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.

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

Jet production in deep inelastic scattering for 120 < Q2 < 3600 GeV’ has been studied using data from an integrated luminosity of 3.2 pbb’ collected with the ZEUS detector at HERA. Jets are identified with the JADE algorithm. A cut on the angular distribution of parton emission in the y*-parton centre-of-mass system minimises the experimental and theoretical uncertainties in the determination of the jet rates. The jet rates, when compared to 0( (Y,=) perturbative QCD calculations, allow a precise determination of czs (Q) in three Q*-intervals. The values are consistent with a running of crys(Q), as expected from QCD. Extrapolating to Q = 1’14~0 yields a,( Mzo) = 0.117 f. 0.005 (stat) ?igi (syst,,,) f 0.007 (sys&,). r Supported by Worldlab, Lausanne, Switzerland. 2 Also at IROE Florence, Italy. 3 Now at Univ. of Salerno and INFN Napoli, Italy. 4 Supported by EU HCM contract ERB-CHRX-CT93-0376. 5 Now at Miibelhaus Kramm, Essen. 6 Now a self-employed consultant. 7 Now also at University of Torino. * Alexander von Humboldt Fellow. 9 Alfred P. Sloan Foundation Fellow. to Presently at Columbia Univ., supported by DAAD/HSPIIAUFE. tl Now at Inst. of Computer Science, Jagellonian Univ., Cracow. I2 Now at Comma-Soft, Bonn. I3 Visitor from Florida State University. l4 Now at Univ. of Mainz. ii Supported by European Community Program PRAXIS XXI. I6 Now at Dr. Seidel Informationssysteme, Frankfurt/M. I7 Now at Inst. of Accelerating Systems & Applications (IASA), Athens. I8 Now at Mercer Management Consulting, Munich. I9 Now at Univ. de Strasbourg. 2o Now at Andrews University, Barrien Springs, U.S.A. 2’ Now with OPAL Collaboration, Faculty of Physics at Univ. of Freibug. ” Now at SAS-Institut GmbH, Heidelberg. 23 Partially supported by DESY. 24 Now at GSI Darmstadt. 25 Also supported by NSERC. 26 Now at DESY. 27 Now at Institute for Cosmic Ray Research, University of Tokyo. 28 Partially supported by CAM. 2g Now at Carleton University, Ottawa, Canada. 3o Now at Department of Energy, Washington. 3’ Now at HEP Div., Argonne National Lab., Argonne, IL, USA. 32 Now at Oxford Magnet Technology, Eynsham, Oxon. 33 In part supported by Argonne National Laboratory. 34 On leave and partially supported by DESY 1993-95. 35 Supported by a MINERVA Fellowship. 36 Now at Centre for Subatomic Research, Univ.of Alberta, Canada and TRIUMF, Vancouver, Canada. 37 Supported by the Polish State Committee for Scientific Re206 Zeus Collaboration/Physics Letters B 363 (1995) 201-216

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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.

The reaction γ p → J/ψ p has been studied in ep interactions using the ZEUS detector at HERA. The cross section for elastic J/ψ photoproduction has been measured as a function of the photon-proton centre of mass energy W in the range 40 < W < 140 GeV at a median photon virtuality Q of 5 × 10−5 GeV. The photoproduction cross section, σγp→J/ψp, is observed to rise steeply with W . A fit to the data presented in this paper to determine the parameter δ in the form σγp→J/ψp ∝W δ yields the value δ = 0.92±0.14±0.10. The differential cross section dσ/d|t| is presented over the range |t| < 1.0 GeV where t is the square of the fourmomentum exchanged at the proton vertex. dσ/d|t| falls exponentially with a slope parameter of 4.6 ± 0.4 −0.6 GeV −2. The measured decay angular distributions are consistent with s-channel helicity conservation. 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, S. De Pasquale, I. Gialas, 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, 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. 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*(+)/- 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 $$\sigma _{e^ + p \to \bar \nu X} (Q^2 > 200 GeV^2 ; \eta _{\max }< 2.5) = 0.8_{ - 0.7}^{ + 1.8} \pm 0.1 pb$$

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 \(\sigma _{e^ + p \to \bar \nu X} (Q^2 > 200 GeV^2 ; \eta _{\max }< 2.5) = 0.8_{ - 0.7}^{ + 1.8} \pm 0.1 pb\)

In deep inelastic neutral current scattering of positrons and protons at the center of mass energy of 300 GeV, we observe, with the ZEUS detector, events with a high energy neutron produced at very small scattering angles with respect to the proton direction. The events constitute a fixed fraction of the deep inelastic, neutral current event sample independent of Bjorken x and Q2 in the range 3 · 10−4 < xBJ < 6 · 10−3 and 10 < Q2 < 100 GeV2.

Photoproduction events which have two or more jets have been studied in the $W_{\gamma p}$ range 135~GeV $< W_{\gamma p} <$ 280~GeV with the ZEUS detector at HERA. A class of events is observed with little hadronic activity between the jets. The jets are separated by pseudorapidity intervals ($\Delta\eta$) of up to four units and have transverse energies greater than 6~GeV. A gap is defined as the absence between the jets of particles with transverse energy greater than 300~MeV. The fraction of events containing a gap is measured as a function of $\Delta\eta$. It decreases exponentially as expected for processes in which colour is exchanged between the jets, up to a value of $\Delta\eta \sim 3$, then reaches a constant value of about 0.1. The excess above the exponential fall-off can be interpreted as evidence for hard diffractive scattering via a strongly interacting colour singlet object.

A measurement of inclusive charged particle distributions in deep inelasticep scattering forγ*p centre-of-mass energies 75<W<175 GeV and momentum transfer squared 10<Q 2<160 GeV2 from the ZEUS detector at HERA is presented. The differential charged particle rates in theγ*p centre-of-mass system as a function of the scaled longitudinal momentum,x F, and of the transverse momentum,p* t and 〈p* t 2〉, as a function ofx F, W andQ 2 are given. Separate distributions are shown for events with (LRG) and without (NRG) a rapidity gap with respect to the proton direction. The data are compared with results from experiments at lower beam energies, with the naive quark parton model and with parton models including perturbative QCD corrections. The comparison shows the importance of the higher order QCD processes. Significant differences of the inclusive charged particle rates between NRG and LRG events at the sameW are observed. The value of 〈p* t 2〉 for LRG events with a hadronic massM x, which excludes the forward produced baryonic system, is similar to the 〈p* t 2〉 value observed in fixed target experiments atW≈M x.

We present measurements of the structure functionF 2 ine + p scattering at HERA in the range 3.5 GeV2<Q 2<5000 GeV2. A new reconstruction method has allowed a significant improvement in the resolution of the kinematic variables and an extension of the kinematic region covered by the experiment. AtQ 2<35 GeV2 the range inx now spans 6.3·10−5<x<0.08 providing overlap with measurements from fixed target experiments. At values ofQ 2 above 1000 GeV2 thex range extends to 0.5. Systematic errors below 5% have been achieved for most of the kinematic region. The structure function rises asx decreases; the rise becomes more pronounced asQ 2 increases. The behaviour of the structure function data is well described by next-to-leading order perturbative QCD as implemented in the DGLAP evolution equations.

A search for excited states of the standard model fermions was performed using the ZEUS detector at the HERA electron-proton collider, operating at a centre of mass energy of 296 GeV. In a sample corresponding to an integrated luminosity of 0.55 pb−1, no evidence was found for any resonant state decaying into final states composed of a fermion and a gauge boson. Limits on the coupling strength times branching ratio of excited fermions are presented for masses between 50 GeV and 250 GeV, extending previous search regions significantly.