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.     

Experimental study of hard photon radiation processes at HERA

We present an experimental study of theepe+p andepe+X processes using data recorded by the H1 detector in 1993 at the electron-proton collider HERA. These processes are employed to measure the luminosity with an accuracy of 4.5 %. A subsample of theepe+X events in which the hard photon is detected at angles 0.45 mrad with respect to the incident electron direction is used to verify experimentally the size of radiative corrections to theepeX inclusive cross section and to investigate the structure of the proton in theQ ² domain down to 2 GeV², lower than previously attained at HERA.Supported by the Bundesministerium für Forschung und Technologie, FRG under contract numbers 6AC17P, 6AC47P, 6DO57I, 6HH17P, 6HH27I, 6HD17I, 6HD27I, 6KI17P, 6MP17I, and 6WT87P     

Forward jet production in deep inelastic scattering at HERA

The production of forward jets has been measured in deep inelastic ep collisions at HERA. The results are presented in terms of single differential cross sections as a function of the Bjorken scaling variable (x_Bj) and as triple differential cross sections d³σ/dx_BjdQ² , where Q² is the four momentum transfer squared and is the squared transverse momentum of the forward jet. Also cross sections for events with a di-jet system in addition to the forward jet are measured as a function of the rapidity separation between the forward jet and the two additional jets. The measurements are compared with next-to-leading order QCD calculations and with the predictions of various QCD-based models.     

Inclusive Jet Production in γp and γγ Processes: Direct and Resolved Photon Cross Sections in Next-To-Leading Order QCD

The production of jets in low Q² ep scattering (photoproduction) and in low Q² ey⁺ey⁻ scattering (γγ scattering) allows for testing perturbative QCD and for measuring the proton and photon structure functions. This requires exact theoretical predictions for one- and two-jet cross sections. We describe the theoretical formalism, giving sufficient details, for calculating the direct and resolved processes in γp and γγ reactions in next-to-leading order QCD. We present the complete analytical results for the Born terms, the virtual, and the real corrections. To separate singular and regular regions of phase space we use the phase space slicing method with an invariant mass cut-off. In this way, all soft and collinear singularities are either canceled or absorbed into the structure functions. Using a flexible Monte Carlo program, we evaluate the cross sections numerically and perform various tests and comparisons with other calculations. We consider the scale dependence of our results and compare them to data from the experiments H1 and ZEUS at HERA and OPAL at LEP.     

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 Q² < 1 GeV², and in deep inelastic scattering, where 2 < Q² < 100 GeV². 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.Received: 7 February 2005, Revised: 26 April 2005, Published online: 31 May 2005     

Inclusive particle photoproduction to next-to-leading order

We study the inclusive photoproduction of neutral and charged pions and at HERA, via the resolved photon mechanism, in QCD to next-to-leading order. We present various distributions of phenomenological interest and study the theoretical uncertainties due to the mass scales, and to photon and proton sets of structure functions. A new set of fragmentation functions for charged pions is also presented.     

Signals for virtual leptoquark exchange at HERA

We study the effects of virtual leptoquarks on charged current and neutral current processes at theep collider HERA. We present the areas of parameter space that can be excluded at HERA by searching for deviations from Standard Model expectations. The best results are obtained by examining the ratio of neutral current to charged current cross sections, R=_NC/_NC, where, with 200 pb^–1 of integrated luminosity for unpolarizede ^– ande ⁺ beams, HERA can search for leptoquarks with masses up to 800 GeV, with leptoquark coupling strengths of order _em.Research supported by an SSC Fellowship from the Texas National Research Laboratory Commission     

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     

Dijet photoproduction of massless charm jets at next-to-leading order of QCD

We compute the charm dijet photoproduction cross section at next-to-leading order of QCD in the zero-mass variable flavour number scheme, i.e. with active charm quarks in the proton and photon. The results are compared to recent measurements from the ZEUS experiment at HERA. The predictions for various distributions agree with the data, in particular for large momentum fractions of the partons in the photon, where direct photon processes dominate. At low momentum fractions, the predictions are quite sensitive to the charm content in the photon. The experimental data are shown to favour parameterizations with a substantial charm-quark density such as the one proposed by Cornet et al.     

Inclusive two-jet production at HERA: direct and resolved cross sections in next-to-leading order QCD

We have calculated inclusive two-jet cross sections in next-to-leading order QCD for low Q ² ep collisions superimposing direct and resolved contributions. Infrared and collinear singularities in the virtual and real contributions are cancelled with the phase space slicing method. Various inclusive two-jet distributions have been computed. The results are compared with recent data from the ZEUS collaboration at HERA.     

Deep inelastic Compton scattering at theep collider HERA

Deep inelastic Compton scattering at HERA is investigated with the intention of determining the usefulness of this process in pinning down the photon and proton structure functions. We examine the contributions of the various _s and _s ² subprocesses that arise from the nonelementary structure of the photon atS _p =30000 GeV². An assessment is made of the sensitivity of this process to the various parton and photon structure functions presently available. In particular we study the possibility of directly measuring the gluon content of the proton and the photon as well as the quark fragmentation into a photon with this process. Theep laboratory frame helps to separate out the different contributions and in particular may allow a quantitative study of the box diagram term.     

Parton distribution functions at LO,NLO and NNLO with correlated uncertainties between orders

Sets of parton distribution functions (PDFs) of the proton are reported for the leading (LO), next-to-leading (NLO) and next-to-next-to-leading-order (NNLO) QCD calculations. The parton distribution functions are determined with the HERAFitter program using the data from the HERA experiments and preserving correlations between uncertainties for the LO, NLO and NNLO PDF sets. The sets are used to study cross-section ratios and their uncertainties when calculated at different orders in QCD. A reduction of the overall theoretical uncertainty is observed if correlations between the PDF sets are taken into account for the ratio of \(WW\) di-boson to \(Z\) boson production cross sections at the LHC.     

Inclusive dijet production at HERA: direct photon cross sections in next-to-leading order QCD

We have calculated inclusive two-jet cross sections in next-to-leading order QCD for direct photo-production in lowQ ² ep collisions at HERA. Infrared and collinear singularities in real and virtual contributions are cancelled with the phase space slicing method. Analytical formulas for the different contributions giving the dependence on the slicing parameter are presented. Various one and two-jet distributions have been computed demonstrating the flexibility of the method.     

Inclusive jet production with virtual photons in next-to-leading order QCD

We present a next-to-leading order calculation for the virtual photoproduction of one and two jets inep collisions. Soft and collinear singularities are extracted using the phase space slicing method. The collinear photon initial state singularity depends logarithmically on the mass of the virtual photon and is absorbed into the virtual photon structure function. An $\overline {MS} $ factorization scheme is defined similarly to the real photon case. Numerical results are presented for HERA conditions using the Snowmass jet definition for inclusive single jet and dijet cross sections. We study the dependence of these cross sections on the transverse energies and rapidities of the jets. Finally, we compare the ratio of the experimentally defined resolved and direct cross sections with recent ZEUS data as a function of the photon virtualityP ².     

HERA分解光子过程中的双喷注产生

在次领头级(NLO)近似下计算了HERA分解光子过程中的双喷注截面.结果表明:在分解光子过程中,NLO修正量(当不变质量大于20GeV时)约为LO的0.5到1.这一结果可解释ZEUS的数据分析结果:“LOQCD理论值比实验值低1.5到2个因子.”当双喷注不变质量M_JJ<30GeV=,快度y_JJ<—1时,光子的胶子分布决定了双喷注截面.利用这一特点可测量光子的胶子分布.当双喷注不变质量较大时(例如:M_JJ>30GeV),-1JJ<0时,光子的夸克分布决定了双喷注截面,利用这一特点可测量光子的夸克分布.     

Diquark production inep collisions

We examine the production of single diquarks (D) and diquark pairs \((D\bar D)\) inep collisions using the effective photon approximation. Cross-sections for both mechanisms are found to be quite sensitive to the assumed charged of the diquark but can lead to significant production rates in all cases at HERA and futureep colliders. Backgrounds from QCD jet production are found to be potentially significant in the case of singleD production where the signal to background is approximately one-to-one whereas the production of \(D\bar D\) pairs is relatively back-ground free.     

Top-quark pair production near threshold at LHC

The next-to-leading order analysis for the cross section for hadroproduction of top-quark pairs close to threshold is presented. Within the framework of non-relativistic QCD a significant enhancement compared to fixed-order perturbation theory is observed which originates from the characteristic remnant of the 1S peak below production threshold of top-quark pairs. The analysis includes all color-singlet and color-octet configurations of top-quark pairs in S-wave states and, for the dominant configurations, it employs all-order soft-gluon resummation for the hard parton cross section. Numerical results for the Large Hadron Collider at TeV and TeV and also for the Tevatron are presented. The possibility of a top-quark mass measurement from the invariant-mass distribution of top-quark pairs is discussed.     

Inclusive particle production at HERA

We calculate in next-to-leading order inclusive cross sections of single-particle production via resolved photons inep collisions at HERA. Transverse-momentum and rapidity distributions are presented and the scale dependence is studied. The results are compared with first experimental data from the H1 Collaboration at HERA.     

Dijet production in neutral current deep inelastic scattering at HERA

Dijet cross sections in neutral current deep inelastic ep scattering have been measured in the range with the ZEUS detector at HERA using an integrated luminosity of 38.4 pb. The cross sections, measured in the Breit frame using the jet algorithm, are compared with next-to-leading-order perturbative QCD calculations using proton parton distribution functions. The uncertainties of the QCD calculations have been studied. The predictions are in reasonable agreement with the measured cross sections over the entire kinematic range. Received: 11 September 2001 / Published online: 20 December 2001     

Inclusive photoproduction of <Emphasis Type="Italic">D</Emphasis><Superscript>*</Superscript> mesons with massive charm quarks

We have calculated the next-to-leading order cross sections for the inclusive production of D ^* mesons in collisions at HERA in two approaches using massive or massless charm quarks. The usual massive theory for the direct cross section with charm quarks only in the final state was transformed into a massive theory with subtraction by subtracting the mass divergent and additional finite terms calculated earlier in connection with the process . This theory approaches the massless theory with increasing transverse momentum. The difference between the massive and the massless approach with subtraction is studied in detail in those kinematic regions relevant for comparison with experimental data. With these results and including the resolved cross section which is dominated by the part originating from the charm in the photon, we compute the fully inclusive cross section and compare it with preliminary data from the ZEUS collaboration at HERA. We find on average good agreement.Received: 5 October 2004, Published online: 26 November 2004