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.

A. Andresen a, A. Bamberger ', A. Baigende c, F. Barreiro `', U. Behrens `, S. Bentvelsen f, M. Brkic 9, B. Burow 9, A. CaldwelI h, G . Cases d°§§, F. Corriveau ',', J .A . Crittenden c, A. Dake f, A. Dannemann °, J. Dawson ', K. Dierks ', G. Drews 1 , J. Engelen f, H . Fawcett W. Fiisken k, A. Fürtjes ', D . Gilkinson ', L. Hagge ', R . Hamatsu in, D. Hanna I, J . Hartmann e, T. Hasegawa ", D . Hasell k , M. Hazumi ", L. Hervás `', E. Hilgei c, U. Holm e, T. Ishii ", P. de Jong f, H. Kammerlocher c, S . Kasai ", S. Kitamuia ', R. Manner ', P . Kooijman f, U. Kötz ', M. Kriimer c, B . Krebs e, W. Krüger a, J. Krüger a, M. Kuze ", G. Levman 9, T. Lindenberg b, H. v.d . Lugt f, J. Martin 9, J. Mayer P, J . Mitchell ', P. Patel ', T. Poser b, K. Prange `, S . Ritz h , M. Rohde ', T. Romanowski °, E. Ros °, H.-L. Schneider `, D. Schäcke b, W. Schulz ', F. Selorike ', W. Sippach h, G. Smith P, G. Stairs 9, J. Straver f, G. Theisen 6, H. Tiecke f, K. Tokushuku ", B. Trdndle b, T. Tsurugai a, H. Uijterwaal ', R . Ullmann b, W. Vogel ', K. Wick e, T. Woeriiger `', S . Yamada ", C. Youngman ' University of Hamburg, II. Institute for Experimental Physics . Germany n University of Freiburg, Germany ` University of Bonn, Germany " Unn ersidad Autónoma de Madrid, Spain `' University of Hamburg, I. Institute for Experimental Physics, Germany i NIKHEF-H, Amsterdam, The Netherlands x University of Toronto, Canada + i' Columbia University, USA + ' Argonne National Laboratory, USA ° ' DESY, Hamburg, Germany York University, Canada i McGill University, Canada "' Tokyo Metropolitan Unn ersity, Japan " INS, University of Tokyo, Japan " Ohio State Unit ersity, USA P University of Manitoba, Canada +

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.

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