Collider HERA Research Articles

Investigation of the inelastic electroproduction of J/ψ mesons at the HERA collider within the semihard QCD approach

The results of theoretical calculations within the semihard (kT-factorization) approach are compared with the latest data obtained in the ZEUS experiment at the HERA collider for the inelastic electroproduction of J/ψ mesons in the kinematical region specified by the inequalities 2 GeV2 < Q2 < 80 GeV2, 50 GeV < W < 250 GeV, 0.2 < z < 0.9, and −1.6 < yψlab < 1.3. The dependence of these theoretical results on the choice of parametrization among three widely used parametrizations of unintegrated gluon distributions is studied.

Technical Report: PETRA III: DESY's New High Brilliance Third Generation Synchrotron Radiation Source

At present, the storage ring DORIS III (4.5 GeV, 150 mA) serves as the main source for synchrotron radiation experiments at DESY. DORIS III is a second generation storage ring with a rather large emittance of 404 nmrad, thus producing high flux X-ray beams of comparatively low brilliance. In order to follow the increasing user request for high brilliance beams, DESY will rebuild the storage ring PETRA into a very low emittance synchrotron radiation source called PETRA III. PETRA (positron electron tandem ring accelerator) is a storage ring with 2.304 km circumference, which was built in 1976 as an e+ - e− collider. Currently PETRA is used as a pre-accelerator for the proton-lepton collider HERA. In parallel it also serves one undulator beamline in parasitic mode.

Longitudinal spin transfer to theΛhyperon in semiinclusive deep-inelastic scattering

The transfer of polarization from a high-energy positron to a {lambda}{sup 0} hyperon produced in semiinclusive deep-inelastic scattering has been measured. The data have been obtained by the HERMES experiment at DESY using the 27.6 GeV longitudinally polarized positron beam of the HERA collider and unpolarized gas targets internal to the positron (electron) storage ring. The longitudinal spin-transfer coefficient is found to be D{sub LL{sup '}}{sup {lambda}}=0.11{+-}0.10(stat){+-}0.03(syst) at an average fractional energy carried by the {lambda}{sup 0} hyperon =0.45. The dependence of D{sub LL{sup '}}{sup {lambda}} on both the fractional energy z and the fractional longitudinal momentum x{sub F} is presented.

Institute of Theoretical and Experimental Physics (ITEP, Moscow) in the H1 experiment

A group of researchers from the Institute of Theoretical and Experimental Physics (ITEP, Moscow) took part at almost all stages of the H1 experiment performed at the HERA collider (Hamburg) in order to study lepton-proton interactions at high energies. Several subdetectors of the H1 detector were developed, designed, and constructed at the ITEP industrial workshop and domestic enterprises. In particular, the ITEP staff participated in assembling and tunning the equipment, servicing the detector and the data acquisition system, and analyzing and presenting the results. Researchers from ITEP have been playing a crucial role at many stages of the experiment to the present day.

The hadronic final state at HERA

The electron-proton collider HERA offers unique capabilities to study the properties of the hadronic final state. This paper summarizes selected results highlights from the colliding beam experiments H1 and ZEUS.

Deep inelastic beauty production at HERA in thekT-factorization approach

We calculate the cross section of beauty production in electron-proton deep inelastic scattering at HERA collider in the framework of the kt-factorization approach. The unintegrated gluon distributions in a proton are obtained from the full CCFM, from unified BFKL-DGLAP evolution equations as well as from the Kimber-Martin-Ryskin prescription. We investigate different production rates and study the b-quark contribution to the inclusive proton structure function F2(x,Q2) at small x and at moderate and high values of Q2. Our theoretical results are compared with the recent experimental data taken by the H1 and ZEUS collaborations. We demonstrate the importance of leading ln 1/x contributions in description of the HERA data.

Measurement of charm and beauty dijet cross sections in photoproduction at HERA using the H1 vertex detector

A measurement of charm and beauty dijet photoproduction cross sections at the ep collider HERA is presented. Events are selected with two or more jets of transverse momentum $p_t^{jet}_{1(2)}>11(8)$ GeV in the central range of pseudo-rapidity $-0.9<\eta^{jet}_{1(2)}<1.3$. The fractions of events containing charm and beauty quarks are determined using a method based on the impact parameter, in the transverse plane, of tracks to the primary vertex, as measured by the H1 central vertex detector. Differential dijet cross sections for charm and beauty, and their relative contributions to the flavour inclusive dijet photoproduction cross section, are measured as a function of the transverse momentum of the leading jet, the average pseudo-rapidity of the two jets and the observable $x_{\gamma}^{obs}$. Taking into account the theoretical uncertainties, the charm cross sections are consistent with a QCD calculation in next-to-leading order, while the predicted cross sections for beauty production are somewhat lower than the measurement.

Unintegrated gluon distributions in D*± and dijet associated photoproduction at HERA

We consider the photoproduction of D*± mesons associated with two hadron jets at HERA collider in the framework of the kT-factorization approach. The unintegrated gluon densities in a proton are obtained from the full CCFM, from unified BFKL-DGLAP evolution equations as well as from the Kimber–Martin–Ryskin prescription. Resolved photon contributions are reproduced by the initial-state gluon radiation. We investigate different production rates and make a comparison with the recent experimental data taken by the ZEUS collaboration. Special attention is given to the specific dijet correlations which can provide unique information about non-collinear gluon evolution dynamics.

Beauty photoproduction at HERA:kT-factorization versus experimental data

We calculate the cross section of charm photoproduction at HERA collider in the framework of the kT-factorization QCD approach. Our analysis cover the inclusive charm production as well as charm and associated jet production processes. Both photon-gluon and gluon-gluon fusion mechanisms are taken into account. The unintegrated gluon densities in a proton and in a photon obtained from the full CCFM, from unified BFKL-DGLAP evolution equations as well as from the Kimber-Martin-Ryskin prescription are used. Our theoretical results are compared with the recent experimental data taken by the H1 and ZEUS collaborations at HERA. Special attention is put on the specific angular correlations which can provide unique information about non-collinear gluon evolution dynamics.

ELECTROWEAK MEASUREMENTS

The measurements of electroweak sector of the Standard Model are presented, including most recent results from LEP, Tevatron and HERA colliders. The robustness of the Standard Model is illustrated with the precision measurements, the electroweak fits and the comparisons to the results obtained from low energy experiments. The status of the measurements of the W boson properties and rare production processes involving weak bosons at colliders is examined, together with the measurements of the electroweak parameters in ep collisions.

Probing SmallxParton Densities in UltraperipheralAAandpACollisions at the CERN Large Hadron Collider

We calculate photoproduction rates for several hard processes in ultraperipheral proton-lead and lead-lead collisions at the CERN Large Hadron Collider (LHC) with $\sqrt{{s}_{NN}}=8.8$ and 5.5 TeV, respectively, which could be triggered in the large LHC detectors. We use ATLAS as an example. The lead ion is treated as a source of (coherently produced) photons with energies and intensities greater than those of equivalent $ep$ collisions at the DESY collider HERA. We find very large rates for both inclusive and diffractive production that will extend the HERA $x$ range by nearly an order of magnitude for similar virtualities. We demonstrate that it is possible to reach the kinematic regime where nonlinear effects are larger than at HERA.

Vertexing in the H1 experiment

Abstract The H1 experiment at the ep collider HERA has been equipped with a silicon strip detector, mainly for the purpose of heavy flavor physics. First results have recently become final. The vertexing techniques and strategies employed in H1 are reviewed.

Multijet production in neutral current deep inelastic scattering at HERA and determination of $\alpha_s$

Multijet production rates in neutral current deep inelastic scattering have been measured in the range of exchanged boson virtualities 10 < Q2 < 5000 GeV2. The data were taken at the ep collider HERA with centre-of-mass energy sqrt(s) = 318 GeV using the ZEUS detector and correspond to an integrated luminosity of 82.2 pb-1. Jets were identified in the Breit frame using the k_T cluster algorithm in the longitudinally invariant inclusive mode. Measurements of differential dijet and trijet cross sections are presented as functions of jet transverse energy E_{T,B}{jet}, pseudorapidity eta_{LAB}{jet} and Q2 with E_{T,B}{jet} > 5 GeV and -1 < eta_{LAB}{jet} < 2.5. Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant alpha_s(M_Z), determined from the ratio of the trijet to dijet cross sections, is alpha_s(M_Z) = 0.1179 pm 0.0013(stat.) {+0.0028}_{-0.0046}(exp.) {+0.0064}_{-0.0046}(th.)

Prompt photon photoproduction at DESY HERA in thekT-factorization approach

We present calculations of the prompt photon photoproduction at HERA collider in the k_T-factorization approach. Both direct and resolved contributions are taken into account. The conservative error analisys is performed. The unintegrated parton densities in a proton and in a photon are determined using the Kimber-Martin-Ryskin prescription. We investigate both inclusive and associated with jet prompt photon photoproduction rates. In particular, we study the angular correlations between produced photon and hadronic jet in the transverse momentum plane which can provide a unique information about non-collinear evolution dynamics. We compare our theoretical predictions with recent experimental data taken by the H1 and ZEUS collaborations.

Measurement of charm and beauty photoproduction at HERA using [formula omitted] correlations

A measurement of charm and beauty photoproduction at the electron proton collider HERA is presented based on the simultaneous detection of a D*± meson and a muon. The correlation between the D* meson and the muon serves to separate the charm and beauty contributions and the analysis provides comparable sensitivity to both. The total and differential experimental cross sections are compared to LO and NLO QCD calculations. The measured charm cross section is in good agreement with QCD predictions including higher order effects while the beauty cross section is higher.

Measurement of beauty production at HERA using events with muons and jets

A measurement of the beauty production cross section in ep collisions at a centre-of-mass energy of 319 GeV is presented. The data were collected with the H1 detector at the HERA collider in the years 1999-2000. Events are selected by requiring the presence of jets and muons in the final state. Both the long lifetime and the large mass of b-flavoured hadrons are exploited to identify events containing beauty quarks. Differential cross sections are measured in photoproduction, with photon virtualities Q^2 < 1 GeV^2, and in deep inelastic scattering, where 2 < Q^2 < 100 GeV^2. The results are compared with perturbative QCD calculations to leading and next-to-leading order. The predictions are found to be somewhat lower than the data.

Recent ZEUS results on Vector Mesons

An overview of the recent experimental results on diffractive vector meson production in ep scattering measured by the ZEUS experiment at the HERA collider is presented. The results are compared to pQCD-based theoretical models.

MULTIJET PRODUCTION IN NEUTRAL CURRENT DEEP INELASTIC SCATTERING AT HERA AND DETERMINATION OF αs

Multijet production rates in neutral current deep inelastic scattering (DIS) have been measured in the range of exchanged boson virtualities 10&lt;Q2&lt;5000 GeV2. The data were taken at the ep collider HERA with centre-of-mass energy [Formula: see text] using the ZEUS detector and correspond to an integrated luminosity of 82.2 pb-1. Jets were identified in the Breit frame using the kTcluster algorithm in the longitudinally invariant inclusive mode. Measurements of differential multijet cross sections are presented as functions of jet transverse energy [Formula: see text], pseudorapidity [Formula: see text] and Q2with [Formula: see text] and [Formula: see text]. Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant αs(MZ), determined from the ratio of the trijet to dijet cross sections, is [Formula: see text].

Heavy flavour production at HERA

Recent results are presented on the production of charm and bottom quarks in electron-proton collisions at the HERA collider. The larger part of this report covers the analysis of the full HERA-I data set. The results are compared to calculations in perturbative quantum chromo dynamics at next-to-leading order and Monte Carlo predictions at leading order. Prospects for additional measurements after the collider and detector upgrades are discussed.

Top-pions and single top production at HERA and THERA

We study single top quark production at the HERA and THERA colliders as coming from the FCNC vertices tcγ and tcZ that appear at one loop level in topcolor-assisted technicolor models (TC2). In contrast with previous expectations, we find that the production cross section is of order 10−6 pb for HERA and 10−4 pb for THERA (even lower that the SM prediction). Therefore, none of the two colliders are capable to probe the pseudoscalar top-pion or the scalar top-Higgs predicted in TC2 models via single top production.