Received 5 September 2006DOI:https://doi.org/10.1103/PhysRevD.74.059906©2006 American Physical Society

Energetic neutrons produced in ep collisions at HERA have been studied with the ZEUS detector in the photoproduction regime at a mean photon-proton center-of-mass energy of 220 GeV. The neutrons carry a large fraction 0.64 < x(L) < 0.925 of the incoming proton energy, and the four-momentum transfer squared at the proton-neutron vertex is small, vertical bar t vertical bar < 0.425 GeV(2). The x(L) distribution of the neutrons is measured in bins of t. The (1 - x(L)) distributions in the t bins studied satisfy a power law dN/dx(L) proportional to (1 - x(L))(a(t)), with the powers a(t) following a linear function of t: a(t) = 0.88 +/- 0.09(stat.)(-0.39)(+0.34) (syst.) - (2.81 +/- 0.42(stat.)(-0.62)(+1.13) (syst.) GeV(-2))t. This result is consistent with the expectations of pion-exchange models, in which the incoming proton fluctuates to a neutron-pion state, and the electron interacts with the pion. (c) 2005 Elsevier B.V. All rights reserved.

Cross sections for ${e}^{+}p$ neutral current deep inelastic scattering have been measured at a center-of-mass energy of $\sqrt{s}=318\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$ with the ZEUS detector at HERA using an integrated luminosity of $63.2\text{ }{\mathrm{p}\mathrm{b}}^{\ensuremath{-}1}$. The double-differential cross section, ${d}^{2}\ensuremath{\sigma}/dxd{Q}^{2}$, is presented for $200\text{ }{\mathrm{G}\mathrm{e}\mathrm{V}}^{2}<{Q}^{2}<30\text{ }000\text{ }{\mathrm{G}\mathrm{e}\mathrm{V}}^{2}$ and for $0.005200\text{ }{\mathrm{G}\mathrm{e}\mathrm{V}}^{2}$. The effect of $Z$-boson-exchange is seen in $d\ensuremath{\sigma}/dx$ measured for ${Q}^{2}>10\text{ }000\text{ }{\mathrm{G}\mathrm{e}\mathrm{V}}^{2}$. The data presented here were combined with ZEUS ${e}^{+}p$ neutral current data taken at $\sqrt{s}=300\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$ and the structure function ${F}_{2}^{\mathrm{e}\mathrm{m}}$ was extracted. All results agree well with the predictions of the Standard Model.

Jet substructure and differential cross sections for jets produced in the photoproduction and deep inelastic ep scattering regimes have been measured with the ZEUS detector at HERA using an integrated luminosity of 82.2 pb−1. The substructure of jets has been studied in terms of the jet shape and subjet multiplicity for jets with transverse energies ETjet>17 GeV. The data are well described by the QCD calculations. The jet shape and subjet multiplicity are used to tag gluon- and quark-initiated jets. Jet cross sections as functions of ETjet, jet pseudorapidity, the jet–jet scattering angle, dijet invariant mass and the fraction of the photon energy carried by the dijet system are presented for gluon- and quark-tagged jets. The data exhibit the behaviour expected from the underlying parton dynamics. A value of αs(MZ) of αs(MZ)=0.1176±0.0009(stat.)−0.0026+0.0009(exp.)−0.0072+0.0091(th.) was extracted from the measurements of jet shapes in deep inelastic scattering.

A search for events containing isolated tau leptons and large missing transverse momentum, not originating from the tau decay, has been performed with the ZEUS detector at the electron-proton collider HERA, using 130 pb^-1 of integrated luminosity. A search was made for isolated tracks coming from hadronic tau decays. Observables based on the internal jet structure were exploited to discriminate between tau decays and quark- or gluon-induced jets. Three tau candidates were found, while 0.40 +0.12 -0.13 were expected from Standard Model processes, such as charged current deep inelastic scattering and single W-boson production. To search for heavy-particle decays, a more restrictive selection was applied to isolate tau leptons produced together with a hadronic final state with high transverse momentum. Two candidate events survive, while 0.20 +-0.05 events are expected from Standard Model processes.

The photoproduction of D*(+/-)(2010) mesons associated with a leading neutron has been observed with the ZEUS detector in ep collisions at HERA using an integrated luminosity of 80 pb(-1). The neutron carries a large fraction, x(L) > 0.2, of the incoming proton beam energy and is detected at very small production angles, theta(n) < 0.8 mrad, an indication of peripheral scattering. The D* meson is centrally produced with pseudorapidity \eta\ < 1.5, and has a transverse momentum p(T) > 1.9 GeV, which is large compared to the average transverse momentum of the neutron of 0.22 GeV The ratio of neutron-tagged to inclusive D* production is 8.85 +/- 0.93(stat.)(-0.61)(+0.48)(syst.)% in the photon-proton center-of-mass energy range 130 < W < 280 Gev The data suggest that the presence of a hard scale enhances the fraction of events with a leading neutron in the final state. (C) 2004 Elsevier B.V. All rights reserved.

A search for physics beyond the Standard Model has been performed with high-Q^2 neutral current deep inelastic scattering events recorded with the ZEUS detector at HERA. Two data sets, e^+ p \to e^+ X and e^- p \to e^- X, with respective integrated luminosities of 112 pb^-1 and 16 pb^-1, were analyzed. The data reach Q^2 values as high as 40000 GeV^2. No significant deviations from Standard Model predictions were observed. Limits were derived on the effective mass scale in eeqq contact interactions, the ratio of leptoquark mass to the Yukawa coupling for heavy leptoquark models and the mass scale parameter in models with large extra dimensions. The limit on the quark charge radius, in the classical form factor approximation, is 0.85 10^-16 cm.

First measurements of cross sections for isolated prompt photon production in deep inelastic ep scattering have been made using the ZEUS detector at the HERA electron-proton collider using an integrated luminosity of 121 pb(-1). A signal for isolated photons in the transverse energy and rapidity ranges 5 < E-T(y) < 10 GeV and -0.7 < eta(gamma) < 0.9 was observed for virtualities of T the exchanged photon of Q(2) > 35 GeV2. Cross sections are presented for inclusive prompt photons and for those accompanied by a single jet in the range E-jet(T) greater than or equal to 6 GeV and 1.5 less than or equal to eta(jet) < 1.8. Calculations at order alpha(3)alpha(s), describe the data reasonably well. (C) 2004 Elsevier B.V. All rights reserved.

Inclusive production of D*± (2010) mesons in deep inelastic scattering has been measured with the ZEUS detector at DESY HERA using an integrated luminosity of 81.9 pb−1. The decay channel D*+→D0π+ with D0→K−π+ and corresponding antiparticle decay were used to identify D* mesons. Differential D* cross sections with 1.5

Cross sections for e^+p neutral current deep inelastic scattering have been measured at a centre-of-mass energy of sqrt{s}=318 GeV with the ZEUS detector at HERA using an integrated luminosity of 63.2 pb^-1. The double-differential cross section, d^2sigma/dxdQ^2, is presented for 200 GeV^2"200 GeV^2. The effect of Z-boson exchange is seen in dsigma/dx measured for Q^2>10000 GeV^2. The data presented here were combined with ZEUS e^+p neutral current data taken at sqrt{s}=300 GeV and the structure function F_2^{em} was extracted. All results agree well with the predictions of the Standard Model."

The photoproduction of bottom quarks in events with two jets and a muon has been measured with the ZEUS detector at HERA using an integrated luminosity of 110 pb(-1). The fraction of jets containing b quarks was extracted from the transverse momentum distribution of the muon relative to the closest jet. Differential cross sections for bottom production as a function of the transverse momentum and pseudorapidity of the muon, of the associated jet and of x(gamma)(jets), the fraction of the photon's momentum participating in the hard process, are compared with MC models and QCD predictions made at next-to-leading order. The latter give a good description of the data.

Bose-Einstein correlations in one and two dimensions have been studied in deep inelastic ep scattering events measured with the ZEUS detector at HERA using an integrated luminosity of 121 pb. The correlations are independent of the virtuality of the exchanged photon, Q, in the range 0.1 < Q < 8000 GeV. There is no significant difference between the correlations in the current and target regions of the Breit frame for Q > 100 GeV. The two-dimensional shape of the particle-production source was investigated, and a significant difference between the transverse and the longitudinal dimensions of the source is observed. This difference also shows no Q dependence. The results demonstrate that Bose-Einstein interference, and hence the size of the particle-production source, is insensitive to the hard subprocess. The ZEUS Collaboration S. Chekanov, M. Derrick, D. Krakauer, J.H. Loizides, S. Magill, S. Miglioranzi, B. Musgrave, J. Repond, R. Yoshida Argonne National Laboratory, Argonne, Illinois 60439-4815, USA n M.C.K. Mattingly Andrews University, Berrien Springs, Michigan 49104-0380, USA P. Antonioli, G. Bari, M. Basile, L. Bellagamba, D. Boscherini, A. Bruni, G. Bruni, G. Cara Romeo, L. Cifarelli, F. Cindolo, A. Contin, M. Corradi, S. De Pasquale, P. Giusti, G. Iacobucci, A. Margotti, A. Montanari, R. Nania, F. Palmonari, A. Pesci, G. Sartorelli, A. Zichichi University and INFN Bologna, Bologna, Italy e G. Aghuzumtsyan, D. Bartsch, I. Brock, S. Goers, H. Hartmann, E. Hilger, P. Irrgang, H.P. Jakob, O. Kind, U. Meyer, E. Paul, J. Rautenberg, R. Renner, A. Stifutkin, J. Tandler, K.C. Voss, M. Wang, A. Weber Physikalisches Institut der Universität Bonn, Bonn, Germany b D.S. Bailey, N.H. Brook, J.E. Cole, G.P. Heath, T. Namsoo, S. Robins, M. Wing H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom m M. Capua, A. Mastroberardino, M. Schioppa, G. Susinno Calabria University, Physics Department and INFN, Cosenza, Italy e J.Y. Kim, Y.K. Kim, J.H. Lee, I.T. Lim, M.Y. Pac Chonnam National University, Kwangju, Korea g A. Caldwell, M. Helbich, X. Liu, B. Mellado, Y. Ning, S. Paganis, Z. Ren, W.B. Schmidke, F. Sciulli Nevis Laboratories, Columbia University, Irvington on Hudson, New York 10027 o J. Chwastowski, A. Eskreys, J. Figiel, A. Galas, K. Olkiewicz, P. Stopa, L. Zawiejski Institute of Nuclear Physics, Cracow, Poland i L. Adamczyk, T. Bo ld, I. Grabowska-Bo ld, D. Kisielewska, A.M. Kowal, M. Kowal, T. Kowalski, M. Przybycień, L. Suszycki, D. Szuba, J. Szuba Faculty of Physics and Nuclear Techniques, AGH-University of Science and Technology, Cracow, Poland p A. Kotański, W. S lomiński Department of Physics, Jagellonian University, Cracow, Poland

A resonance search has been made in the K(S)(0)p and K-S(0)(p) over bar invariant-mass spectrum measured with the ZEUS detector at HERA using an integrated luminosity of 121 pb(-1). The search was performed in the central rapidity region of inclusive deep inelastic scattering at an ep centre-of-mass energy of 300-318 GeV for exchanged photon virtuality, Q(2), above 1 GeV2. Recent results from fixed-target experiments give evidence for a narrow baryon resonance decaying to K(+)n and K(S)(0)p, interpreted as a pentaquark. The results presented here support the existence of such state, with a mass of 1521.5 +/- 1.5(stat.)(-1.7)(+2.8)(syst.) MeV and a Gaussian width consistent with the experimental resolution of 2 MeV The signal is visible at high Q(2) and, for Q(2) > 20 GeV2, contains 221 +/- 48 events. The probability of a similar signal anywhere in the range 1500-1560 MeV arising from fluctuations of the background is below 6 x 10(-5). (C) 2004 Elsevier B.V. All rights reserved.

A search for single-top production, ep→etX, has been performed with the ZEUS detector at HERA using data corresponding to an integrated luminosity of 0.37 fb−1. No evidence for top production was found, consistent with the expectation from the Standard Model. Limits were computed for single-top production via flavour changing neutral current transitions involving a neutral electroweak vector boson, γ or Z. The result was combined with a previous ZEUS result yielding a total luminosity of 0.50 fb−1. A 95% credibility level upper limit of 0.13 pb was obtained for the cross section at the centre-of-mass energy of s=315 GeV.

Production of D*+/-(2010) mesons in diffractive deep inelastic scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 82 pb(-1). Diffractive events were identified by the presence of a large rapidity gap in the final state. Differential cross sections have been measured in the kinematic region 1.5 < Q(2) < 200 GeV2 0.02 < y < 0.7, x(P) < 0.035, beta < 0.8, P-T (D*+/-) > 1.5 GeV and \eta(D*+/-)\ < 1.5. The measured cross sections are compared to theoretical predictions. The results are presented in terms of the open-charm contribution to the diffractive proton structure function. The data demonstrate a strong sensitivity to the diffractive parton densities. (C) 2003 Published by Elsevier B.V.

Inclusive phi-meson production in neutral current deep inelastic e+p scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 45 pb^-1. The phi mesons were studied in the range 10<Q2<100 GeV2, where Q2 is the virtuality of the exchanged photon, and in restricted kinematic regions in the transverse momentum, p_T, pseudorapidity, eta, and the scaled momentum in the Breit frame, x_p. Monte Carlo models with the strangeness-suppression factor as determined by analyses of e+e- annihilation events overestimate the cross sections. A smaller value of the strangeness-suppression factor reduces the predicted cross sections, but fails to reproduce the shapes of the measured differential cross sections. High-momentum phi mesons in the current region of the Breit frame give the first direct evidence for the strange sea in the proton at low x.

The subjet multiplicity has been measured in neutral current e(+) p interactions at Q(2) > 125 GeV2 with the ZEUS detector at HERA using an integrated luminosity of 38.6 pb(-1). Jets were identified in the laboratory frame using the longitudinally invariant k(T) cluster algorithm. The number of jet-like substructures within jets, known as the subjet multiplicity, is defined as the number of clusters resolved in a jet by reapplying the jet algorithm at a smaller resolution scale y(cut). Measurements of the mean subjet multiplicity, , for jets with transverse energies E-T,E-jet > 15 GeV are presented. Next-to-leading-order perturbative QCD calculations describe the measurements well. The value of alpha(s) (M-Z), determined from at y(cut) = 10(-2) for jets with 25 < E-T,E-jet < 71 GeV, is alpha(s) (M-Z) = 0.1187 +/- 0.0017(stat.)(-0.0009)(+0.0024)(syst.)(-0.0076)(+0.0093)(th.). (C) 2003 Published by Elsevier Science B.V.