Collider HERA Research Articles

Observation and differential cross section measurement of neutral current DIS events with an empty hemisphere in the Breit frame

The Breit frame provides a natural frame to analyze lepton–proton scattering events. In this reference frame, the parton model hard interactions between a quark and an exchanged boson defines the coordinate system such that the struck quark is back-scattered along the virtual photon momentum direction. In Quantum Chromodynamics (QCD), higher order perturbative or non-perturbative effects can change this picture drastically. As Bjorken-x decreases below one half, a rather peculiar event signature is predicted with increasing probability, where no radiation is present in one of the two Breit-frame hemispheres and all emissions are to be found in the other hemisphere. At higher orders in αs\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha _{s}$$\\end{document} or in the presence of soft QCD effects, predictions of the rate of these events are far from trivial, and that motivates measurements with real data. We report on the first observation of the empty current hemisphere events in electron–proton collisions at the HERA collider using data recorded with the H1 detector at a center-of-mass energy of 319 GeV. The fraction of inclusive neutral-current DIS events with an empty hemisphere is found to be 0.0112±3.9%stat±4.5%syst±1.6%mod\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.0112 \\pm 3.9\\%_\ ext {stat} \\pm 4.5\\%_{\ ext {syst}} \\pm 1.6\\%_{\ ext {mod}}$$\\end{document} in the selected kinematic region of 150<Q2<1500GeV2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$150<Q^{2} <1500\\, \ extrm{GeV}^2 $$\\end{document} and inelasticity 0.14<y<0.7\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.14<y<0.7$$\\end{document}. The data sample corresponds to an integrated luminosity of 351.1 pb-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}^{-1}$$\\end{document}, sufficient to enable differential cross section measurements of these events. The results show an enhanced discriminating power at lower Bjorken-x among different Monte Carlo event generator predictions.

Probing the Onset of Maximal Entanglement inside the Proton in Diffractive Deep Inelastic Scattering.

It has been proposed that at small Bjorken x, or equivalently at high energy, hadrons represent maximally entangled states of quarks and gluons. This conjecture is in accord with experimental data from the electron-proton collider HERA at the smallest accessible x. In this Letter, we propose to study the onset of the maximal entanglement inside the proton using diffractive deep inelastic scattering. It is shown that the data collected by the H1 Collaboration at HERA allow one to probe the transition to the maximal entanglement regime. By relating the entanglement entropy to the entropy of final-state hadrons, we find a good agreement with the H1 data using both the exact entropy formula as well as its asymptotic expansion which indicates the presence of a nearly maximally entangled state. Finally, future opportunities at the Electron Ion Collider are discussed.

Measurement of jet production in deep inelastic scattering and NNLO determination of the strong coupling at ZEUS

A new measurement of inclusive-jet cross sections in the Breit frame in neutral current deep inelastic scattering using the ZEUS detector at the HERA collider is presented. The data were taken in the years 2004–2007 at a centre-of-mass energy of 318GeV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$318\\,\\,\ extrm{GeV}$$\\end{document} and correspond to an integrated luminosity of 347pb-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$347\\,\\,\ extrm{pb}^{-1}$$\\end{document}. The jets were reconstructed using the kt\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$k_t$$\\end{document}-algorithm in the Breit reference frame. They have been measured as a function of the squared momentum transfer, Q2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Q^2$$\\end{document}, and the transverse momentum of the jets in the Breit frame, p⊥,Breit\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$p_{\\perp ,\ ext {Breit}}$$\\end{document}. The measured jet cross sections are compared to previous measurements and to perturbative QCD predictions. The measurement has been used in a next-to-next-to-leading-order QCD analysis to perform a simultaneous determination of parton distribution functions of the proton and the strong coupling, resulting in a value of αs(MZ2)=0.1142±0.0017(experimental/fit)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\alpha _s(M_Z^2) = 0.1142 \\pm 0.0017 \ ext {~(experimental/fit)}$$\\end{document}-0.0007+0.0006(model/parameterisation)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}_{-0.0007}^{+0.0006} \ ext {~(model/parameterisation)}$$\\end{document}-0.0004+0.0006(scale)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${}_{-0.0004}^{+0.0006} \ ext {~(scale)}$$\\end{document}, whose accuracy is improved compared to similar measurements. In addition, the running of the strong coupling is demonstrated using data obtained at different scales.

Determination of diffractive PDFs from a global QCD analysis of inclusive diffractive DIS and dijet cross-section measurements at HERA

We present an updated set of {\tt SKMHS} diffractive parton distribution functions (PDFs). In addition to the diffractive deep-inelastic scattering (diffractive DIS) data sets, the recent diffractive dijet cross sections measurements by the H1 experiment from the HERA collider are added to the data sample. The new set of diffractive PDFs, entitled {\tt SKMHS23} and {\tt SKMHS23-dijet}, are presented at next-to-leading order (NLO) and next-to-next-to-leading order (NNLO) accuracy in perturbative QCD. Since the gluons directly contribute to jet production through the boson-gluon fusion process, the data on diffractive dijet production in inclusive DIS help to constrain the gluon density, allowing for the determination of both the quark and gluon densities with better accuracy. The NLO and NNLO theory predictions calculated using both {\tt SKMHS23} and {\tt SKMHS23-dijet} are compared to the analyzed data showing excellent agreements. The effect arising from the inclusion of diffractive dijet data and higher order QCD corrections on the extracted diffractive PDFs and data/theory agreements are clearly examined and discussed.

Determination of the parton distribution functions of the proton using diverse ATLAS data from pp collisions at sqrt{s} = 7, 8 and 13 TeV

This paper presents an analysis at next-to-next-to-leading order in the theory of quantum chromodynamics for the determination of a new set of proton parton distribution functions using diverse measurements in pp collisions at sqrt{s} = 7, 8 and 13 TeV, performed by the ATLAS experiment at the Large Hadron Collider, together with deep inelastic scattering data from ep collisions at the HERA collider. The ATLAS data sets considered are differential cross-section measurements of inclusive W^{pm } and Z/gamma ^* boson production, W^{pm } and Z boson production in association with jets, tbar{t} production, inclusive jet production and direct photon production. In the analysis, particular attention is paid to the correlation of systematic uncertainties within and between the various ATLAS data sets and to the impact of model, theoretical and parameterisation uncertainties. The resulting set of parton distribution functions is called ATLASpdf21.

АНАЛИЗ МЕТОДОВ И ПРАКТИЧЕСКОЕ ПРИМЕНЕНИЕ ХЕШ-ФУНКЦИЙ

One of the first exotic phenomena observed in the 80s of the last century was an event registered by a calorimetric type installation. In this event, an anomalous relationship was observed between charged and neutral hadrons, arising from the collision of a high-energy particle with a carbon nucleus. In accordance with the principle of isotopic invariance, the number of neutral pions should be equal to the number of charged pions. The event, which was recorded by Japanese physicists, contained only charged peonies with no neutral [1]. This event was named "Centaur". Until now, such events have not been observed in accelerator experiments. The article presents the results of searching for such events in experiments at the DESY HERA collider. For the candidates for Centaur events and for all other events, constructed distributions by the type of fitability of charged tracks in CTD. Since the absence of neutral particles in events is explained by methodological reasons, "Centaur" events were not detected in electron-proton interactions at the ZEUS facility.

Preservation through modernisation: The software of the H1 experiment at HERA

The lepton–proton collisions produced at the HERA collider represent a unique high energy physics data set. A number of years after the end of collisions, the data collected by the H1 experiment, as well as the simulated events and all software needed for reconstruction, simulation and data analysis, were migrated into a preserved operational mode at DESY. A recent modernisation of the H1 software architecture has been performed, which will not only facilitate on going and future data analysis efforts with the new inclusion of modern analysis tools, but also ensure the long-term availability of the H1 data and associated software. The present status of the H1 software stack, the data, simulations and the currently supported computing platforms for data analysis activities are discussed.

Measurement of exclusive {varvec{{{{pi ^+pi ^-}}}}} and {varvec{{{{rho ^0}}}}} meson photoproduction at HERA

Exclusive photoproduction of {{rho ^0}} (770) mesons is studied using the H1 detector at the ep collider HERA. A sample of about 900,000 events is used to measure single- and double-differential cross sections for the reaction gamma p rightarrow pi ^{+}pi ^{-}Y. Reactions where the proton stays intact ({{{m_Y}} {=}m_p}) are statistically separated from those where the proton dissociates to a low-mass hadronic system (m_p{<}{{m_Y}} {<}10~{{text {GeV}}} ). The double-differential cross sections are measured as a function of the invariant mass m_{pi pi } of the decay pions and the squared 4-momentum transfer t at the proton vertex. The measurements are presented in various bins of the photon–proton collision energy {{W_{gamma p}}} . The phase space restrictions are 0.5le m_{pi pi } le 2.2~{{text {GeV}}} , vert tvert le 1.5~{{text {GeV}^2}} , and 20 le W_{gamma p} le 80~{{text {GeV}}} . Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the {m_{pi pi }} dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the {{rho ^0}} (770) meson mass and width at m_rho = 770.8{}^{+2.6}_{-2.7}~({text {tot.}})~{{text {MeV}}} and Gamma _rho = 151.3 {}^{+2.7}_{-3.6}~({text {tot.}})~{{text {MeV}}} , respectively. The model is used to extract the {{rho ^0}} (770) contribution to the pi ^{+}pi ^{-} cross sections and measure it as a function of t and {W_{gamma p}}. In a Regge asymptotic limit in which one Regge trajectory alpha (t) dominates, the intercept alpha (t{=}0) = 1.0654 {}^{+0.0098}_{-0.0067}~({text {tot.}}) and the slope alpha ^prime (t{=}0) = 0.233 {}^{+0.067 }_{-0.074 }~({text {tot.}}) ~{{text {GeV}^{-2}}} of the t dependence are extracted for the case m_Y{=}m_p.

Transverse energy\u2013energy correlations of jets in the electron\u2013proton deep inelastic scattering at HERA

We study the event shape variables, transverse energy–energy correlation TEEC (cos phi ) and its asymmetry ATEEC (cos phi ) in deep inelastic scattering (DIS) at the electron–proton collider HERA, where phi is the angle between two jets defined using a transverse-momentum (k_T) jet algorithm. At HERA, jets are defined in the Breit frame, and the leading nontrivial transverse energy–energy correlations arise from the 3-jet configurations. With the help of the NLOJET++, these functions are calculated in the leading order (LO) and the next-to-leading order (NLO) approximations in QCD at the electron–proton center-of-mass energy sqrt{s}=314 GeV. We restrict the angular region to -0.8 le cos phi le 0.8, as the forward- and backward-angular regions require resummed logarithmic corrections, which we have neglected in this work. Following experimental jet-analysis at HERA, we restrict the DIS-variables x, y=Q^2/(x s), where Q^2=-q^2 is the negative of the momentum transfer squared q^2, to 0 le x le 1, 0.2 le y le 0.6, and the pseudo-rapidity variable in the laboratory frame (eta ^mathrm{{lab}}) to the range -1 le eta ^mathrm{{lab}} le 2.5. The TEEC and ATEEC functions are worked out for two ranges in Q^2, defined by 5.5,mathrm{GeV}^2 le Q^2 le 80,mathrm{GeV}^2, called the low-Q^2-range, and 150,mathrm{GeV}^2 le Q^2 le 1000,mathrm{GeV}^2, called the high-Q^2-range. We show the sensitivity of these functions on the parton distribution functions (PDFs), the factorization (mu _F) and renormalization (mu _R) scales, and on alpha _s(M_Z^2). Of these the correlations are stable against varying the scale mu _F and the PDFs, but they do depend on mu _R. For the choice of the scale mu _R= sqrt{langle E_Trangle ^2 +Q^2}, advocated in earlier jet analysis at HERA, the shape variables TEEC and ATEEC are found perturbatively robust. These studies are useful in the analysis of the HERA data, including the determination of alpha _s(M_Z^2) from the shape variables.

Exclusive photoproduction of excited quarkonia in ultraperipheral collisions

In this paper, we discuss the exclusive photoproduction of ground and excited states of $\psi(1S,2S)$ and $\Upsilon(1S,2S)$ in ultraperipheral collisions (UPCs). Using the potential model in order to obtain the vector meson wave function, we find a good agreement of our calculations with data from the LHC and HERA colliders for $J/\psi (1S,2S) $ and $\Upsilon(1S)$ in $\gamma p$ collisions. We extend the calculations to the nuclear target case applying them to $AA$ UPCs with the use of the shadowing and finite coherence length effects fitted to the data. Our results are compared to the recent LHC data, in both incoherent ($J/\Psi(1S)$ at 2.76 TeV) and coherent ($J/\Psi(1S)$ at 2.76 and 5.02 TeV) processes. We also show the corresponding predictions for the excited states, in the hope that future measurements could provide more detailed information about the vector meson wave functions and nuclear effects.

Analysis of Isolated Photons in Photoproduction in PYTHIA

Collision of particles at high energies at accelerators is the main source of data used to obtain deeper understanding of the fundamental interactions and the structure of the matter. Processes of isolated photon production have provided many tests of theoretical descriptions of the universe on scales smaller than the proton. This work is dedicated to the analysis of the large amount of collision data that has been accumulated at ZEUS in 2004-2007 period and new methods of processing isolated photons that have been proposed. The authors develop software algorithms that allow obtaining the signal of isolated photons from the data collected on the ZEUS detector at electron-proton collider HERA, calculating the differential cross sections, and comparing the measured data with PYTHIA Monte Carlo predictions. Taking into account the features of the ZEUS detector, the photon signal is separated from the background events and the number of isolated photons is calculated. Computational mathematical and numerical methods have been used to simulate the interaction of particles in the detector. Monte Carlo predictions for differential cross sections as functions of the pseudorapidity and transverse energy of the photon ηg, ETg and the jet ηjet, ETjet, and the fraction of the photon momentum хgmeas carried by the interacting parton have been calculated and compared with the experimental data. The results of the study are compared with the previous studies and show for the first time that all isolated photon HERA measurements are consistent with each other. New results show improved uncertainties. The formation of isolated inclusive photons and photons with the accompanying jet was measured in photoproduction with ZEUS detector at HERA collider using the integrated luminosity of 374 ± 7 pb-1. For the first time, more complex Monte-Carlo simulation models of isolated photons for ZEUS detector were generated and applied, and the description of the photon signal was improved. It has been found that PYTHIA describes the shape of the cross section as a function of ηg well enough, but does not fully reproduce the shape of ETg, ETjet, and the middle region of хgmeas, while ηjet is described not very well. The reason for this discrepancy can be the lack of corrections of higher orders in the predictions for cross sections of direct photons. Scaling of the cross sections obtained with PYTHIA improves the description of ETg and ηg. The unsatisfactory description of ηjet indicates that further studies are required.

Mass dependence of vector meson photoproduction off protons and nuclei within the energy-dependent hot-spot model

We study the photoproduction of vector mesons off proton and off nuclear targets. We work within the colour dipole model in an approach that includes subnucleon degrees of freedom, so-called hot spots, whose positions in the impact-parameter plane change event-by-event. The key feature of our model is that the number of hot spots depends on the energy of the photon–target interaction. Predictions are presented for exclusive and dissociative production of ρ0, J/ψ, and ϒ(1S) off protons, as well as for coherent and incoherent photoproduction of ρ0 off nuclear targets, where Xe, Au, and Pb nuclei are considered. We find that the mass dependence of dissociative production off protons as a function of the energy of the interaction provides a further handle to search for saturation effects at HERA, the LHC and future colliders. We also find that the coherent photonuclear production of ρ0 is sensitive to fluctuations in the subnucleon degrees of freedom at RHIC and LHC energies.

Phenomenology of leading nucleon production in ep collisions at HERA in the framework of fracture functions

In recent years, several experiments at the $e^-p$ collider HERA have collected high precision deep inelastic scattering (DIS) data on the spectrum of leading nucleon carrying a large fraction of the proton's energy. In this paper, we have analyzed recent experimental data on the production of forward proton and neutron in DIS at HERA in the framework of a perturbative QCD. We propose a technique based on the fractures functions framework, and extract the nucleon fracture functions (nucleon FFs) ${\cal M}_2^{(n/p)} (x, Q^2; x_L)$ from global QCD analysis of DIS data measured by ZEUS collaboration at HERA. We have shown that an approach based on the fracture functions formalism allows us phenomenologically parametrize the nucleon FFs. Considering both leading neutron as well as leading proton production data at HERA, we present the results for the separate parton distributions for all parton species, including valence quark densities, the anti-quark densities, the strange sea distribution, and the gluon distribution functions. We proposed several parameterizations for the nucleon FFs and open the possibility of these asymmetries. The obtained optimum set of nucleon FFs is accompanied by Hessian uncertainty sets which allow one to propagate uncertainties to other observables interest. The extracted results for the $t$-integrated leading neutron $F_2^{\rm LN(3)} (x, Q^2; x_L)$ and leading proton $F_2^{\rm LP(3)} (x, Q^2; x_L)$ structure functions are in good agreement with all data analyzed, for a wide range of fractional momentum variable $x$ as well as the longitudinal momentum fraction $x_L$.

QCD Analysis of Leading-Neutron Production at HERA: Determination of Neutron Fracture Functions

The last two decades have seen a growing trend towards the experimental efforts at the electron-proton collider HERA in which have collected high precision data on the spectrum of leading neutron (LN) and leading-proton (LP) carrying a large fraction of the proton’s energy. In our recent study [Phys. Rev. D 95 (2017), 074011], we have proposed an approach based on the Fractures Functions (FF) formalism and have extracted the neutron Fracture Functions (neutron FFs) from a global QCD analysis of LN production data measured by H1 and ZEUS collaborations at HERA. We have shown that considering the approach based on the framework of Fracture Functions, one could phenomenologically parametrize the neutron FFs at the input scale. In order to access the uncertainties for the obtained neutron FFs as well as the LN structure functions and cross section, associated with the uncertainties in the data, we have made an extensive use of the “Hessian method”. Our theory predictions based on the obtained neutron FFs are in satisfactory agreement with all LN data analyzed, for a wide range of [Formula: see text] and [Formula: see text].

Hard Diffraction at Colliders

The phenomenon of hard diffraction is presented. The discussion includes: the present understanding of the process, experimental techniques, past measurements performed at the SPS, HERA and Tevatron colliders, the first LHC data and prospects for new results.

Experimental investigation of ρ0 photoproduction on the pion in the H1 experiment

Experimental results on quasielastic photoproduction of the ρ0 meson in association with a neutron, obtained at the HERA collider, are presented. The total and differential cross sections of the γp → ρ0 nπ+ reaction at the positron–proton center-of-mass energy of √s =319 GeV are measured. The data collected with the H1 detector in 2006 and 2007 correspond to an integrated luminosity of 1.16 pb−1. The kinematic region of the photon–proton cms energy of 20 < W γp <100 GeV, photon virtuality of Q 2 < 2 GeV2, and the ρ0 transverse momentum below 1 GeV/c is analyzed. Secondary neutrons with energies x L > 0.35 (in proton-energy units) and emission angles below 0.75 mrad are selected. The model of double peripheral exchange, in which the ρ0 is elastically produced via the photon interaction with the virtual pion from the proton–neutron vertex, is employed for interpreting the results. The cross section for the ρ0 elastic photoproduction on the pion, γπ+→ ρ0π+, is extracted in the one-pion-exchange approximation. The magnitude of the cross section suggests that the γp → ρ0 nπ+ reaction is significantly affected by absorption.

The Proton as Seen by the HERA Collider

Deep-inelastic electron–proton (ep) scattering at the HERA collider has been very important in the investigation of the partonic structure of the proton. The neutral- and charged-current cross sections, measured over a large kinematic range, are one of the legacies of the first and so far only ep collider. Here I discuss the combination of H1 and ZEUS data. The HERA data alone can provide parton distribution functions (PDFs) within the framework of perturbative QCD. I also discuss the family of sets of PDFs called HERAPDF2.0 as well as the interpretation of such proton PDFs that characterize proton interactions in momentum space. I then introduce other possible applications of the precision cross sections. In addition, some measurements at HERA provide hints about spatial aspects of the proton. Finally, I present a brief discussion of the connection to nuclear physics.

Combined electroweak and QCD fit to HERA data

A simultaneous Electroweak and QCD fit of electroweak parameters and parton distribution functions to HERA data on deep inelastic scattering is presented. The input data are neutral current and charged current inclusive cross sections measured by the H1 and ZEUS collaborations at the ep collider HERA. The polarisation of the electron beam was taken into account for the ZEUS and H1 data recorded between 2004 and 2007. Results are presented on the vector and axial-vector couplings of the Z boson to u- and d-type quarks. The values are in agreement with Standard Model predictions. The results on a_u and v_u represent the most precise measurements from a single process.

Search for QCD instanton-induced processes at HERA in the high- $$\pmb {Q^2}$$ Q 2 domain

Signals of QCD instanton-induced processes are searched for in neutral current deep-inelastic scattering at the electron-proton collider HERA in the kinematic region defined by the Bjorken-scaling variable $x > 10^{-3}$, the inelasticity $0.2< y < 0.7$ and the photon virtuality $150 < Q^2 < 15000$ GeV$^2$. The search is performed using H1 data corresponding to an integrated luminosity of ~$351$ pb$^{-1}$. No evidence for the production of QCD instanton-induced events is observed. Upper limits on the cross section for instanton-induced processes between $1.5$~pb and $6$~pb, at $95\%$~ confidence level, are obtained depending on the kinematic domain in which instantons could be produced. Compared to earlier publications, the limits are improved by an order of magnitude and for the first time are challenging theory predictions.

Dynamic color screening in diffractive deep inelastic scattering

We present a novel Monte-Carlo implementation of dynamic colour screening via multiple exchanges of semi-soft gluons as a basic QCD mechanism to understand diffractive electron-proton scattering at the HERA collider. Based on the kinematics of individual events in the standard QCD description of deep inelastic scattering at the parton level, which at low $x$ is dominantly gluon-initiated, the probability is evaluated for additional exchanges of softer gluons resulting in an overall colour singlet exchange leading to a forward proton and a rapidity gap as the characteristic observables for diffractive scattering. The probability depends on the impact parameter of the soft exchanges and varies with the transverse size of the hard scattering subsystem and is therefore influenced by different QCD effects. We account for matrix elements and parton shower evolution either via conventional DGLAP $\log Q^2$-evolution with collinear factorisation or CCFM small-$x$ evolution with $k_\perp$-factorisation and discuss the sensitivity to the gluon density distribution in the proton and the importance of large log\,$x$-contributions. The overall result is that, with only two model parameters which have theoretically motivated values, a satisfactory description of the observed diffractive cross-section at HERA is obtained in a wide kinematical range.