PL9700208 28 Section V ,.,.-; ZEUS EXPERIMENT AT HERA ' ZEUS Collaboration 2 Group from the Institute of Nuclear Physics includes: P. Borzeniski, J. Chwastowski, A. Eskreys, K. Piotrzkowski, M. Przybycieri, M. Zachara, L. Zawiejski (physicists) and J. Andruszkow, B. Dajarowski, W. Daniluk, P. Jurkiewicz, A. Kotarba, K. Oliwa. W. Wierba (engineers and technicians) Third year of HERA operation was a very successful one for the HERA crew. Experiments (ZEUS and HI) collected more than 3pb~1 of the integrated luminosity compared to ()00nb~^ in the previous year. Higher luminosity means also higher rates and higher background. That created new problems for our experiment at the beginning of the running period. The common effort of all groups taking care of the different detector components resulted in a smooth and successful running of the ZEUS experiment. The main responsibility of the Cracow group is the reliable performance of the Luminosity Monitor, which has been designed and built by our group. That means the maintenance of the detector, continuous development of the online and offline software tools and the luminosity measurement itself. A very detailed offline analysis of the 1993 luminosity data was done in 1994. This resulted in much better understanding of the systematic effects influencing the luminosity measurement. The systematic error in the determination of the integrated luminosity is now below 2.5%. Modification of the hardware setup, especially replacement of the photomultipliers in the calorimeters, made during the 1993/1994 winter shutdown, allowed for more stable opera- tion in the 1994 running period. Much improvement has also been achieved in both offline and online software. Several corrections for the systematic effects have been included into the online luminosity measurement scheme what makes this measurement much more accurate. The offline procedure, including calibration of the calorimeters and the luminosity recalculation, has been automatized. Much effort has been put into the development of the Monte Carlo simulation of the beam line structure. Members of the Cracow INP group have been taking part in the ZEUS data taking shifts. They have also contributed to the development of the general ZEUS online and offline software. Our physicists have actively participated in the physical analysis of the collected data and have significantly contributed to studies of the photoproduction processes, where the luminosity monitor used for tagging of almost real photons plays a very important role. Both HERA experiments (ZEUS and HI) have continued studies started with small data samples from 1992, using much larger data samples collected in 1993. This enriched statistics 'Polish ZEUS groups activity is partially supported by the State Committee for Scientific Research (grant No SPUB/P3/202/94) and Foundation for German-Polish Collaboration (Project No 506/92); 2Participating institutions: Argonne National Laboratory, University and INFN (Bologna), L'niversitat Bonn, Bristol University, Brookhaven National Laboratory, Calabria University and 1NFN (Cosenza), Columbia Univer- sity, Institute of Nuclear Physics (Cracow), Faculty of Physics and Nuclear Techniques of the Academy of Mining and Metallurgy (Cracow), Jagellonian University (Cracow), DESY (Hamburg), DESY-Zeuthen (Zeuthen), Uni- versity and INFN (Florence), INFN (Frascati), Universitat Freiburg, University of Glasgow, Hamburg University (I. and II. Institutes of Physics), Imperial College London, University of Iowa (Iowa City), Institut fiir Kern- physik (Julich), Korea University (Seoul), Louisiana State University (Baton Rouge), Univer. Autonoma Madrid, University of Manitoba, McGill University (Montreal), Moscow State University, NIKIIEF (Amsterdam), Ohio State University (Columbus), University of Oxford, University and INFN (Padova), Pennsylvania State University (University Park), Univ. 'La Sapienza' (Rome), Rutherford Appleton Laboratory (Chilton.Didcot), University of California (Santa Cruz), Universitat-Gesamthochschule Siegen, Tel-Aviv University, University of Tokyo, Tokyo Metropolitan University, University and INFN (Torino), University of Toronto, University College London, Vir- ginia Polytechnic Institute (Blacksburg), Warsaw University, Institute for Nuclear Studies (Warsaw). Weizmann Institute (Rehovot). University of Wisconsin (Madison), York University (North York). PL9700209 Section V 29 allowed also new analyses of the processes with smaller cross-sections to be made. Results of the ZEUS physics analysis, based mainly on the data collected in two previous years, have been published in 9 papers. New interesting data concern many aspects of the electron proton interactions and the much larger data sample permits more accurate measurements. We would like to review here briefly on the published results of two different analyses dealing with different type of processes: photoproduction and deep inelastic scattering. Observation of Direct Processes in Photoproduction at HERA 3 ZEUS Collaboration M. Derrick et al. 4 Electron proton scattering is dominated by the exchange of almost real photons. Although most of the photoproduction cross section is due to soft processes, a fraction of the 7p collisions at HERA energies is expected to contain high-pj processes. In lowest-order QCD, these hard processes are of two main types [1, 2], as shown in Fig. 1. In the direct processes, the photon participates as a point-like particle, interacting with a gluon (jg —* qq, photon gluon fusion) or a quark (jq —* gq, QCD Compton scattering). In the resolved processes, the photon behaves as a source of partons which can scatter off those in the proton. The unscattered constituents of the photon then give rise to a hadronic system, known as the photon remnant, going approximately in the direction of the original photon. a) b) Figure 1: Schematic diagrams showing examples of (a) a direct process and (b) a resolved process. Hard scattering in photoproduction should produce multi-jet structures with features similar to hadron-hadron collisions [3]. QCD-based models of these processes predict that the resolved processes should dominate over the direct for a wide range of jet transverse energy [2, 4]. The presence of hard scattering in ~jp collisions has already been observed at HERA [5, 6], with evidence for multi-jet structures and for the presence of the resolved contribution. The main components of the ZEUS detector [5, 7] used in this analysis are the high resolution uranium-scintillator calorimeter, the central tracking detector and the luminosity monitor. The analyzed data were collected during 1992, when 820 GeV protons were colliding with 26.7 GeV electrons. The hard photoproduction data sample was selected using criteria described in [5] and 3Phys. Lett. B322 (1994) 287, 4INP authors: P. Borzemski, J. Chwastowski, A. Eskreys, K. Piotrzkowski, M. Zachara, and L. Zawiejski..