Measurement of Three-jet Distributions in Photoproduction at HERA

The cross section for the photoproduction of events containing three jets with a three-jet invariant mass of M3J > 50 GeV has been measured with the ZEUS detector at HERA. The three-jet angular distributions are inconsistent with a uniform population of the available phase space but are well described by parton shower models andO(αα s ) pQCD calculations. Comparisons with the parton shower model indicate a strong contribution from initial state radiation as well as a sensitivity to the effects of colour coherence. The ZEUS Collaboration J. Breitweg, S. Chekanov, M. Derrick, D. Krakauer, S. Magill, B. Musgrave, 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. Castellini1, L. Cifarelli2, 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 Garcia3, A. Zichichi University and INFN Bologna, Bologna, Italy f C. Amelung, A. Bornheim, I. Brock, K. Coböken, J. Crittenden, R. Deffner, M. Eckert, M. Grothe4, H. Hartmann, K. Heinloth, L. Heinz, E. Hilger, H.-P. Jakob, A. Kappes, U.F. Katz, R. Kerger, E. Paul, M. Pfeiffer, H. Schnurbusch, A. Weber, H. Wieber Physikalisches Institut der Universität Bonn, Bonn, Germany c D.S. Bailey, O. Barret, W.N. Cottingham, B. Foster, R. Hall-Wilton, G.P. Heath, H.F. Heath, J.D. McFall, D. Piccioni, 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. Pac5 Chonnam National University, Kwangju, Korea h A. Caldwell6, N. Cartiglia, Z. Jing, W. Liu, B. Mellado, J.A. Parsons, S. Ritz7, R. Sacchi, S. Sampson, F. Sciulli, Q. Zhu Columbia University, Nevis Labs., Irvington on Hudson, N.Y., USA q P. Borzemski, J. Chwastowski, A. Eskreys, J. Figiel, K. Klimek, M.B. Przybycień, L. Zawiejski Inst. of Nuclear Physics, Cracow, Poland j L. Adamczyk8, 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 Z. Duliński, A. Kotański Jagellonian Univ., Dept. of Physics, Cracow, Poland k L.A.T. Bauerdick, U. Behrens, H. Beier9, J.K. Bienlein, C. Burgard, K. Desler, G. Drews, U. Fricke, F. Goebel, P. Göttlicher, R. Graciani, T. Haas, W. Hain, G.F. Hartner, D. Hasell10, K. Hebbel, K.F. Johnson11, M. Kasemann12, W. Koch, U. Kötz, H. Kowalski, L. Lindemann, B. Löhr, M. Mart́ınez, J. Milewski13, M. Milite, T. Monteiro14, D. Notz, A. Pellegrino, F. Pelucchi, K. Piotrzkowski, M. Rohde, J. Roldán15, J.J. Ryan16, P.R.B. Saull, A.A. Savin, U. Schneekloth, O. Schwarzer, F. Selonke, M. Sievers, S. Stonjek, B. Surrow14, E. Tassi, D. Westphal17, G. Wolf, U. Wollmer, C. Youngman, W. Zeuner Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany