DESY HERA Research Articles

Diffractive Higgs bosons and prompt photons at hadron colliders

Models for soft color interactions have been successful in describing and predicting diffractive hard scattering processes in ep collisions at DESY HERA and ppbar at the Fermilab Tevatron. Here we present new comparisons of the model to recent diffractive dijet data, also showing good agreement. The topical issue of diffractive Higgs production at the Tevatron and LHC hadron colliders is further investigated. For H->gamma gamma the irreducible background of prompt photon pairs from qqbar->gamma gamma and gg->gamma gamma is always dominating, implying that higher branching ratio decay modes of the Higgs have to be used. However, such prompt photons can be used to test the basic prediction for Higgs production since gg->gamma gamma involves a quark loop diagram similar to gg->H.

Theoretical aspects of inclusive light-hadron production

We summarize the key results of a recent global analysis of inclusive single-charged-hadron production in high-energy colliding-beam experiments. In the framework of the parton model of quantum chromodynamics at next-to-leading order (NLO), fragmentation functions (FFs) for charged pions, charged kaons and (anti)protons are extracted from the experimental data of e+e− annihilation at the Z-boson resonance and at centre-of-mass energy √s = 29 GeV. This fit also yields a new NLO value for the strong-coupling constant, namely α(5)s(MZ) = 0.1170 ± 0.0073. The scaling violations encoded in the FFs through the Altarelli–Parisi evolution equations are tested by confronting e+e−-annihilation data from DESY DORIS, DESY PETRA and CERN LEP2 with NLO predictions based on these FFs. Comparisons of pp̄ data from CERN Spp̄S and the Fermilab Tevatron, γp data from DESY HERA and γγ data from LEP2 with the corresponding NLO predictions allow us to test the universality of the FFs predicted by the factorization theorem.

Erratum: PolarizedJ/ψproduction in deep inelastic scattering at DESY HERA [Phys. Rev. D 63, 034017 (2001)

We perform a calculation on the polarization of J/psi production in deep inelastic scattering in the HERA energy range. For the inclusive production distributions, we find that the color-singlet contributions are consistent with the experimental data in the major region of z (z>0.4). Only in low z regions, there are some hints of the need of the color-octet contributions to describe the experimental data. For the polarization of J/psi in DIS processes, we find the parameter alpha changes with Q^2. Especially, at higher Q^2, difference on alpha between color-singlet and color-octet contributions become more distinctive. In the two regions of lower and larger z, the polarization parameter alpha have different features. These properties can provide important information on the polarization mechanism for J/psi production.

Erratum: Next-to-leading order evolution of generalized parton distributions for DESY HERA and HERMES [Phys. Rev. D65, 056012 (2002)

The QCD evolution of both unpolarized and polarized generalized parton distributions (GPDs) to next-to-leading order (NLO) accuracy is presented, in both the DGLAP and ERBL regions, for two appropriately symmetrized input distributions based on conventional parton density functions. To illustrate the relative size of the NLO corrections a comparison is made with leading order evolution of the same distributions. For the first time, NLO results are given for both small and large values of the skewedness parameter, $\zeta = x_{bj}$, i.e. for all of the kinematic range relevant to HERA and HERMES.

Diffractive Higgs boson production at the Fermilab Tevatron and the CERN Large Hadron Collider.

Improved possibilities to find the Higgs boson in diffractive events, having less hadronic activity, depend on whether the cross section is large enough. Based on the soft color interaction models that successfully describe diffractive hard scattering at DESY HERA and the Fermilab Tevatron, we find that only a few diffractive Higgs events may be produced at the Tevatron, but we predict a substantial rate at the CERN Large Hadron Collider.

Prospects of searching for excited leptons during run II of the Fermilab Tevatron

We present the prospects of searching for excited leptons during run II of the Fermilab Tevatron. We concentrate on single and pair production of excited electrons in the photonic decay channel for both $2 {\mathrm{fb}}^{\ensuremath{-}1}$ (run IIa) and $15 {\mathrm{fb}}^{\ensuremath{-}1}$ (run IIb) using one CDF or $\mathrm{D}\mathrm{\O{}}$ detector equivalent. We expect the Tevatron to improve upon the limits set by CERN LEP and DESY HERA for excited lepton masses above 190 GeV by the end of run IIa.

Measurement of analyzing powers ofπ+andπ−produced on a hydrogen and a carbon target with a22−GeV/cincident polarized proton beam

The analyzing powers of ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ were measured using an incident $22\ensuremath{-}\mathrm{GeV}/c$ transversely polarized proton beam at the Brookhaven Alternating Gradient Synchrotron. A magnetic spectrometer measured ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ inclusive asymmetries on a hydrogen and a carbon target. An elastic polarimeter with a ${\mathrm{CH}}_{2}$ target measured pp elastic-scattering asymmetries to determine the beam polarization using published data for the pp elastic analyzing power. Using the beam polarization determined from the elastic polarimeter and asymmetries from the inclusive spectrometer, analyzing powers ${A}_{N}$ for ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ were determined in the ${x}_{F}$ and ${p}_{T}$ ranges $(0.45--0.8)$ and $(0.3--1.2 \mathrm{GeV}/c),$ respectively. The analyzing power results are similar in both sign and character to other measurements at 200 and $11.7 \mathrm{GeV}/c,$ confirming the expectation that high-energy pion inclusive analyzing powers remain large and relatively energy independent. This suggests that pion inclusive polarimetry may be a suitable method for measuring future beam polarizations at BNL RHIC or DESY HERA. Analyzing powers of ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ produced on hydrogen and carbon targets are the same. Various models to explain inclusive analyzing powers are also discussed.

Search for lepton-flavor violation ine+pcollisions at DESY HERA

A search has been made for lepton-flavor-violating interactions, e+ p -> mu/tau X, at a center-of-mass energy of 300 GeV with an integrated luminosity of 47.7 pb^-1 using the ZEUS detector at HERA. No evidence was found for lepton-flavor violation and constraints were derived on leptoquarks that could mediate such interactions. Some of the constraints are also applicable to lepton-flavor-violating processes mediated by squarks in R-parity-violating supersymmetric models. In some cases involving heavy quarks and especially for the tau channel, the ZEUS constraints are the most stringent published to date.

J/Ψ production in deep inelastic electron scattering at HERA

J/ψ production in deep inelasic electron–proton scattering is calculated in the framework of nonrelativistic quantum chromodynamics. All the calculated differential distributions, except for one, turn out to be very sensitive to the colour octet mechanism. The analytical results disagree with previous analyses, both for the colour singlet and colour octet channels. The theoretical predictions agree reasonably well with recent data taken by the H1 Collaboration at DESY HERA, significantly better than those obtained within the colour singlet model.

Kaluza-Klein states of the standard model gauge bosons: Constraints from high energy experiments

In theories with the standard model gauge bosons propagating in ${\mathrm{TeV}}^{\ensuremath{-}1}$-size extra dimensions, their Kaluza-Klein states interact with the rest of the SM particles confined to the 3-brane. We look for possible signals for this interaction in the present high-energy collider data, and estimate the sensitivity offered by the next generation of collider experiments. Based on the present data from the CERN LEP 2, Fermilab Tevatron, and DESY HERA experiments, we set a lower limit on the extra dimension compactification scale ${M}_{C}>6.8 \mathrm{TeV}$ at the $95%$ confidence level (dominated by the LEP 2 results) and quote expected sensitivities in the Tevatron Run 2 and at the CERN LHC.

Next-to-leading order evolution of generalized parton distributions for DESY HERA and HERMES

The QCD evolution of both unpolarized and polarized generalized parton distributions (GPDs) to next-to-leading order (NLO) accuracy is presented, in both the DGLAP and ERBL regions, for two appropriately symmetrized input distributions based on conventional parton density functions. To illustrate the relative size of the NLO corrections a comparison is made with leading order evolution of the same distributions. For the first time, NLO results are given for both small and large values of the skewedness parameter, $\ensuremath{\zeta}{=x}_{\mathrm{bj}},$ i.e., for all of the kinematic range relevant to DESY HERA and HERMES.

Properties of hadronic final states in diffractive deep inelasticepscattering at DESY HERA

Characteristics of the hadronic final state of diffractive deep inelastic scattering events, ep -> eXp, were studied in the kinematic range 4 < M_X < 35 GeV, 4 < Q^2 < 150 GeV^2, 70 < W < 250 GeV and 0.0003 < x_pom < 0.03 with the ZEUS detector at HERA using an integrated luminosity of 13.8 pb^{-1}. The events were tagged by identifying the diffractively scattered proton using the leading proton spectrometer. The properties of the hadronic final state, X, were studied in its center-of-mass frame using thrust, thrust angle, sphericity, energy flow, transverse energy flow and ``seagull'' distributions. As the invariant mass of the system increases, the final state becomes more collimated, more aligned and more asymmetric in the average transverse momentum with respect to the direction of the virtual photon. Comparisons of the properties of the hadronic final state with predictions from various Monte Carlo model generators suggest that the final state is dominated by qqg states at the parton level.

Color evaporation and elasticΥphotoproduction at DESY HERA

The diffractive photoproduction of vector mesons is usually described by considering the two-gluon (Pomeron) exchange, the nondiagonal parton distributions, and the contribution of the real part to the cross section. In this Brief Report we analyze the diffractive photoproduction of the $\ensuremath{\Upsilon}$ at DESY HERA using an alternative model, the color evaporation model (CEM), where the cross section is simply determined by the boson-gluon cross section and an assumption for the production of the colorless state. We verify that, as in the $J/\ensuremath{\psi}$ case, the HERA data for this process can be well described by the CEM. Moreover, we propose the analysis of the ratio $R={\ensuremath{\sigma}}_{\ensuremath{\Upsilon}}/{\ensuremath{\sigma}}_{J/\ensuremath{\psi}}$ to discriminate between the different approaches.

Heavy quark production at the Fermilab Tevatron and DESYepcollider HERA usingktfactorization with the CCFM evolution approach

The application of ${k}_{t}$ factorization supplemented with the Ciafaloni-Catani-Fiorani-Marchesini (CCFM) small-$x$ evolution equation to heavy quark production at the Fermilab Tevatron and at DESY HERA is discussed. The $b\overline{b}$ production cross sections at the Tevatron can be consistently described using the ${k}_{t}$-factorization formalism together with the unintegrated gluon density obtained within the CCFM evolution approach from a fit to HERA ${F}_{2}$ data. Special attention is drawn to the comparison with measured visible cross sections, which are compared to the hadron level Monte Carlo generator CASCADE.

QCD corrections to flavor-changing-neutral-current single top production at the DESY HERA collider

We calculate first-order QCD corrections to the cross section for single top-quark production mediated by flavor-changing neutral currents (FCNC's) in $\mathrm{ep}$ collisions at the DESY HERA collider. We find that the uncertainty due to the choice of the QCD scale is significantly reduced. This study is motivated by the current experimental work for limits on the FCNC single top cross section.

J/ ψ inclusive production in ep deep-inelastic scattering at DESY HERA

We calculate the cross section of J/ ψ plus jet associated production in ep deep-inelastic scattering within the factorization formalism of nonrelativistic quantum chromodynamics. Our analytic results disagree with previous analyses, both for the colour-singlet and colour-octet channels. Our theoretical predictions agree reasonably well with recent data taken by the H1 Collaboration at DESY HERA, significantly better than those obtained within the colour-singlet model.

Photoproduction of the isolated photon at DESY HERA in next-to-leading order QCD

The NLO QCD calculation for the photoproduction of the isolated photon with a large pT at the HERA ep collider is presented. The single resolved photon contribution and the QCD corrections of order alpha_s to the Born term are consistently included. The NNLO contributions, the box and the double resolved photon subprocesses, are sizeable and are taken into account in addition. The importance of the isolation cut, as well as the influence of other experimental cuts on pT and final photon rapidity distributions are discussed in detail. The investigation of the renormalization scale dependence is performed in order to estimate the size of missing higher order QCD corrections. Results are compared with experimental data and with the prediction of a different NLO calculation.

Erratum: Semi-inclusive hadron production at DESY HERA: The effect of QCD gluon resummation [Phys. Rev. D61, 014003 (2000)

We present a formalism that improves the applicability of perturbative QCD in the current region of semi-inclusive deep inelastic scattering. The formalism is based on all-order resummation of large logarithms arising in the perturbative treatment of hadron multiplicities and energy flows in this region. It is shown that the current region of semi-inclusive DIS is similar to the region of small transverse momenta in vector boson production at hadron colliders. We use this resummation formalism to describe transverse energy flows and charged particle multiplicity measured at the electron-proton collider HERA. We find good agreement between our theoretical results and experimental data for the transverse energy flows.

Diffractiveφand ρ production in a perturbative QCD model

The elastic leptoproduction of $\ensuremath{\varphi}$ measured by the H1 Collaboration at DESY HERA is described by a perturbative QCD model, based on open $s\overline{s}$ production and parton hadron duality, proposed by Martin et al. We observe that both the total cross section and the ratio of the longitudinal and transverse cross sections are well reproduced with an effective strange-quark mass ${m}_{s}\ensuremath{\sim}370\ensuremath{-}400 \mathrm{MeV}$ for various gluon distribution functions. The possible connection of the effective mass and the momentum dependent dynamical mass associated with dynamical breaking of chiral symmetry is discussed. The $\ensuremath{\rho}$ leptoproduction data are also well reproduced with an effective quark mass $\ensuremath{\sim}315 \mathrm{MeV}.$

Energy dependence of the contribution of pion exchange to large-rapidity-gap events in deep inelastic scattering

We study the energy dependence of the contribution of pion exchange to large-rapidity-gap events in deep inelastic scattering. The results show that this contribution can be quite significant at low energy and that the LRG events observed by the E665 Collaboration in $\ensuremath{\mu}\mathrm{Xe}$ and $\ensuremath{\mu}\mathrm{D}$ interactions at 490 GeV can be reasonably well described in terms of meson exchange. We also show that the distribution of the maximum rapidity for all hadrons is quite different from that for charged hadrons only and that the former exhibits also shoulderlike structure for events at 490 GeV similar to that at DESY HERA.