0 O ct 1 99 9 Measurement of inclusive prompt photon photoproduction at HERA ZEUS

First inclusive measurements of isolated prompt photons in photoproduction at the HERA ep collider have been made with the ZEUS detector, using an integrated luminosity of 38.4 pb. Cross sections are given as a function of the pseudorapidity and the transverse energy (η , E γ T ) of the photon, for E γ T > 5 GeV in the γp centreof-mass energy range 134–285 GeV. Comparisons are made with predictions from Monte Carlo models having leading-logarithm parton showers, and with next-toleading-order QCD calculations, using currently available parameterisations of the photon structure. For forward η (proton direction) good agreement is found, but in the rear direction all predictions fall below the data. DESY 99-161 Oct. 1999 The ZEUS Collaboration J. Breitweg, S. Chekanov, M. Derrick, D. Krakauer, S. Magill, B. Musgrave, A. Pellegrino, J. Repond, R. Stanek, R. Yoshida Argonne National Laboratory, Argonne, IL, USA p M.C.K. Mattingly Andrews University, Berrien Springs, MI, USA G. Abbiendi, F. Anselmo, P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, G. Cara Romeo, G. Castellini, L. Cifarelli, F. Cindolo, A. Contin, N. Coppola, M. Corradi, S. De Pasquale, P. Giusti, G. Iacobucci, G. Laurenti, G. Levi, A. Margotti, T. Massam, R. Nania, F. Palmonari, A. Pesci, A. Polini, G. Sartorelli, Y. Zamora Garcia, A. Zichichi University and INFN Bologna, Bologna, Italy f C. Amelung, A. Bornheim, I. Brock, K. Coböken, J. Crittenden, R. Deffner, H. Hartmann, K. Heinloth, E. Hilger, H.-P. Jakob, A. Kappes, U.F. Katz, R. Kerger, E. Paul, J. Rautenberg, H. Schnurbusch, A. Stifutkin, J. Tandler, K.Ch. Voss, A. Weber, H. Wieber Physikalisches Institut der Universität Bonn, Bonn, Germany c D.S. Bailey, O. Barret, N.H. Brook, B. Foster, G.P. Heath, H.F. Heath, J.D. McFall, D. Piccioni, E. Rodrigues, J. Scott, R.J. Tapper H.H. Wills Physics Laboratory, University of Bristol, Bristol, U.K. o M. Capua, A. Mastroberardino, M. Schioppa, G. Susinno Calabria University, Physics Dept.and INFN, Cosenza, Italy f H.Y. Jeoung, J.Y. Kim, J.H. Lee, I.T. Lim, K.J. Ma, M.Y. Pac Chonnam National University, Kwangju, Korea h A. Caldwell, W. Liu, X. Liu, B. Mellado, R. Sacchi, S. Sampson, F. Sciulli Columbia University, Nevis Labs., Irvington on Hudson, N.Y., USA q J. Chwastowski, A. Eskreys, J. Figiel, K. Klimek, K. Olkiewicz, M.B. Przybycień, P. Stopa, L. Zawiejski Inst. of Nuclear Physics, Cracow, Poland j L. Adamczyk, B. Bednarek, K. Jeleń, D. Kisielewska, A.M. Kowal, T. Kowalski, M. Przybycień, E. Rulikowska-Zarȩbska, L. Suszycki, J. Zaja̧c Faculty of Physics and Nuclear Techniques, Academy of Mining and Metallurgy, Cracow, Poland j A. Kotański Jagellonian Univ., Dept. of Physics, Cracow, Poland k