Jet Production at HERA
I review the status of perturbative QCD calculations for jet production in eP-scattering at HERA. I will discuss possibilities of combining fixed order, especially higher order, calculations and parton showers.
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arXiv subjects
B. Pötter
Publications and source records attributed to B. Pötter.
Combining Parton Showers with Next-to-Leading Order QCD Matrix Elements in Deep-Inelastic eP Scattering
We have implemented a systematic procedure for combining parton shower algorithms with next-to-leading order QCD calculations for the case of jet production in deep-inelastic electron-proton scattering. Using this method we have computed inclusive jet cross sections and jet shapes for the case of single-jet production and compared them to data from the ZEUS collaboration at HERA. We found good agreement between the data and our calculations, both for the jet shapes and the inclusive spectra.
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Combining QCD Matrix Elements at Next-to-Leading Order with Parton Showers in Electroproduction
We present a method to combine next-to-leading order (NLO) matrix elements in QCD with leading logarithmic parton showers by applying a suitably modified version of the phase-space-slicing method. The method consists of subsuming the NLO corrections into a scale-dependent phase-space-slicing parameter, which is then automatically adjusted to cut out the leading order, virtual, soft and collinear contributions in the matrix element calculation. In this way a positive NLO weight is obtained, which can be redistributed by a parton shower algortihm. As an example, we display the method for single-jet inclusive cross sections at O(alpha_s) in electroproduction. We numerically compare the modified version of the phase-space-slicing method with the standard approach and find very good agreement on the percent level.
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Fragmentation Functions for Pions, Kaons, and Protons at Next-to-Leading Order
We present new sets of fragmentation functions for charged pions, charged kaons, and protons, both at the leading and next-to-leading orders. They are fitted to the scaled-momentum distributions of these hadrons measured in e+e- annihilation on the Z-boson resonance at CERN LEP1 and SLAC SLC. These data partly come as light-, charm-, bottom-quark-enriched and gluon-jet samples, which allows us to treat all partons independently, after imposing the SU(2) flavour symmetry relations. In order to gain sensitivity to the scaling violation in fragmentation, we also include data from SLAC PEP, with center-of-mass energy root(s)=29 GeV, in our fits. This allows us to also determine the strong-coupling constant, with a competitive error. LEP1 data on the longitudinal cross section as well as DESY DORIS and PETRA data at lower energies nicely agree with theoretical predictions based on our fragmentation functions.
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Strong Coupling Constant from Scaling Violations in Fragmentation Functions
We present a new determination of the strong coupling constant alpha_s through the scaling violations in the fragmentation functions for charged pions, charged kaons, and protons. In our fit we include the latest e+e- annihilation data from CERN LEP1 and SLAC SLC on the Z-boson resonance and older, yet very precise data from SLAC PEP at center-of-mass energy sqrt(s)=29 GeV. A new world average of alpha_s is given.
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JetViP 2.1: the hbook version
We present an update of the JetViP 1.1 program for performing fixed NLO calculations in jet production including direct and resolved components in a continuous range of photon virtuality Q^2. The new version allows to access the full event record on the parton level. The program is set up such that hbook can be used to fill histograms. The phase space generator has been optimized and the azimuthal dependence of the cross sections is taken into account in LO. We comment on recent comparisons between various NLO programs for jet production at HERA. We demonstrate that the \Sum E_T cut for dijet cross sections is not infrared safe.
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Inclusive Single- and Dijet Rates in Next-to-Leading Order QCD for gamma^*proton and gamma^*gamma Collisions
We present one- and two-jet inclusive cross sections for gamma*gamma 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 MS-bar 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.
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Forward Jet Production at HERA
We discuss forward jet production data recently published by the H1 and ZEUS collaborations at HERA. We review how several Monte-Carlo models compare to the data. QCD calculations based on the BFKL formalism and on fixed NLO perturbation theory with and without resolved virtual photons are described.
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Probing the parton densities of virtual photons with the reaction gamma^*gamma->jets at LEP
We present a next-to-leading order calculation of jet production in gamma^*gamma collisions from e+e- scattering in a region where the virtuality Q^2 of the probing virtual photon is small compared to the transverse jet energy. The calculation is based on the phase-space slicing method. The initial state singularity of the virtual photon is factorized into the structure function of the virtual photon, using the MS-bar factorization scheme for virtual photons. Numerical results are presented for LEP2 conditions. The perturbative stability of the pure direct virtual photon approach is compared to that of including resolved virtual photons in different regions of Q^2. We make predictions for cross sections which suggest that different parametrizations of virtual photon parton densities should be distinguishable by measurements of jet cross sections at LEP.
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Forward Jet Production at small x in Next-to-Leading Order QCD
The production of forward jets of transverse energy E_T\simeq Q and large momentum fraction x_jet >> x is calculated in next-to-leading order including consistently direct and resolved virtual photon contributions. The predictions are compared to recent ZEUS and H1 data. Good agreement with the data is found.
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Low and High Q^2 Jet Production in γ^*γ$-Scattering at e+e- Colliders
I review recent results on NLO calculations in QCD for inclusive jet production in γ^*γ-scattering at e+e- colliders. I discuss the region of large Q^2 (DIS regime) and the transition to small Q^2 (virtual photoproduction regime). The limit to real photoproduction is performed and compared with existing calculations.
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Virtual Photon Structure from Low Q^2 Jet Production
We review next-to-leading order calculations of one- and two-jet production in ep collisions at HERA for photon virtualities in the range 1<Q^2<100 GeV^2. Soft and collinear singularities are extracted using the phase space slicing method. Numerical results are presented for HERA conditions with the Snowmass jet definition. The transition between photoproduction and deep-inelastic scattering is studied. We discuss the comparison with recent H1 data of the dijet rate and differential dijet cross sections with special attention to the region, in which two jets have equal transverse momenta.
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Jet Cross Sections in gamma^*-gamma Scattering at e+e- Colliders in NLO QCD
Recent results from NLO QCD calculations for inclusive jet cross sections in gamma^*-gamma scattering at e+e- colliders, especially for LEP, are reported. The virtuality Q^2 of the virtual photon is non-zero and can be unlimited large. The virtuality of the second photon is zero and the spectrum is calculated with the Weizs"acker-Williams approximation. Four components of the cross sections have to be distinguished, involving direct and resolved real and virtual photon contributions. Since Q^2 is non-zero, the virtual photon structure function is needed to calculate the contributions involving a resolved virtual photon.
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JetViP 1.1: Calculating One- and Two-Jet Cross Sections with Virtual Photons in NLO QCD
JetViP is a computer program for the calculation of inclusive single- and dijet cross sections in eP- and e-gamma-scattering in NLO QCD. The virtuality of the photon, radiated by the incoming electron, can be chosen in a continuous range, reaching from photoproduction into deep inelastic scattering. The various contributions to the full jet cross section, including the resolved photon contributions, are implemented. The calculation is based on the phase-space-slicing method.
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Jet Production in Deep Inelastic Electron-Photon Scattering at e+e- Colliders in Next-to-Leading Order QCD
A complete next-to-leading order QCD calculation of deep inelastic electron-photon scattering including direct and resolved real photon components is presented. Soft and collinear singularities are extracted using the phase space slicing method. Application of the results for the prediction of single- and dijet cross sections at the LEP e+e- collider are presented, using the Snowmass jet definition. The dependence of the cross sections on the transverse momentum and on the rapidities of the jets are discussed.
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Transition from Photoproduction to Deep-Inelastic Scattering in Jetproduction at HERA in NLO QCD
We present results from NLO QCD calculations on the production of jets in ep collisions at HERA in a continuous range of photon virtuality Q^2. Special attention is given to the collinear singularities from the virtual photon and the resolved virtual photon component of the cross sections. Comparisons with dijet data from H1 are shown and the infrared sensitivity of dijet cross sections is discussed.
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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 in $ep$ 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 virtuality $P^2$.
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