DESY Ep Collider HERA Research Articles

J/ψProduction at HERA

We consider the J/psi photoproduction data collected at the DESY ep collider HERA in the light of next-to-leading order predictions for the color-singlet yield and polarization. We find that, while the shapes of inclusive distributions in the transverse momentum and inelasticity are well reproduced, the experimental rates are larger than those given by the color-singlet contribution alone. Furthermore, the next-to-leading order calculation predicts the J/psi's to be mostly longitudinally polarized at high transverse momentum in contrast with the trend of the preliminary data from the ZEUS Collaboration.

InclusiveKS0KS0Resonance Production inepCollisions at HERA

Inclusive K_{S};{0}K_{S};{0} production in ep collisions at the DESY ep collider HERA was studied with the ZEUS detector using an integrated luminosity of 0.5 fb;{-1}. Enhancements in the mass spectrum were observed and are attributed to the production of f_{2}(1270)/a_{2};{0}(1320), f_{2};{'}(1525) and f_{0}(1710). Masses and widths were obtained using a fit which takes into account theoretical predictions based on SU(3) symmetry arguments, and are consistent with the Particle Data Group values. The f_{0}(1710) state, which has a mass consistent with a glueball candidate, was observed with a statistical significance of 5 standard deviations. However, if this state is the same as that seen in gammagamma-->K_{S};{0}K_{S};{0}, it is unlikely to be a pure glueball state.

Geometric scaling in inclusive charm production.

We show that the cross section for inclusive charm production exhibits geometric scaling in a large range of photon virtualities. In the DESY ep collider HERA kinematic domain the saturation momentum Q(2)(sat)(x) stays below the hard scale micro(2)(c)=4m(2)(c), implying charm production probing mostly the color transparency regime and unitarization effects being almost negligible. We derive our results considering two saturation models which are able to describe the HERA data for the proton structure function at small values of the Bjorken variable x. A striking feature is the scaling on tau identical with Q(2)/Q(2)(sat)(x) above te saturation limit, corroborating recent theoretical studies.

Resummation of transverse momentum and mass logarithms in DIS heavy-quark production

Differential distributions for heavy quark production depend on the heavy quark mass and other momentum scales, which can yield additional large logarithms and inhibit accurate predictions. Logarithms involving the heavy quark mass can be summed in heavy quark parton distribution functions in the Aivazis-Collins-Olness-Tung (ACOT) factorization scheme. A second class of logarithms involving the heavy-quark transverse momentum can be summed using an extension of the Collins-Soper-Sterman (CSS) formalism. We perform a systematic summation of logarithms of both types, thereby obtaining an accurate description of heavy-quark differential distributions at all energies. Our method essentially combines the ACOT and CSS approaches. As an example, we present angular distributions for bottom quarks produced in parity-conserving events at large momentum transfers Q at the DESY ep collider HERA.

The nonlinear QCD dynamical equations with higher order effective coupling

Through studying the nonlinear QCD dynamical equations with higher order effective coupling, we find that shadowing effect is substantially weaker than that neglecting higher order effective coupling. This conclusion is very important for explaining the data from DESY ep collider HERA.

Diffractive dijets with a leading antiproton in &pmacr;p collisions at sqrt

We report results from a study of events with a leading antiproton of beam momentum fraction 0.905<x(F)<0.965 and 4-momentum transfer squared |t|<3 GeV2 produced in &pmacr;p collisions at sqrt[s] = 1800 GeV at the Fermilab Tevatron collider. Approximately 2% of the events contain two jets of transverse energy E(jet)(T)>7 GeV. Using the dijet events, we evaluate the diffractive structure function of the antiproton and compare it with expectations based on results obtained in diffractive deep inelastic scattering experiments at the DESY ep collider HERA.

Polarization in diffractive electroproduction of light vector mesons

We study in perturbative QCD the helicity amplitudes of the process ${\ensuremath{\gamma}}^{*}\stackrel{\ensuremath{\rightarrow}}{p}\ensuremath{\rho}p$ at large virtualities Q of the photon ${\ensuremath{\gamma}}^{*}.$ We estimate all spin-flip amplitudes taking into account an important effect of the scale behavior of the gluon density. The transition of a transverse virtual photon to a longitudinal vector meson is not small at typical conditions at the DESY ep collider HERA. This helicity nonconserving amplitude leads by interference to a measurable effect in the distribution of the angle between the electron scattering and the meson production planes.

Neutral kaon production in e+e-, ep, and pp-bar collisions at next-to-leading order.

We present new sets of fragmentation functions for neutral kaons, both at leading and next-to-leading order. They are fitted to data on inclusive K^0 production in e^+e^- annihilation taken by MARK II at PEP (\sqrt s = 29 GeV) and by ALEPH at LEP. Our fragmentation functions lead to a good description of other e^+e^- data on inclusive K^0 production at various energies. They also nicely agree with the K_S^0 transverse-momentum spectra measured by H1 at the DESY ep collider HERA, by UA5 at the CERN Sp\bar pS Collider, and by CDF at the Fermilab Tevatron.

The H1 backward calorimeter BEMC and its inclusive electron trigger

A sandwich type lead-scintillator electromagnetic calorimeter with wavelength shifter optical readout has been successfully operated at the DESY ep collider HERA in the H1 detector for three years. The mechanical design of the calorimeter together with the associated electronics and the inclusive electron trigger as well as its performance and stability in test beams and at the ep collider HERA are described in detail.

Next-to-leading-order corrections to inclusive hadron photoproduction.

We present a complete calculation of next-to-leading-order corrections to inclusive hadron photoproduction. We take into account all contributions from resolved and unresolved photons and apply the results to ${\mathrm{\ensuremath{\pi}}}^{0}$ production at CERN LEP 2 as well as at the DESY ep collider HERA where we study in detail the scale dependence of the cross section. In addition compact analytical expressions for the matrix elements for all the direct contributions to both cross sections are presented. We also make a comparison with existing single tag data on inclusive charged hadron production.

Bounds on vector leptoquarks.

We derive bounds on vector leptoquarks coupling to the first generation, using data from low energy experiments as well as from high energy accelerators. Similarly to the case of scalar leptoquarks, we find that the strongest indirect bounds arise from atomic parity violation and universality in leptonic \ensuremath{\pi} decays. These bounds are considerably stronger than the first direct bounds of the DESY ep collider HERA, restricting vector leptoquarks that couple with electromagnetic strength to right-handed quarks to lie above 430 GeV or 460 GeV, and leptoquarks that couple with electromagnetic strength to left-handed quarks to lie above 1.3 TeV, 1.2 TeV, and 1.5 TeV for the SU(2${)}_{\mathit{W}}$ singlet, doublet, and triplet, respectively.

The eventbuilder of the ZEUS detector

The ZEUS detector, one of the two detectors at DESY's ep-collider HERA, requires a fairly complex data acquisition system to cope with 250,000 readout channels and a bunch crossing interval of 96 ns. The highly parallel system is split up into 17 component subsystems and a three level trigger system. Central to the system is the Eventbuilder: the Eventbuilder combines the data flows of the component subsystems and distributes the event data records to the six branches of processor nodes of the third level trigger system. The Eventbuilder is an asynchronous packet-switching transputer network using transputer links for bulk data transfer. It achieves a sustained data throughput of 24 MB/sec over a distance of 100 m. A high speed 64X64 custom made crossbar switch allows dynamical linking of any component to any branch of processor nodes. Implementation and system operation are described, and experience gained in the design and integration is presented.

Longitudinal structure function at intermediate x and the gluon density

Calculations are presented of the longitudinal structure function F L ( x, Q 2). We use next-to-leading order expressions in QCD (O( α 2 s )) plus parton densities determined previously from global fits to data on deep inelastic lepton scattering, prompt photon production, and bottom quark production. Anticipated data from the DESY ep collider HERA should provide discriminating information on the gluon density, particularly for values of x < 0.05.

Reexamining the jet contribution to the photoproduction cross section.

We reexamine the contribution of minijet production to the total photoproduction cross section. We show that previous calculations showing a dramatic increase of the cross section with energy have overestimated the minijet contribution at very high energies (${\mathit{E}}_{\ensuremath{\gamma}}$&gt;${10}^{6}$ GeV). We present revised quantitative predictions. Understanding the high energy behavior of the photoproduction cross section should be a high priority as it is dictated by the gluon structure of high energy photons which will be studied in detail at the DESY ep collider HERA. It is also essential for understanding the development of extensive air showers.

Radiative zeros and extra neutral gauge bosons at lepton-hadron colliders.

We consider the process ${e}^{\ifmmode\pm\else\textpm\fi{}}$p\ensuremath{\rightarrow}${e}^{\ifmmode\pm\else\textpm\fi{}}$\ensuremath{\gamma}X. In the first section, we look for radiative zeros and find that they are absent in ${e}^{\mathrm{\ensuremath{-}}}$p collisions but possibly observable in ${e}^{+}$p collisions. In the second section we investigate the effects of the exchange of an extra neutral gauge boson on the process at hand. We find that, given appropriate cuts and using expected luminosities at the DESY ep collider HERA, a 2-standard-deviation effect could arise from a light neutral gauge boson. Z' masses up to \ensuremath{\sim}300 GeV/${c}^{2}$ could be probed at the 1-standard-deviation level.

Production of supersymmetric particles in elastic ep collisions.

The reactions ep\ensuremath{\rightarrow}pe\ifmmode \tilde{}\else \~{}\fi{}Z${\ifmmode \tilde{}\else \~{}\fi{}}_{i}$ and ep\ensuremath{\rightarrow}p\ensuremath{\nu}\ifmmode \tilde{}\else \~{}\fi{}W${\ifmmode \tilde{}\else \~{}\fi{}}_{\mathrm{\ensuremath{-}}}$ are investigated, where Z${\ifmmode \tilde{}\else \~{}\fi{}}_{i}$ and W${\ifmmode \tilde{}\else \~{}\fi{}}_{\mathrm{\ensuremath{-}}}$ are a generic neutralino and the lightest chargino eigenstate, respectively. We present both exact results and a modified effective-photon approximation. We show that ten fully measured e\ifmmode \tilde{}\else \~{}\fi{}Z${\ifmmode \tilde{}\else \~{}\fi{}}_{1}$ events with e\ifmmode \tilde{}\else \~{}\fi{}\ensuremath{\rightarrow}eZ${\ifmmode \tilde{}\else \~{}\fi{}}_{1}$ decay, where the Z${\ifmmode \tilde{}\else \~{}\fi{}}_{1}$ escape detection, are enough to determine ${m}_{e}$\ifmmode \tilde{}\else \~{}\fi{} and ${m}_{Z}$${\ifmmode \tilde{}\else \~{}\fi{}}_{1}$ with statistical errors of 2 GeV; the detection of the outgoing proton in a forward proton spectrometer is crucial. e\ifmmode \tilde{}\else \~{}\fi{}Z${\ifmmode \tilde{}\else \~{}\fi{}}_{2}$ and \ensuremath{\nu}\ifmmode \tilde{}\else \~{}\fi{}W${\ifmmode \tilde{}\else \~{}\fi{}}_{\mathrm{\ensuremath{-}}}$ production, resulting mostly in final states with one or three charged leptons plus missing ${p}_{T}$, are also discussed. Within the framework of minimal supergravity models, these elastic processes make the DESY ep collider HERA competitive with or even superior to the contemporary colliders CERN LEP I and Fermilab Tevatron as far as searches for supersymmetry are concerned.

Measuring the parton content of the photon at the DESY ep collider HERA.

We discuss the possibility to measure the parton, and especially gluon, content of the photon at the forthcoming DESY ep collider HERA. We first reemphasize that the production of two jets with high transverse momentum ${p}_{T}$ is dominated by events where the photon is resolved into quarks and gluons for ${p}_{T}$\ensuremath{\lesssim}40 GeV. We then show that these ``resolved'' contributions, and in particular those that are initiated by a gluon from the photon, are especially important at large jet rapidities. Rapidity distributions also allow us to isolate the direct photon-gluon fusion contributions to bb\ifmmode\bar\else\textasciimacron\fi{} production. This makes it possible to use this process to determine the gluon content of the proton. However, a direct measurement of the gluon content of the photon will only be possible if the spectator jet from the photon can be detected. We find that this jet typically has an energy of 10--12 GeV and estimate its opening angle to be around 15\ifmmode^\circ\else\textdegree\fi{}, which should allow for its detection.

Effects of the hadronic structure of the photon at the DESY ep collider HERA.

We investigate the production of hadronic jets with high transverse momenta p/sub T/ at the forthcoming DESY ep collider HERA. We find that events where the exchanged quasireal photon is resolved into quarks and gluons dominate the two-jet cross section for p/sub T/approx. <40 GeV. We also find that this class of events contributes about 20% to the total bb-bar and cc-bar production. The possibility to use these processes to measure the gluon content of the photon is discussed.

Comparison of discovery limits for E6 neutral gauge bosons at future colliders.

We study and compare the phenomenology of extra ${\mathrm{E}}_{6}$ neutral gauge bosons at ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$, ep, and hadron colliders. Our purpose is to compare the discovery reach of the various colliders and see how they compare in studying the phenomenology of extra neutral gauge bosons. We find that of the future colliders, hadron colliders probe the highest mass scales for the existence of extra neutral gauge bosons although precision measurements at ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ colliders, the SLAC Linear Collider will put tight constraints on models predicting extra neutral gauge bosons. If deviations from the standard model are observed at ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ colliders at \ensuremath{\surd}As=${\mathrm{M}}_{\mathit{Z}}^{0}$, other measurements, especially those at the DESY ep collider HERA, will be necessary to unravel the physics. We also find that high-precision, low-energy neutral-current experiments will be competitive with ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ and ep colliders for detecting the presence of extra ${\mathrm{E}}_{6}$ neutral gauge bosons.

Nonstandard-Higgs-scalar- and pseudoscalar-boson production in ep and e+e- colliders.

Nonstandard Higgs bosons in the mass range 40--150 GeV/${c}^{2}$ are studied for two-Higgs-doublet extensions of the standard electroweak model. Upper bounds on production rates are compared for ep and ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ colliders. Under favorable conditions our calculations indicate that about 100 events per year production rates at the DESY ep collider HERA for nonstandard Higgs bosons are obtainable. Electron-positron annihilations give smaller rates but cleaner signals.