Parton showers in leptoproduction events

We present a model that includes the production of arbitrarily many jets in lepton-hadron events, using the leading log formalism for parton shower evolution. The main problem encountered here, which has not previously been illuminated by studies ofe ⁺ e ^? annihilation or Drell-Yan/Z ⁰/W ^± production, is the choice of kinematics in the space-like shower evolution. In our preferred solution, the standard definition of Bjorkenx is preserved during the construction of initial and final state showers—a nontrivial constraint. The resulting model is described in detail, including some first investigations of its properties.     

Double-hadron leptoproduction in the nuclear medium

The first measurements of double-hadron production in deep-inelastic scattering within the nuclear medium were made with the HERMES spectrometer at DESY HERA using a 27.6 GeV positron beam. By comparing data for deuterium, nitrogen, krypton, and xenon nuclei, the influence of the nuclear medium on the ratio of double-hadron to single-hadron yields was investigated. Nuclear effects on the additional hadron are clearly observed, but with little or no difference among nitrogen, krypton, or xenon, and with smaller magnitude than effects seen on previously measured single-hadron multiplicities. The data are compared with models based on partonic energy loss or prehadronic scattering and with a model based on a purely absorptive treatment of the final-state interactions. Thus, the double-hadron ratio provides an additional tool for studying modifications of hadronization in nuclear matter.     

Comment on azimuthal dependence in leptoproduction

It is pointed out that in the conventional calculation a large part of 〈cosφ〉 in leptoproduction comes from configurations in which the outgoing partons have small p_T. This indicates that the conventional QCD prediction for 〈cosφ〉 is not reliable in the experimentally available Q² regions at present.     

On high energy leptoproduction

The particle distributions and the event structures in high energy leptoproduction are considered in a model where perturbative QCD is used to compute the cross sections to order α _s and the Lund jet model is used for the soft hadronization process. Since complete events are generated by a Monte Carlo program not only single-particle spectra can be studied but also various correlations, thus making a more detailed comparison between theory and experiment possible. The model is found to be in agreement with data from the European Muon Collaboration. We indicate how a one-particle trigger can be used to study an enriched sample of two forward jet events, and exhibit a useful experimental test for the existence of such events with the properties predicted by perturbative QCD. We also indicate how to gain further information on the confinement and soft fragmentation mechanisms.     

Nuclear effects in leptoproduction of secondaries

We estimate the energy losses in the case of deep-inelastic scattering on nuclear targets in terms of the effective change of the virtual photon energy. Our phenomenological results are in reasonable agreement with theoretical calculations. The difference in secondary production processes in hard and soft interactions is discussed. The text was submitted by the authors in English.     

Fast-hadron formation mechanism in leptoproduction in nuclei

The mechanism of fast-hadron leptoproduction in nuclei in deep inelastic region is considered. Different possible mechansims of fast-hadron formation in the virtual-photon fragmentation region and the process of transition of the quark (quark-gluon system) through nuclear matter is analysed. The range of consistency of the SLAC and EMC CERN data on meson leptoproduction in different nuclei is obtained to be 0.2 GeV²m _c ² 0.5 GeV². It is shown that the kinematic region 0.05x _B 0.3 and (3–5)Q ²/m _c ² 100 is the most favourable for an investigation of the growth law (evolution) of the transverse dimensions of quark (quark-gluon system) produced by the virtual photon.     

Transverse spin effects in diffractive hadron leptoproduction

We consider double spin asymmetries for longitudinally polarized leptons and transversely polarized protons in diffractive vector meson and production at high energies within the two-gluon model. The connection of the two-gluon approach with skewed gluon distributions is discussed. The asymmetry predicted for meson production is quite small. The asymmetry for production contains two independent terms which are large and can be used to obtain information on the polarized skewed gluon distributions in the proton. Received: 19 February 2002 / Revised version: 19 March 2002 / Published online: 22 May 2002     

Investigations of azimuthal asymmetry in semi-inclusive leptoproduction

We consider the azimuthal asymmetries in semi-inclusive deep inelastic leptoproduction arising due to both perturbative and nonperturbative effects at HERMES energies and show that the order corrections to and are significant. We also reconsider the results of perturbative effects for at large momentum transfers [1] using the more recent sets of scale-dependent distribution and fragmentation functions, which bring up to 18% difference in . In the same approach we calculate the as well. Received: 24 October 1997 / Revised version: 10 February 1998 / Published online: 30 March 1998     

Angular energy flow in leptoproduction and QCD

The perturbative QCD prediction of the angular energy flow in deep inelastic leptoproduction is discussed. A numerical estimate of its magnitude is given in the Breit frame and in the c.m. frame of the virtual photon and proton.     

QCD and final-state jet measures in leptoproduction

Various infrared-safe measures of jet-like-ness in leptoproduction are studied in QCD perturbation theory using a general formalism. This quantifies the effects predicted in a previous paper. The nucleon remnants jet is found to be distinctly narrower than the quark jet.     

Jet cross sections in leptoproduction from QCD

We have calculated the longitudinal and other polarization dependent cross sections for jet production in deep inelasticep, vp, and \(\bar vp\) scattering up to order α _s of the quark-gluon coupling constant. Fragmentation of final state partons into hadrons is taken into account. Distributions in thrust,p ₂ ^{T in} andp ₂ ^{T out} are predicted for all three reactions and various values ofW andQ.     

The pion pole in hard exclusive vector-meson leptoproduction

Exploiting a set of generalized parton distributions (GPDs) derived from analyses of hard exclusive leptoproduction of ρ ⁰, φ and π ⁺ mesons, we investigate the ω spin density matrix elements (SDMEs) recently measured by the HERMES Collaboration. It turns out, from our study, that the pion pole is an important contribution to ω production. It will be treated as a one-particle exchange since its evaluation from the GPD \(\tilde E\) considerably underestimates its contribution. As an intermediate step of our analysis, we extract the πω transition form factor for photon virtualities less than 4 GeV². From our approach we achieve results for the ω SDMEs in good agreement with the HERMES data. The role of the pion pole in exclusive ρ ⁰ and φ leptoproduction is discussed too.     

Generalized parton distributions from meson leptoproduction

Generalized parton distributions (GPDs) extracted from exclusive meson leptoproduction within the handbag approach are briefly reviewed. Only the GPD E is discussed in some detail. Applications of these GPDs to virtual Compton scattering (DVCS) and to Ji’s sum rule are also presented.     

QCD predictions for jet transverse momentum in leptoproduction

QCD predictions for distributions in the jet variable Σ_T  [Σ_hadp_T|]² are calculated for incident ν, $\text{ν}$ and μ. Estimates of the nonperturbative background are constructed.     

QCD effects on the event structure in leptoproduction

Perturbative QCD corrections to leptoproduction events can be introduced either in the form of matrix elements or of parton showers. Each of these approaches has its advantages and disadvantages, making a comparison of the two interesting. At present energies, both methods can be made to agree reasonably well with data, whereas differences appear at higher energies. The influence of these QCD effects on the expected event structure at ep colliders, HERA in particular, is investigated in detail. This includes multiplicity and momentum distributions, transverse momentum flow and correlations, as well as jet properties.     

Pion electromagnetic structure functions from pion exchange in leptoproduction

The pion-exchange contribution to e+p→e′+n+X is calculated using different models, both for pion exchange and for the pion electromagnetic structure functions. Numerical estimates of this contribution to deep inelastic scattering off protons are given.     

Hadron final states in deep-inelastic leptoproduction on nuclei

We discuss some aspects of the formation of inclusive hadron final states in deep-inelastic electro-, muo- and neutrinoproduction on nuclear targets.The inclusive final states are known to possess a remarkable Q² independence in interactions with free nucleons. Nevertheless, we prove that the nuclear spectra and mean multiplicities should have a rather specific and appreciable dependence on Q². In leptoproduction on heavy nuclei the yields of particles in nucleus fragmentation rise by a factor of two, as at the fixed energy transfer v one passes from incoherent scattering to diffraction scattering, and the total multiplicities change by 20–40%. The effects predicted seem to be detectable with the current and the near-future experimental facilities. Fermilab bubble chamber neutrino experiments and the coming SPS muon and neutrino experiments deserve special attention in this respect. We also comment on the possibilities offered by leptoproduction on nuclei for testing some absorption models.