Dijet angular distributions in direct and resolved at HERA

Abstract Jet photoproduction, where the two highest transverse energy (Eg) jets have E$ above 6 GeV and a jet-jet invariant mass above 23 GeV, has been studied with the ZEUS detector at the HERA ep collider. Resolved and direct photoproduction samples have been separated. The cross section as a function of the angle between the jet-jet axis and the beam direction in the dijet rest frame has been measured for the two samples. The measured angular distributions differ markedly from each other. They agree with the predictions of QCD calculations, where the different angular distributions reflect the different spins of the quark and gluon exchanged in the hard subprocess. l Also at IROE Florence, Italy. 2 Now at Univ. of Salerno and INFN Napoli, Italy. 3 Supported by Worldlab, Lausanne, Switzerland. 4 Now as MINERVA-Fellow at Tel-Aviv University. 5 Now at Univ. of California, Santa Cruz. 6 Now at VDI-Technologiezentrum Dusseldorf. 7 Now at Commasoft, Bonn. *Also at University of Torino and Alexander von Humboldt Fellow. g Alexander von Humboldt Fellow. I0 Alfred P. Sloan Foundation Fellow. l1 Now at University of Washington, Seattle. l2 Now at California Institute of Technology, Los Angeles. l3 Supported by an EC fellowship number ERBFMBICT 950172. l4 Now at Inst. of Computer Science, Jagellonian Univ., Cracow. l5 Visitor from Florida State University. t6 Now at DESY Computer Center. l7 Supported by European Community Program PRAXIS XXI. I8 Now at Univ. de Strasbourg. I9 Present address: Dipartimento di Fisica, Univ. “La Sapienza”, Rome. 2o Now at ATLAS Collaboration, Univ. of Munich. 21 Also supported by NSERC, Canada. 22 Supported by an EC fellowship. 23 PPARC Post-doctoral Fellow. 24 Now at Park Medical Systems Inc., Lachine, Canada. 25 Partially supported by DESY. 26 Now at Philips Natlab, Eindhoven, NL. *7 Now at Department of Energy, Washington. 28 Also at University of Hamburg, Alexander von Humboldt Re- search Award. 29 Now at Lawrence Berkeley Laboratory, Berkeley. 3o Now at Yale University, New Haven, CT. 31 Supported by a MINERVA Fellowship. 32 Supported by the Japan Society for the Promotion of Science (JSPS). 33 Present address: Tokyo Metropolitan College of Allied Medical Sciences, Tokyo 116, Japan. 34 Supported by the Polish State Committee for Scientific Re- search, grant No. 2PO3B09308. 35 Supported by the Polish State Committee for Scientific Re- search, grant No. 2P03B09208. 36 Supported by the Natural Sciences and Engineering Research Council of Canada