A. Eskreys, J. Figiel, A. Galas, M. Gil, K. Olkiewicz, P. Stopa, L. Zawiejski The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Poland i L. Adamczyk, T. Bo ld, I. Grabowska-Bo ld, D. Kisielewska, J. Lukasik, M. Przybycień, L. Suszycki Faculty of Physics and Applied Computer Science, AGH-University of Science and Technology, Cracow, Poland p A. Kotański, W. S lomiński Department of Physics, Jagellonian University, Cracow, Poland