Collider HERA Research Articles

Testing the equivalent photon approximation of the proton in the process $ep \rightarrow \nu W X$

The accuracy of the equivalent photon approximation (EPA) of the proton in describing the inelastic process $ep\to \nu W X$ is investigated. In particular, the scale dependence of the corresponding inelastic photon distribution is discussed. Furthermore, an estimate of the total number of events, including the ones coming from the elastic and quasi-elastic channels of the reaction, is given for the HERA collider.

EVENTS WITH ISOLATED CHARGED LEPTONS AND LARGE MISSING TRANSVERSE MOMENTUM AT HERA

Striking events with isolated charged leptons, large missing transverse momentum and large transverse momentum of the hadronic final state [Formula: see text] were observed at the electron proton collider HERA in a data sample corresponding to an integrated luminosity of about 130 pb-1. The H1 collaboration observed 11 events with isolated electrons or muons and with [Formula: see text]. Only 3.4±0.6 events were expected from Standard Model (SM) processes. Six of these events have [Formula: see text], while 1.3±0.3 events were expected. The ZEUS collaboration observed good agreement with the SM. However, ZEUS found two events with a similar event topology, but tau leptons instead of electrons or muons in the final state. Only 0.2±0.05 events were expected from SM processes. For various hypotheses, the compatibility of the experimental results was investigated with respect to the SM and with respect to possible explanations beyond the SM. Prospects for the high-luminosity HERA-II data taking period are given.

A radiation monitor for the ZEUS detector at HERA

A radiation monitoring and automatic beam-dump system has been installed in the interaction region of the ZEUS experiment during the last upgrade of the electron/positron-proton collider HERA. It is used to prevent the ZEUS Silicon Microvertex Detector (MVD) from beam-background induced radiation damage during HERA operation. The dose rate measurement is based on the readout of radiation induced signal currents from silicon PIN diodes. A precise temperature measurement allows to correct for temperature induced changes in the offset leakage current of the diodes. The online dose rate measurement is complemented by a system of radiation field effect transistors (RadFETs), which are sensitive to accumulated surface damage from ionizing radiation. The integrated dose is obtained from the change in the threshold voltage of the RadFET devices. In addition, thermo-luminescense dosimeters (TLDs) measuring the integrated dose of photon and neutron irradiation are used to control and calibrate the online dose measurements. The radiation monitoring system has been designed to measure the dose on a time scale from milliseconds up to the lifetime of the experiment of several years. It provides online information on the actual level of background radiation to both the HERA and ZEUS shift crews and triggers an automatic dump of the lepton beam in case of excessively high instantaneous dose rates.

Suppressing the background process to QED Compton scatteringfor delineating the photon content of the proton

We investigate the QED Compton process (QEDCS) in $e p \rightarrow e \gamma p$ and $e p \rightarrow e \gamma X$ , together with the major background coming from virtual Compton scattering (VCS), where the photon is emitted from the hadronic vertex. We suggest new kinematical constraints which suppress the VCS background and are furthermore suitable for the extraction of the equivalent photon content of the proton at the HERA collider. We show that the cross section, commonly expressed in terms of the proton structure functions, is reasonably well described by the equivalent photon approximation of the proton, also in the inelastic channel in the proposed kinematical region.

Energy dependence of Cronin momentum in saturation model for p + A and A + A collisions

We investigate the QED Compton process (QEDCS) in $e p \to e \gamma p$ and $e p \to e \gamma X$, together with the major background coming from the virtual Compton scattering (VCS), where the photon is emitted from the hadronic vertex. We suggest new kinematical constraints which suppress the VCS background and are furthermore suitable for the extraction of the equivalent photon content of the proton at the HERA collider. We show that the cross section, commonly expressed in terms of the proton structure functions, is reasonably well described by the equivalent photon approximation of the proton, also in the inelastic channel in the proposed kinematical region.

Neutral top-pion πt0 and tγ(z) production at the HERA and THERA colliders

In the context of top-color assisted technicolor (TC2) models, we calculate the contributions of the neutral top-pion πt0 to tγ and tz production via the processes ep→γc→tγ and ep→γc→tz at the HERA and THERA colliders. Our results show that the cross sections σtγ(s) and σtz(s) are very small at the HERA collider with s=320 GeV. However, in most of the parameter space, σtγ(s) or σtz(s) is in the range of about 0.1–1 pb at the THERA collider with s=1000 GeV.

Color dipoles andρ,φelectroproduction

We present a detailed comparison of a variety of predictions for diffractive light vector meson production with the data collected at the DESY HERA collider. All our calculations are performed within a dipole model framework and make use of different models for the meson light-cone wave function. There are no free parameters in any of the scenarios we consider. Generally we find good agreement with the data using rather simple Gaussian motivated wave functions in conjunction with dipole cross sections which have been fitted to other data.

Neutral top-pion $pi;t0 and t$gamma;(z) production at the HERA and THERA colliders

In the context of top-color assisted technicolor (TC2) models, we calculate the contributions of the neutral top-pion π t 0 to tγ and tz production via the processes ep → γc → tγ and ep → γc → tz at the HERA and THERA colliders. Our results show that the cross sections σ tγ ( s ) and σ tz ( s ) are very small at the HERA collider with s =320 GeV. However, in most of the parameter space, σ tγ ( s ) or σ tz ( s ) is in the range of about 0.1–1 pb at the THERA collider with s =1000 GeV.

Search for QCD-instanton induced events in deep inelastic ep scattering at HERA

A search for QCD-instanton-induced events in deep inelastic ep scattering has been performed with the ZEUS detector at the HERA collider, using data corresponding to an integrated luminosity of 38 pb^{-1}. A kinematic range defined by cuts on the photon virtuality, Q^2 > 120 GeV^2, and on the Bjorken scaling variable, x > 10^{-3}, has been investigated. The QCD-instanton induced events were modelled by the Monte Carlo generator QCDINS. A background-independent, conservative 95% confidence level upper limit for the instanton cross section of 26 pb is obtained, to be compared with the theoretically expected value of 8.9 pb.

Investigation of the Balitsky-Fadin-Kuraev-Lipatov dynamics of gluon distributions in the inelastic production of J/ψ mesons at the HERA collider

The inelastic photo-and electroproduction of J/ψ mesons at the HERA collider are considered within the semihard (kT-factorization) QCD approach and the color-singlet model. The total, differential, and double-differential cross sections for the inelastic production of J/ψ mesons are investigated versus the Pomeron intercept Δ, which is the basic parameter of low-x physics; also studied here is the spin alignment parameter α versus the square of the transverse momentum, pψT2, and the variable z. The theoretical results obtained in the present study are compared with the latest experimental data of the H1 and ZEUS Collaborations. It is shown that experimental investigations of the polarization properties of J/ψ mesons at the HERA collider for Q2<1 GeV2 may provide an additional test of the Balitsky-Fadin-Kuraev-Lipatov dynamics of gluon distributions.

Estimating the rate of photon + jet events to determine the gluon distribution in the Tevatron Run II

It is shown that experiments at an integrated luminosity of Lint=3 fb−1 in the Tevatron Run II will make it possible to collect about one million photon + jet events selected according to the criteria proposed previously. Such statistics would allow one to cover the kinematical region that is specified by the inequalities 2×10−3<x<1.0 and 1.6×103≤Q2≤2×104 (GeV/c)2 and which was not studied in previous experiments devoted to measuring proton structure functions. This region contains values of Q2 that are on average an order of magnitude higher than any ever attained in experiments at the HERA collider.

DEEP INELASTIC LEPTON-NUCLEON SCATTERING AT HERA

Data from the HERA collider experiments, H1 and ZEUS, have been fundamental to the rapid recent development of our understanding of the partonic composition of the proton and of QCD. This report focuses on inclusive measurements of neutral and charged current cross sections at HERA, using the full available data taken to date. The present precision on the proton parton densities and the further requirements for future measurements at the Tevatron and LHC are explored. Emphasis is also placed on the region of very low Bjorken-x and Q2. In this region, the 'confinement' transition takes place from partons to hadrons as the relevant degrees of freedom and novel or exotic QCD effects associated with large parton densities are most likely to be observed. Finally, prospects for the second phase of HERA running are discussed.

SEARCHES FOR NEW PHENOMENA AT COLLIDERS

An overview of recent experimental results on searches for new phenomena at the LEP, HERA and Tevatron high energy colliders is presented, including in particular new results obtained from the analysis of the Run II data at the Tevatron. No significant evidence for physics beyond the Standard Model has been found and limits at the 95% confidence level have been set on the mass and couplings of several new particles. The complementarity between the different experiments is discussed, as well as future prospects for ongoing and future experiments.

A Fast Track Trigger for the H1 collaboration

A new Fast Track Trigger was built for the H1 Experiment at the electron–proton collider HERA. This trigger allows the full three-dimensional reconstruction of all charged tracks measured in the central drift chamber within 25 μs. Particles are reconstructed down to 100 MeV in a high track multiplicity environment with a momentum resolution of better than 3% for 1 GeV tracks. This new device is thus ideally suited to trigger on exclusive final states by identifying particle resonances in e.g. open heavy quark or vector meson production. The trigger makes use of highly parallel track reconstruction algorithms implemented in modern electronics by exploiting high density FPGAs with embedded content addressable memory functionalities for track linking and Floating Point DSPs for track fitting. First results from the commissioning of the frontend system are presented.

Isolated tau leptons in events with large missing transverse momentum at HERA

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.

Diffractive vector meson production with a large momentum transfer

We summarise recent progress in the computation of helicity amplitudes for diffractive vector meson production at large momentum transfer and their comparison to data collected at the HERA collider.

FCNC top quark production via anomaloustqVcouplings beyond leading order

We calculate flavor-changing neutral-current (FCNC) processes involving top-quark production via anomalous tqV couplings at the Tevatron and HERA colliders. We cover the FCNC processes p pbar -> t Z, p pbar -> t gamma, p pbar -> t t, and e p -> e t. We go beyond leading order and include the effects of soft-gluon corrections through next-to-next-to-leading order. We demonstrate the stabilisation of the cross section with respect to the variation of QCD scale, and we investigate the reach of the Tevatron and HERA colliders.

Vector meson photoproduction from the BFKL equation 2. Phenomenology

Diffractive vector meson photoproduction accompanied by proton dissociation is studied for large momentum transfer. The process is described by the non-forward BFKL equation which we use to compare to data collected at the HERA collider.

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.

Search for (singly and doubly) charged Higgses

Recent results on searches for singly and doubly charged Higgs bosons at LEP, Tevatron and Hera colliders are reported. These results do not show any deviation from the Standard Model expetations allowing to set lower limits on the mass of the charged Higgs bosons.