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.

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

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

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.

A small electromagnetic sampling calorimeter, installed in the ZEUS experiment in 1995, significantly enhanced the acceptance for very low x and low Q^2 inelastic neutral current scattering, e^{+}p \to e^{+}X, at HERA. A measurement of the proton structure function F_2 and the total virtual photon-proton (\gamma^*p) cross-section is presented for 0.11 \le Q^{2} \le 0.65 GeV^2 and 2 \times 10^{-6} \le x \le 6 \times 10^{-5}, corresponding to a range in the \gamma^{*}p c.m. energy of 100 \le W \le 230 GeV. Comparisons with various models are also presented.

Using the ZEUS detector at HERA, we have studied the reaction e + p → e + X for Q 2 > 5000 GeV 2 with a 20.1 pb −1 data sample collected during the years 1994 to 1996. For Q 2 below 15000 GeV 2 , the data are in good agreement with Standard Model expectations. For Q 2 > 35000 GeV 2 , two events are observed while 0.145 ± 0.013 events are expected. A statistical analysis of a large ensemble of simulated Standard Model experiments indicates that with probability 6.0%, an excess at least as unlikely as that observed would occur above some Q 2 cut. For x > 0.55 and y > 0.25, four events are observed where 0.91+ - 0.08 events are expected. A statistical analysis of the two-dimensional distribution of the events in x and y yields a probability of 0.72% for the region x > 0.55 and y > 0.25 and a probability of 7.8% for the entire Q 2 > 5000 GeV 2 data sample. The observed excess above Standard Model expectations is particularly interesting because it occurs in a previously unexplored kinematic region.

A small electromagnetic sampling calorimeter, installed in the ZEUS experiment in 1995, significantly enhanced the acceptance for very low x and low Q(2) inelastic neutral current scattering, e(+)p --> e(+)X, at HERA. A measurement of the proton structure function F-2 and the total virtual photon-proton (gamma*p) cross-section is presented for 0.11 less than or equal to Q(2) less than or equal to 0.65 GeV2 and 2 x 10(-6) less than or equal to x less than or equal to 6 x 10(-5), corresponding to a range in the gamma*p c.m. energy of 100 less than or equal to W less than or equal to 230 GeV. Comparisons with various models are also presented. (C) 1997 Published by Elsevier Science B.V.

Events containing an isolated prompt photon with high transverse energy, together with a balancing jet, have been observed for the first time in photoproduction at HERA.The data were taken with the ZEUS detector, in a $\gamma p$ centre of mass energy range 120--250 GeV. The fraction of the incoming photon energy participating in the production of the prompt photon and the jet, $x_\gamma$, shows a strong peak near unity, consistent with LO QCD Monte Carlo predictions. In the transverse energy and pseudorapidity range $5\le \eTg < 10$ GeV, $-0.7 \le \eta^\gamma < 0.8$, $\eTj\ge 5$ GeV, and $-1.5\le \eta^{jet}\le 1.8$, with $\xgO > 0.8,$ the measured cross section is $15.3 \pm 3.8 \pm 1.8 pb$, in good agreement with a recent NLO calculation.

Charged particle production has been measured in deep inelastic scattering (DIS) events over a large range of x and Q^2 using the ZEUS detector. The evolution of the scaled momentum, x_p, with Q^2, in the range 10 to 1280 GeV^2, 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^2.

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(2) < 4 GeV2. The cross section sigma(ep) --> D*X integrated over the kinematic region p(D)*(perpendicular to) 3 GeV acid -1.5 < eta(D)* < 1.0 is (10.6 +/- 1.7(stat.)+/-(1.6)(1.3)(syst.)) nb. Differential cross sections as functions of p(D)*(perpendicular to), eta(D)* 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. (C) 1997 Published by Elsevier Science B.V.

Jet photoproduction, where the two highest transverse energy (E(T)(jet)) jets have E(T)(jet) 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.

The production of $\phi$ mesons in the reaction $e^{+}p \rightarrow e^{+} \phi p$ ($\phi \rightarrow K^{+}K^{-}$), for $7 < Q^2 < 25$ 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 $Q^2$, the results show that the $\gamma^*p \rightarrow \phid p$ cross section rises strongly with $W$. This behaviour is similar to that previously found for the $\gamma^*p \rightarrow \rho^0 p$ 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 $\sigma (\phi) / \sigma (\rho^0)$, which has previously been determined by ZEUS to be 0.065 $\pm$ 0.013 (stat.) in photoproduction at a mean $W$ of 70 GeV, is measured to be 0.18 $\pm $ 0.05 (stat.) $\pm$ 0.03 (syst.) at a mean $Q^2$ of 12.3 GeV$^2$ and mean $W$ of $\approx$ 100 GeV and is thus approaching at large $Q^2$ the value of 2/9 predicted from the quark charges of the vector mesons and a flavour independent production mechanism.

Deep inelastic charged-current reactions have been studied ine + p ande − p collisions at a center of mass energy of about 300GeV in the kinematic regionQ 2>200GeV2 andx>0.006 using the ZEUS detector at HERA. The integrated cross sections forQ 2>200GeV2 are found to be $$\sigma _{e^ + p \to \bar \nu X} = 30.3_{ - 4.2 - 2.6}^{ + 5.5 + 1.6} pb$$ and $$\sigma _{e^ - p \to \nu X} = 54.7_{ - 9.8 - 3.4}^{ + 15.9 + 2.8} pb$$. Differential cross sections have been measured as functions of the variablesx, y andQ 2. From the measured differential cross sectionsdσ/dQ 2, theW boson mass is determined to be $$M_W = 79_{ - 7 - 4}^{ + 8 + 4} GeV$$. Measured jet rates and transverse energy profiles agree with model predictions. A search for charged-current interactions with a large rapidity gap yielded one candidate event, corresponding to a cross section of $$\sigma _{e^ + p \to \bar \nu X} (Q^2 > 200 GeV^2 ; \eta _{\max }< 2.5) = 0.8_{ - 0.7}^{ + 1.8} \pm 0.1 pb$$