The reaction γp → ωp ( ω → π + π − π 0 and π 0 → γγ ) has been studied in ep interactions using the ZEUS detector at photon-proton centre-of-mass energies between 70 and 90 GeV and | t | < 0 . 6 GeV 2 , where t is the squared four momentum transferred at the proton vertex. The elastic ω photoproduction cross section has been measured to be σ γp → ωp = 1 . 21 ± 0 . 12 ± 0 . 23 µ b. The differential cross section dσ γp → ωp /d | t | has an exponential shape e − b | t | with a slope b = 10 . 0 ± 1 . 2 ± 1 . 3 GeV − 2 . The angular distributions of the decay pions are consistent with s -channel helicity conservation. When compared to low energy data, the features of ω photoproduction as measured at HERA energies are in agreement with those of a soft diffractive process. Previous measurements of the ρ 0 and φ photoproduction cross sections at HERA show a similar behaviour.

Inclusive photoproduction of D in ep collisions at HERA has been measured with the ZEUS detector for photon-proton centre of mass energies in the range 115 < W < 280 GeV and photon virtuality Q < 4GeV. The cross section σep→D∗X integrated over the kinematic region p D ⊥ > 3 GeV and −1.5 < ηD∗ < 1.0 is (10.6 ± 1.7(stat .) ± 1.3 (syst .)) nb. Differential cross sections as functions of p ∗ ⊥ , η ∗ and W are given. The data are compared with two next-to-leading order perturbative QCD predictions. For a calculation using a massive charm scheme the predicted cross sections are smaller than the measured ones. A recent calculation using a massless charm scheme is in agreement with the data. DESY-97-026 The ZEUS Collaboration J. Breitweg, M. Derrick, D. Krakauer, S. Magill, D. Mikunas, B. Musgrave, J. Repond, R. Stanek, R.L. Talaga, R. Yoshida, H. Zhang Argonne National Laboratory, Argonne, IL, USA p M.C.K. Mattingly Andrews University, Berrien Springs, MI, USA 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, M. Corradi, I. Gialas, P. Giusti, G. Iacobucci, G. Laurenti, G. Levi, A. Margotti, T. Massam, R. Nania, F. Palmonari, S. De Pasquale, 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, M. Eckert, L. Feld, M. Grothe, H. Hartmann, K. Heinloth, L. Heinz, E. Hilger, H.-P. Jakob, U.F. Katz, E. Paul, M. Pfeiffer, Ch. Rembser, J. Stamm, R. Wedemeyer Physikalisches Institut der Universität Bonn, Bonn, Germany c D.S. Bailey, S. Campbell-Robson, W.N. Cottingham, B. Foster, R. Hall-Wilton, M.E. Hayes, G.P. Heath, H.F. Heath, D. Piccioni, D.G. Roff, R.J. Tapper H.H. Wills Physics Laboratory, University of Bristol, Bristol, U.K. o M. Arneodo, R. Ayad, M. Capua, A. Garfagnini, L. Iannotti, M. Schioppa, G. Susinno Calabria University, Physics Dept.and INFN, Cosenza, Italy f J.Y. Kim, J.H. Lee, I.T. Lim, M.Y. Pac Chonnam National University, Kwangju, Korea h A. Caldwell, N. Cartiglia, Z. Jing, W. Liu, J.A. Parsons, S. Ritz, S. Sampson, F. Sciulli, P.B. Straub, Q. Zhu Columbia University, Nevis Labs., Irvington on Hudson, N.Y., USA q P. Borzemski, J. Chwastowski, A. Eskreys, Z. Jakubowski, M.B. Przybycień, M. Zachara, L. Zawiejski Inst. of Nuclear Physics, Cracow, Poland j L. Adamczyk, B. Bednarek, K. Jeleń, D. Kisielewska, T. Kowalski, M. Przybycień, E. RulikowskaZarȩbska, L. Suszycki, J. Zaja̧c Faculty of Physics and Nuclear Techniques, Academy of Mining and Metallurgy, Cracow, Poland j Z. Duliński, A. Kotański A. Kotański Jagellonian Univ., Dept. of Physics, Cracow, Poland k

Charged particle production has been measured in deep inelastic scattering (DIS) events over a large range of x and Q using the ZEUS detector. The evolution of the scaled momentum, xp, with Q , in the range 10 to 1280 GeV, has been investigated in the current fragmentation region of the Breit frame. The results show clear evidence, in a single experiment, for scaling violations in scaled momenta as a function of Q. DESY 97-183

Dijet production by almost real photons has been studied at HERA with the ZEUS detector. Jets have been identiied using the cone algorithm. A cut on x OBS , the fraction of the photon energy participating in the production of the two jets of highest transverse energy, is used to deene cross sections sensitive to the parton distributions in the proton and in the photon. The dependence of the dijet cross sections on pseudorapidity has been measured for x OBS 0:75 and x OBS < 0:75. The former is sensitive to the gluon momentum density in the proton. The latter is sensitive to the gluon in the photon. The cross sections are corrected for detector acceptance and compared to leading order QCD calculations.

Jet photoproduction, where the two highest transverse energy (E jet T) jets have E jet T 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 diier markedly from each other. They agree with the predictions of QCD calculations, where the diierent angular distributions reeect the diierent spins of the quark and gluon exchanged in the hard subprocess.

Exclusive production of 0 and J== mesons in e + p collisions has been studied with the ZEUS detector in the kinematic range 0:25 < Q 2 < 50 GeV 2 , 20 < W < 167 GeV for the 0 data and 2 < Q 2 < 40 GeV 2 , 50 < W < 150 GeV for the J== data. Cross sections for exclusive 0 and J== production have been measured as a function of Q 2 , W and t. The spin-density matrix elements r 04 00 , r 1 1?1 and Re r 5 10 have been determined for exclusive 0 production as well as r 04 00 and r 04 1?1 for exclusive J== production. The results are discussed in the context of theoretical models invoking soft and hard phenomena.

This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with ?L > 0.97, where xL is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of t, the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section ont is measured in the interval 0.073 t| 2 and is found to be described by an exponential shape with the slope parameterb = 7.2 ± 1.1(stat.)-0.9+0.7(syst.) GeV-2. The diffractive structure function FD (4) is presented as 0.9 a function of ?H ? 1 - ?L and ß, the momentum fraction of the struck quark with respect to ?H, and averaged over thet interval 0.073 2 and the photon virtuality range 5 Q2 2. In the kinematic range 4 × 104 p p dependence ofFD(4) is fitted with a form (1/?p)a , yieldinga - 1.00 ± 0.09 (stat.)-0.05+0.11(syst.). Upon integration overL, the structure functionF2D(3) is determined in a kinematic range extending to higher ?p and lower ß compared to our previous analysis; the results are discussed within the framework of Regge theory.

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 Research 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 Research, grant No. 2PO3B09308. 35 Supported by the Polish State Committee for Scientific Research, grant No. 2P03B09208. 36 Supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) 37 Supported by the FCAR of Quebec, Canada. 38 Supported by the German Federal Ministry for Education and Science, Research and Technology (BMBF) , under contract numbers 056BN191, 056FR19R 056HHl91, 056HH291, 056SI791. 39 Supported by the MINERVA Gesellschaft fiir Forschung GmbH, the Israel Academy of Science and the U.S.-Israel Binational Science Foundation. a Supported by the German Israeli Foundation, and by the Israel Academy of Science. 41 Supported by the Italian National Institute for Nuclear Physics 406 ZEUS Collaboration/Physics Letters B 384 (1996) 401-413

The production of ~b mesons in the reaction e+p --+ e+~bp (qb ~ K + K ) , for 7 < Q2 < 25 GeV 2 and for virtual photon-proton centre of mass energies (W) in the range 42-134 GeV, has been studied with the ZEUS detector at HERA. When compared to lower energy data at similar Q2, the results show that the 7*P --~ ~bp cross section rises strongly with W. This behaviour is similar to that previously found for the y*p ~ pOp cross section. This strong dependence cannot be explained by production through soft pomeron exchange. It is, however, consistent with perturbative QCD expectations, where it reflects the rise of the gluon momentum density in the proton at small x. The ratio of tr(qb)fir(p°), which has previously been determined by ZEUS to be 0.065 + 0.013 (stat.) in photoproduction at a mean W of 70 GeV, is measured to be 0.18 + 0.05 (stat.) -40.03 (syst.) at a mean Q2 of 12.3 GeV 2 and mean W of ~ 100 GeV and is thus approaching at large Q2 the value of 2 / 9 predicted from the quark charges of the vector mesons and a flavour independent production mechanism. 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 Tei-Aviv University. 5 now at Univ. of California, Santa Cruz. 6 now at VDI-Technologiezentrum DiJsseldorf. 7 now at ESG, Miinchen. 8 also at University of Torino and Alexander yon Humboldt Fellow. 9 Alexander von Humboldt Fellow. 10 Alfred E Sloan Foundation Fellow. it now at University of Washington, Seattle. 12 now at California Institute of Technology, Los Angeles. 13 supported by an EC fellowship number ERBFMBICT 950172. 14 now at Inst. of Computer Science, Jagellonian Univ., Cracow. 15 visitor from Florida State University. z6 now at DESY Computer Center. 17 supported by European Community Program PRAXIS XXI. 18 now at Univ. de Strasbourg. 19 present address: Dipartimento di Fisica, Univ. "La Sapienza', Rome. 2o also supported by NSERC, Canada. 21 supported by an EC fellowship. 22 PPARC Post-doctoral Fellow. 23 now at Park Medical Systems Inc., Lachine, Canada. 24 partially supported by DESY. 25 now at Philips Natlab, Eindhoven, NL. 26 now at Department of Energy, Washington. 27 also at University of Hamburg, Alexander von Humboldt Research Award. 28 now at Lawrence Berkeley Laboratory, Berkeley. 29 now at Yale University, New Haven, CT. 30 supported by a MINERVA Fellowship. 31 supported by the Japan Society for the Promotion of Science (JSPS). 32 present address: Tokyo Metropolitan College of Allied Medical Sciences, Tokyo 116, Japan. 33 supported by the Polish State Committee for Scientific Research, grant No. 2P03B09308. 34 supported by the Polish State Committee for Scientific Research, grant No. 2P03B09208. 35 supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). 36 supported by the FCAR of Qurbec, Canada. ZEUS Collaboration/Physics Letters B 380 (1996) 220-234 225

Elastic and proton-dissociative ρ photoproduction (γp → ρp, γp → ρN , respectively, with ρ → π+π−) has been studied in ep interactions at HERA for photon-proton centre-of-mass energies in the range 50 < W < 100 GeV and for |t| < 0.5 GeV, where t is the square of the four-momentum transfer at the proton vertex; the results on the protondissociative reaction are presented for masses of the dissociated proton system in the range M N < 0.1W . For the elastic process, the π+π− invariant mass spectrum has been investigated as a function of t. As in fixed target experiments, the ρ resonance shape is asymmetric; this asymmetry decreases with increasing |t|, as expected in models in which the asymmetry is ascribed to the interference of resonant and non-resonant π+π− production. The cross section has been studied as a function of W ; a fit to the resonant part with the form W a gives a = 0.16± 0.06 (stat.) +0.11 −0.15 (syst.). The resonant part of the γp → π+π−p cross section is 11.2 ± 0.1 (stat.) +1.1 −1.2 (syst.) μb at 〈W 〉 = 71.7 GeV. The t dependence of the cross section can be described by a function of the type Aρ exp (−bρ|t| + cρt) with bρ = 10.9 ± 0.3 (stat.) +1.0 −0.5 (syst.) GeV−2 and cρ = 2.7 ± 0.9 (stat.) +1.9 −1.7 (syst.) GeV−4. The t dependence has also been studied as a function of W and a value of the slope of the pomeron trajectory α IP ′ = 0.23 ± 0.15 (stat.) +0.10 −0.07 (syst.) GeV−2 has been deduced. The ρ spin density matrix elements r 00, r 04 1−1 and Re[r04 10 ] have been measured and found to be consistent with expectations based on s-channel helicity conservation. For proton-dissociative π+π− photoproduction in the ρ mass range, the distributions of the two-pion invariant mass, W and the polar and azimuthal angles of the pions in the helicity frame are the same within errors as those for the elastic process. The t distribution has been fitted to an exponential function with a slope parameter 5.8 ± 0.3 (stat.) ± 0.5 (syst.) GeV−2. The ratio of the elastic to proton-dissociative ρ photoproduction cross section is 2.0 ± 0.2 (stat.) ± 0.7 (syst.). DESY 97-237 November 1997 The ZEUS Collaboration J. Breitweg, M. Derrick, D. Krakauer, S. Magill, D. Mikunas, B. Musgrave, J. Repond, R. Stanek, R.L. Talaga, R. Yoshida, H. Zhang Argonne National Laboratory, Argonne, IL, USA p M.C.K. Mattingly Andrews University, Berrien Springs, MI, USA F. Anselmo, P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, G. Cara Romeo, G. Castellini, M. Chiarini, L. Cifarelli, F. Cindolo, A. Contin, M. Corradi, S. De Pasquale, I. Gialas, P. Giusti, G. Iacobucci, G. Laurenti, G. Levi, A. Margotti, T. Massam, R. Nania, C. Nemoz, F. Palmonari, A. Pesci, A. Polini, F. Ricci, 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, M. Eckert, M. Grothe, H. Hartmann, K. Heinloth, L. Heinz, E. Hilger, H.-P. Jakob, U.F. Katz, R. Kerger, E. Paul, M. Pfeiffer, Ch. Rembser, J. Stamm, R. Wedemeyer, H. Wieber Physikalisches Institut der Universität Bonn, Bonn, Germany c D.S. Bailey, S. Campbell-Robson, W.N. Cottingham, B. Foster, R. Hall-Wilton, M.E. Hayes, G.P. Heath, H.F. Heath, J.D. McFall, D. Piccioni, D.G. Roff, R.J. Tapper H.H. Wills Physics Laboratory, University of Bristol, Bristol, U.K. o M. Arneodo, R. Ayad, M. Capua, A. Garfagnini, L. Iannotti, M. Schioppa, G. Susinno Calabria University, Physics Dept.and INFN, Cosenza, Italy f J.Y. Kim, J.H. Lee, I.T. Lim, M.Y. Pac Chonnam National University, Kwangju, Korea h A. Caldwell, N. Cartiglia, Z. Jing, W. Liu, B. Mellado, J.A. Parsons, S. Ritz, S. Sampson, F. Sciulli, P.B. Straub, 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. Adamczyk, B. Bednarek, M. Bukowy, A. Czermak, K. Jeleń, D. Kisielewska, 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 G. Abbiendi, L.A.T. Bauerdick, U. Behrens, H. Beier, J.K. Bienlein, G. Cases, O. Deppe, K. Desler, G. Drews, U. Fricke, D.J. Gilkinson, C. Glasman, P. Göttlicher, T. Haas, W. Hain, D. Hasell, K.F. Johnson, M. Kasemann, W. Koch, U. Kötz, H. Kowalski, J. Labs, L. Lindemann, B. Löhr, M. Löwe, O. Mańczak, J. Milewski, T. Monteiro, J.S.T. Ng, D. Notz, K. Ohrenberg, I.H. Park, A. Pellegrino, F. Pelucchi, K. Piotrzkowski, M. Roco, M. Rohde, J. Roldán, J.J. Ryan, A.A. Savin, U. Schneekloth, O. Schwarzer, F. Selonke, B. Surrow, E. Tassi, T. Voß, D. Westphal, G. Wolf, U. Wollmer, C. Youngman, A.F. Żarnecki, W. Zeuner Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany B.D. Burow, H.J. Grabosch, A. Meyer, S. Schlenstedt DESY-IfH Zeuthen, Zeuthen, Germany G. Barbagli, E. Gallo, P. Pelfer University and INFN, Florence, Italy f G. Anzivino, G. Maccarrone, L. Votano INFN, Laboratori Nazionali di Frascati, Frascati, Italy f A. Bamberger, S. Eisenhardt, P. Markun, T. Trefzger, S. Wölfle Fakultät für Physik der Universität Freiburg i.Br., Freiburg i.Br., Germany c J.T. Bromley, N.H. Brook, P.J. Bussey, A.T. Doyle, N. Macdonald, D.H. Saxon, L.E. Sinclair, E. Strickland, R. Waugh Dept. of Physics and Astronomy, University of Glasgow, Glasgow, U.K. o I. Bohnet, N. Gendner, U. Holm, A. Meyer-Larsen, H. Salehi, K. Wick Hamburg University, I. Institute of Exp. Physics, Hamburg, Germany c L.K. Gladilin, D. Horstmann, D. Kçira, R. Klanner, E. Lohrmann, G. Poelz, W. Schott, F. Zetsche Hamburg University, II. Institute of Exp. Physics, Hamburg, Germany c T.C. Bacon, I. Butterworth, J.E. Cole, G. Howell, B.H.Y. Hung, L. Lamberti, K.R. Long, D.B. Miller, N. Pavel, A. Prinias, J.K. Sedgbeer, D. Sideris, R. Walker Imperial College London, High Energy Nuclear Physics Group, London, U.K. o U. Mallik, S.M. Wang, J.T. Wu University of Iowa, Physics and Astronomy Dept., Iowa City, USA p P. Cloth, D. Filges Forschungszentrum Jülich, Institut für Kernphysik, Jülich, Germany J.I. Fleck, T. Ishii, M. Kuze, I. Suzuki, K. Tokushuku, S. Yamada, K. Yamauchi, Y. Yamazaki Institute of Particle and Nuclear Studies, KEK, Tsukuba, Japan g S.J. Hong, S.B. Lee, S.W. Nam, S.K. Park Korea University, Seoul, Korea h F. Barreiro, J.P. Fernández, G. Garćıa, R. Graciani, J.M. Hernández, L. Hervás, L. Labarga, M. Mart́ınez, J. del Peso, J. Puga, J. Terrón, J.F. de Trocóniz Univer. Autónoma Madrid, Depto de F́ısica Teórica, Madrid, Spain n

The exclusive electroproduction of J/psi mesons, ep->epJ/psi, has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15<Q^2<0.8 GeV^2 and 2<Q^2<100 GeV^2 using integrated luminosities of 69 pb^-1 and 83 pb^-1, respectively.The photon-proton centre-of-mass energy was in the range 30<W<220 GeV and the squared four-momentum transfer at the proton vertex |t|<1.The cross sections and decay angular distributions are presented as functions of Q^2, W and t. The effective parameters of the Pomeron trajectory are in agreement with those found in J/psi photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections, sigma_L/sigma_T, grows with Q^2, whilst no dependence on W or t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton.

Exclusive production of ρ0 and J/ψ mesons in e+p collisions has been studied with the ZEUS detector in the kinematic range 0.25 < Q2 < 50 GeV, 20 < W < 167 GeV for the ρ0 data and 2 < Q2 < 40 GeV, 50 < W < 150 GeV for the J/ψ data. Cross sections for exclusive ρ0 and J/ψ production have been measured as a function of Q2, W and t. The spin-density matrix elements r04 00, r 1 1−1 and Re r 5 10 have been determined for exclusive ρ0 production as well as r04 00 and r 04 1−1 for exclusive J/ψ production. The results are discussed in the context of theoretical models invoking soft and hard phenomena. The ZEUS Collaboration J. Breitweg, S. Chekanov, M. Derrick, D. Krakauer, S. Magill, D. Mikunas, B. Musgrave, J. Repond, R. Stanek, R.L. Talaga, R. Yoshida, H. Zhang Argonne National Laboratory, Argonne, IL, USA p M.C.K. Mattingly Andrews University, Berrien Springs, MI, USA 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, M. Eckert, M. Grothe, 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, W.N. Cottingham, B. Foster, R. Hall-Wilton, G.P. Heath, H.F. Heath, J.D. McFall, D. Piccioni, D.G. Roff, J. Scott, R.J. Tapper H.H. Wills Physics Laboratory, University of Bristol, Bristol, U.K. o M. Capua, L. Iannotti, A. Mastroberardino, M. Schioppa, G. Susinno Calabria University, Physics Dept.and INFN, Cosenza, Italy f J.Y. Kim, J.H. Lee, I.T. Lim, M.Y. Pac Chonnam National University, Kwangju, Korea h A. Caldwell, N. Cartiglia, Z. Jing, W. Liu, B. Mellado, J.A. Parsons, S. Ritz, S. Sampson, F. Sciulli, P.B. Straub, 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. Adamczyk, B. Bednarek, M. Bukowy, A.M. Czermak, K. Jeleń, D. Kisielewska, 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

Inclusive photoproduction of $D^{\ast\pm}$ mesons has been measured for photon-proton centre-of-mass energies in the range $130 < W < 280$ GeV and photon virtuality $Q^2 < $ 1 GeV $^{2}$ . The data sample used corresponds to an integrated luminosity of 37 pb $^{-1}$ . Total and differential cross sections as functions of the $D^*$ transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading order (NLO) perturbative QCD calculations using the “massive charm” and “massless charm” schemes. The measured cross sections are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured results in a kinematic region where the resolved photon contribution is significant.

This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with χ L > 0.97, where xL is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of t, the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section on t is measured in the interval 0.073 < | t | < 0.4 GeV 2 and is found to be described by an exponential shape with the slope parameter b = 7.2 ± 1.1(stat.) −0.9 +0.7 (syst.) GeV −2 . The diffractive structure function FD (4) is presented as 0.9 a function of χ H ≃ 1 − χ L and β, the momentum fraction of the struck quark with respect to χ H , and averaged over the t interval 0.073 < |t′ < 0.4 GeV 2 and the photon virtuality range 5 < Q 2 < 20 GeV 2 . In the kinematic range 4 × 10 4 < χ p < 0.03 and 0.015 < β < 0.5, the χ p dependence of F D (4) is fitted with a form (1/χ p ) α , yielding a − 1.00 ± 0.09 (stat.) −0.05 +0.11 (syst.). Upon integration over L , the structure function F 2 D(3) is determined in a kinematic range extending to higher χ p and lower β compared to our previous analysis; the results are discussed within the framework of Regge theory.

We have searched for the production of a selectron and a squark in e(+)p collisions at a center-of-mass energy of 300 GeV using the ZEUS detector at HERA. The selectron and squark are sought in the direct decay into the lightest neutralino in the framework of supersymmetric extensions to the Standard Model which conserve R-parity. No evidence for the production of supersymmetric particles has been found in a data sample corresponding to 46.6 pb(-1) of integrated luminosity. We express upper limits on the product of the cross section times the decay branching ratios as excluded regions in the parameter space of the Minimal Supersymmetric Standard Model. (C) 1998 Elsevier Science B.V. All rights reserved.

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon (ϰγ OBS) and pomeron (β OBS) momentum participating in the production of the dijet system. The observed ϰγ OBS dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section da/dβ OBS increases as β OBS increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron.

A global event shape analysis of the multihadronic final states observed in neutral current deep inelastic scattering events with a large rapidity gap with respect to the proton direction is presented. The analysis is performed in the range $5 \leq Q^2 \leq 185\gev^2$ and $160 \leq W \leq 250\gev$, where $Q^2$ is the virtuality of the photon and $W$ is the virtual-photon proton centre of mass energy. Particular emphasis is placed on the dependence of the shape variables, measured in the $\gamma^*-$pomeron rest frame, on the mass of the hadronic final state, $M_X$. With increasing $M_X$ the multihadronic final state becomes more collimated and planar. The experimental results are compared with several models which attempt to describe diffractive events. The broadening effects exhibited by the data require in these models a significant gluon component of the pomeron.

Dijet cross sections are presented using photoproduction data obtained with the ZEUS detector during 1994. These measurements represent an extension of previous results, as the higher statistics allow cross sections to be measured at higher jet transverse energy (E τ jet ). Jets are identified in the hadronic final state using three different algorithms, and the cross sections compared to complete next-to-leading order QCD calculations. Agreement with these calculations is seen for the pseudorapidity dependence of the direct photon events with E T jet > 6 GeV and of the resolved photon events with E t jet > 11 GeV. Calculated cross sections for resolved photon processes with 6 GeV < E T jet < 11 GeV lie below the data.