Measurement of dijet cross sections in <Emphasis Type="Italic">ep</Emphasis> interactions with a leading neutron at HERA

Measurements are reported of the production of dijet events with a leading neutron in ep interactions at HERA. Differential cross sections for photoproduction and deep inelastic scattering are presented as a function of several kinematic variables. Leading order QCD simulation programs are compared with the measurements. Models in which the real or virtual photon interacts with a parton of an exchanged pion are able to describe the data. Next-to-leading order perturbative QCD calculations based on pion exchange are found to be in good agreement with the measured cross sections. The fraction of leading neutron dijet events with respect to all dijet events is also determined. The dijet events with a leading neutron have a lower fraction of resolved photon processes than do the inclusive dijet data.Received: 31 January 2005, Revised: 7 March 2005, Published online: 28 April 2005     

Inclusive Single- and Dijet Rates in Next-to-Leading Order QCD for γ*p and γ*γ Collisions

We present one- and two-jet inclusive cross sections for γ*γ scattering and virtual photoproduction in ep collisions. The hard cross sections are calculated in next-to-leading order QCD. Soft and collinear singularities are extracted using the phase-space-slicing method. The initial state singularity of the virtual photon depends logarithmically its’ virtuality. This logarithm is large and has to be absorbed into the parton distribution function of the virtual photon. We define for this purpose an factorization scheme similar to the real photon case. We numerically study the dependence of the inclusive cross sections on the transverse energies and rapidities of the outgoing jets and on the photon virtuality. The ratio of the resolved to the direct cross section in ep collisions is compared to ZEUS data.     

Photon structure functions beyond the SUSY threshold

We evolve virtual photon parton densities up to the SUSY threshold and higher using coupled inhomogeneous DGLAP differential equations. Reliable input parameterizations were available from the c-quark threshold. Limited (target photon virtuality) dependence is observed. The difference to the photon structure function is shown to be significant with the introduction of SUSY dependent splitting functions. A negligible difference is observed by letting the gluino mass enter after the squark mass. An effort is made to include the squark threshold effect in such a way that both the renormalization group equations are satisfied and the perturbative calculation is reproduced. Received: 25 August 2000 / Revised version: 1 November 2000 / Published online: 21 December 2000     

Generalized parton distributions of the photon for nonzero skewness

We present a calculation of the generalized parton distributions of the photon when there is nonzero momentum transfer both in the transverse and longitudinal directions. We consider only the contributions when the photon helicity is not flipped and calculate those at leading order in electromagnetic coupling α and zeroth order in the strong coupling αs. We keep the leading logarithmic terms as well as the quark mass terms in the vertex. By taking Fourier transforms of the GPDs with respect to the transverse and longitudinal momentum transfer, we obtain the parton distributions of the photon in position space.     

A detailed next-to-leading order QCD analysis of deeply virtual Compton scattering observables

We present a detailed next-to-leading order (NLO) leading twist QCD analysis of deeply virtual Compton scattering (DVCS) observables, for several different input scenarios, in the scheme. We discuss the size of the NLO effects and the behavior of the observables in skewedness , momentum transfer,t, and photon virtuality, . We present results on the amplitude level for unpolarized and longitudinally polarized lepton probes, and unpolarized and longitudinally polarized proton targets. We make predictions for various asymmetries and for the DVCS cross section and compare with the available data. Received: 30 November 2001 / Revised version: 12 February 2002 / Published online: 15 March 2002     

Measurement of dijet cross sections in photoproduction at HERA

Dijet cross sections as functions of several jet observables are measured in photoproduction using the H1 detector at HERA. The data sample comprises data with an integrated luminosity of 34.9 pb. Jets are selected using the inclusive algorithm with a minimum transverse energy of 25 GeV for the leading jet. The phase space covers longitudinal proton momentum fraction and photon longitudinal momentum fraction in the ranges and . The predictions of next-to-leading order perturbative QCD, including recent photon and proton parton densities, are found to be compatible with the data in a wide kinematical range. Received: 4 January 2002 / Published online: 12 July 2002     

Electroproduction of vector mesons: factorization of end-point contributions

The end-point contributions in the quark longitudinal momentum fraction of the virtual photon ( g^*\gamma^* ) to vector meson (V) impact factor to the diffractive electroproduction amplitude can be factorized in terms of a generalized parton evolution of the target parton distribution. This result is used to model the helicity amplitudes g^* p ? V p\gamma^* p \rightarrow V p in terms of small x generalized parton distributions.     

Real and virtual photon contributions to inelasticep scattering

We investigate direct and resolved photon contributions to hardep scattering at high energies. Some terms of the direct contribution are already included in the resolved photon one, described in terms of the photon structure functions. A suitable subtraction mechanism has to be found in order to avoid double counting. Inelasticep scattering requires the knowledge of photon structure functions over a wide range in photon virtuality, in particular from approximately zero up to almost 10⁵ GeV² at HERA. We analyse the real and virtual photon structure function and we comment on the structure of the photon in the intermediate region of low virtuality. We finally describe howep cross sections can be obtained as functions of the virtuality of the exchanged photon.     

Study of scalar meson f 0(980) from B→f 0(980)K * decays

We show how parton distributions unintegrated over the parton transverse momentum, k _t , may be generated, at NLO accuracy, from the known integrated (DGLAP-evolved) parton densities determined from global data analyses. A few numerical examples are given, which demonstrate that sufficient accuracy is obtained by keeping only the LO splitting functions together with the NLO integrated parton densities. However, it is important to keep the precise kinematics of the process, by taking the scale to be the virtuality rather than the transverse momentum, in order to be consistent with the calculation of the NLO splitting functions.     

Measurement of open beauty production in photoproduction at HERA

The production and semi-leptonic decay of heavy quarks have been studied in the photoproduction process with the ZEUS detector at HERA using an integrated luminosity of 38.5 pb. Events with photon-proton centre-of-mass energies, , between 134 and 269 GeV and a photon virtuality, , less than 1 GeV were selected requiring at least two jets of transverse energy GeV and an electron in the final state. The electrons were identified by employing the ionisation energy loss measurement. The contribution of beauty quarks was determined using the transverse momentum of the electron relative to the axis of the closest jet, . The data, after background subtraction, were fit with a Monte Carlo simulation including beauty and charm decays. The measured beauty cross section was extrapolated to the parton level with the b quark restricted to the region of transverse momentum 5 GeV and pseudorapidity 2. The extrapolated cross section is . The result is compared to a perturbative QCD calculation performed to next-to-leading order. Received: 22 November 2000 / Published online: 5 February 2001     

Generalized parton distributions of the photon

We present a first calculation of the generalized parton distributions of the photon (both polarized and unpolarized) using overlaps of light-front wave functions at leading order in α and zeroth order in αs; for non-zero transverse momentum transfer and zero skewness. We present the novel parton content of the photon in transverse position space.     

Measurement of inclusive D^{\ast\pm} and associated dijet cross sections in photoproduction at HERA

Inclusive photoproduction of mesons has been measured for photon-proton centre-of-mass energies in the range GeV and photon virtuality 1 GeV. The data sample used corresponds to an integrated luminosity of 37 pb. Total and differential cross sections as functions of the transverse momentum and pseudorapidity are presented in restricted kinematical regions and the data are compared with next-to-leading order (NLO) perturbative QCD calculations using the “massive charm” and “massless charm” schemes. The measured cross sections are generally above the NLO calculations, in particular in the forward (proton) direction. The large data sample also allows the study of dijet production associated with charm. A significant resolved as well as a direct photon component contribute to the cross section. Leading order QCD Monte Carlo calculations indicate that the resolved contribution arises from a significant charm component in the photon. A massive charm NLO parton level calculation yields lower cross sections compared to the measured results in a kinematic region where the resolved photon contribution is significant. Received: 9 July 1998 / Published online: 22 October 1998     

Double deeply virtual Compton scattering off the nucleon

We study the double deeply virtual Compton scattering (DDVCS) process off the nucleon, through the scattering of a space-like virtual photon with large virtuality resulting in the production of a time-like virtual photon, decaying into an e(+)e(-) pair. This process is expressed in the Bjorken regime in terms of generalized parton distributions (GPDs), and it is shown that by varying the invariant mass of the lepton pair, one can directly extract the GPDs from the observables. We give predictions for the DDVCS cross section and beam helicity asymmetry and discuss its experimental feasibility.     

Factorization scheme and parton distributions in the polarized virtual photon target

We investigate spin-dependent parton distributions in the polarized virtual photon target in perturbative QCD up to the next-to-leading order (NLO). In the case , where is the mass squared of the probe (target) photon, the parton distributions can be predicted completely up to NLO, but they are factorization-scheme dependent. We analyze the parton distributions in six different factorization schemes and discuss their scheme dependence. We study, in particular, the QCD and QED axial anomaly effects on the first moments of the parton distributions to see the interplay between the axial anomalies and factorization schemes. We also show that the factorization-scheme dependence is characterized by the large-x behaviors of the quark distributions in the virtual photon. The gluon distribution is predicted to be the same up to NLO among the six factorization schemes examined. In particular, the first moment of the gluon distribution is found to be factorization-scheme independent up to NLO. Received: 24 January 2001 / Published online: 18 May 2001     

Photoproduction of large transverse momentum mesons and production of large-pT photons and leptons in proton-proton collisions

The photoproduction of large transverse momentum mesons is examined in a leading particle model using the techniques of the covariant parton model. In the leading particle model the large-p_T meson is produced in a high-energy, wide-angle scattering: γ + q → M + q. Numerical results such as inclusive cross sectioons, angular distributions and particle ratios are presented for the pseudoscalar mesons. The Landshoff-Polking-horne quark-fusion model is then applied to the production of large transverse momentum photons and leptons in proton-proton collisions. In the lepton case this model differs from the usual Drell-Yan mechanism in that the lepton pair has a small invariant mass but comes from a large-p_T virtual photon.     

Diffractive dijet cross sections in photoproduction at HERA

Differential dijet cross sections have been measured with the ZEUS detector for photoproduction events in which the hadronic final state containing the jets is separated with respect to the outgoing proton direction by a large rapidity gap. The cross section has been measured as a function of the fraction of the photon () and pomeron () momentum participating in the production of the dijet system. The observed dependence shows evidence for the presence of a resolved- as well as a direct-photon component. The measured cross section increases as increases indicating that there is a sizeable contribution to dijet production from those events in which a large fraction of the pomeron momentum participates in the hard scattering. These cross sections and the ZEUS measurements of the diffractive structure function can be described by calculations based on parton densities in the pomeron which evolve according to the QCD evolution equations and include a substantial hard momentum component of gluons in the pomeron. Received: 20 April 1998 / Published online: 2 July 1998     

On the scale dependence of the dimension of hadronic matter

We advance the notion of a dynamical, scale-dependent Hausdorff dimension for hadronic matter. The physical motivations behind such a concept are explained. For very small scales, we relate this dimension to the anomalous dimension of the product of two momentum densities. For hadrons, we rely on the precocious onset of Bjorken scaling in e-p inelastic scattering and a heuristic parton model to obtain the Hausdorff dimensional profile for the proton d(q²) as a function of the probing photon momentum q². We find that d reaches a maximum around d = 1 for a range of low q² values. This feature is indicative of a string-like structure of the hadrons at low momentum transfers.     

Glue in the pomeron from nonlinear k ⊥ factorization

We derive the nonlinear k _⊥ factorization for the spectrum of jets in high-mass diffractive deep inelastic scattering as a function of three hard scales—the virtuality of the photon Q ², the transverse momentum of the jet, and the saturation scale Q _A . In contrast to all other hard reactions studied so far, we encounter a clash between the two definitions of the glue in the pomeron from the inclusive spectrum of leading quarks and the small-β evolution of the diffractive cross section. This clash casts a further shadow on customary applications of the familiar collinear factorization to a pQCD analysis of diffractive deep inelastic scattering. The text was submitted by the authors in English.