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.     

Liberated color in leptoproduction

Within a spontaneously broken color gauge theory of integer charge quarks (icq) one expects standard “confined” QCD predictions for leptoproduction experiments (e.g.,μN→μX) to hold for energies below the threshold for color production; above such a threshold, however, the color degree of freedom must be excited leading to a threshold rise in structure functions. We predict the expected rises for μN and charged current (cc) νN and $\text{ν}\text{N}$ scattering within the “naive” parton model. Comparing these predictions with the available data on μN, cc νN and $\text{ν}\text{N}$ as well as e^?e⁺ experiments, we deduce that the lightest spin-parity 1^? color octet meson (like the gluon) as well as the lightest color octet (8, 8) baryon must lie above 7–8 GeV. This in turn closes the only window in the light mass region <3.1 GeV (i.e., the 1.1 to 1.8 GeVregion), whichhadsofarbeenleftopenasapossiblevalueforthephysicalmassofthegluon. Wenextobservethatthetrendofthemostrecent high-Q² high-W Michigan State-Fermilab experiment on μN scattering exhibiting a relatively sharp rise in F₂ above the expectations of confined QCD of order 15% at low x agrees very well with our predictions based on the gauge model of icq. This rise, if confirmed, would favor the hypothesis of gauge icq, and simultaneously imply a threshold for the liberated color octet (8, 8) state at nearly (9±0.6) GeV and a mass for the liberated gluon in the region 8 to 9.5 GeV. We remark that within a left-right symmetric gauge structure leading to icq, charged current scattering of laboratory neutrinos (νN→μ^?X) will excite color only provided that the charged gluons V⁺ mix with V-A gauge particles W_L⁺mrather then with the V+A gauge particles W_R⁺. Thus it is possible that the μN, e^?e⁺ and neutral current scattering of neutrinos (i.e., νN→νX) show signs of color excitation, while the charged current scattering of neutrinos does not. We discuss briefly the implications of the threshold for liberated color octet states possibly lying around 8–10 GeV or higher on future e^?e⁺ experiments as well as $\text{ν}\text{N}\text{,}\text{ν}\text{N}\text{,}\text{p}\text{p}\text{-}\text{and}\text{p}\text{p}\text{-}\text{produced}$ dilepton-search experiments.     

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 transmission profiles in high energy electron channeling

Monte Carlo computer calculations have shown that transmission profiles in high energy electron channeling can be well understood by regarding the direct influence of the string-effect on the electron scattering angle.     

On the particle multiplicity distributions at high energy

An experiment has been performed to study the inclusive photoproduction of π⁰ mesons. Photoproduced π⁰'s were detected in a hodoscope consisting of 14 lead glass Cerenkov counters. To obtain the cross sections at a photon energy of 6 GeV we used the photon-difference method. Data are presented in terms of the Feynman variable $\text{x = q}{}_6{}^1\text{(√s/2)}$ and the trnasverse momentum q⊥. We have measured four q⊥ distributions at x = 0.2, 0.4, 0.6 and 0.8. The results dhow that the π⁰ rate at small values of q⊥ is smaller than the π± rate. The data can be fitted smoothly by a form A exp (?Bq_⊥²) with a value about 3.5 GeV^?2 for the slope parameter B.     

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.     

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     

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.     

On the coalescence model for high energy nuclear reactions

The coalescence model for composite particle spectra from high energy collisions is formulated in a way which clarifies the underlying assumptions and the meaning of the parameters in the model. A density matrix formalism is used to describe a highly excited part formed by a collision and the coalescence volume is found to be related to the internal wave function of the composite particle and the spatial distribution of nucleons in the highly excited part.     

On the energy loss of high energy quarks in a finite-size quark-gluon plasma

The induced gluon emission from a fast quark passing through a finite-size QCD plasma is studied within the light-cone path integral approach. It is shown that the leading log approximation used in previous studies fails when the gluon formation length becomes on the order of the length of the medium traversed by the quark. Calculation of the energy loss beyond the leading log approximation gives the energy loss which grows logarithmically with quark energy, contrary to the energy-independent prediction of the leading log approximation.     

On rigid projectile approximation in high energy nucleus-nucleus scattering

Rigid projectile approximation in high energy nucleus-nucleus collisions is discussed. The results are demonstrated ond- elastic scattering.Dedicated to Professor Ivan Úlehla on the occasion of his sixtieth birthday.     

QCD predictions for semi-inclusive and inclusive leptoproduction

We give analytic predictions for angular distributions of hadrons in semi-inclusive and inclusive leptoproduction deduced from first-order calculations in QCD. The predictions allow tests of QCD and give information about gluon fragmentation functions.     

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.     

On the photon production in nucleus-nucleus collisions at high energy

A modified Landau hydrodynamical model is applied to study hard thermal photon production in central heavy-ion collisions at LHC, RHIC and SPS energies. It is shown that the phase transition from quark-gluon plasma into hadrons in consequence of the thermodynamical expansion is close to the second order phase transition if a resonance production is taken into account. Hard direct photon emission is also investigated with consideration of nuclear shadowing effect on structure function of quarks and gluons. Also π^π photon background is investigated. It is demonstrated that at the LHC energy photon yield from the quark-gluon plasma in the photon transversal momentumk _⊥ range from 5 to 25 GeV/c exceeds both the background and the direct photon yield. This conclusion may be important for the quark-gluon plasma diagnostic aims. It is also shown that for the LHC energy the thermal photon yield in the present model essentially exceeds this yield obtained in the Bjorken scaling model.     

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.