S.I. Troyan

Hard Processes in Quantum Chromodynamics

Abstract The use of Quantum Chromodynamics (QCD) in treating the hadronic world has become an overwhelming trend in particle physics. Owing to the asymptotic freedom of QCD, one can use perturbative methods to describe hard hadronic processes, i.e. those in which small distances as compared to hadron size are important. This paper is devoted to an improved perturbative QCD analysis of a wide class of hard processes. We start with the deep inelastic lepton-hadron scattering and the inclusive e + e − annihilation to hadrons, and show how and to what extent QCD imitates the parton model. We move further to hard semi-inclusive processes, and demonstrate that in this case the QCD predictions differ drastically from those of the parton model. The approach outlined in the paper paves the way for a detailed quantitative description of hard processes (provided QCD is the right theory). Special attention is given to the underlying physics. In particular, a possible influence of the hitherto unknown confinement mechanism on perturbative QCD analysis is discussed.

THE STRING EFFECT AND QCD COHERENCE

Abstract In the framework of the idea of local parton-hadron duality we discuss the asymptotic predictions of QCD perturbation theory for angular distributions of hadron flows in three-jet events, e + e - → q q g → hadrons. The coherence of soft gluon emission provides the QCD explanation of the string effect observed in experiments.

On the Transverse Momentum Distribution of Massive Lepton Pairs

Abstract The experimentally measured q t distributions of massive lepton pairs produced in hadron collisions are compared with the predictions of QCD [1]. We demonstrate that a fair agreement may be achieved without the use of large transverse momenta of the constitunt quarks - owing to a specific double-logarithmic form factor which manifests itself in this process. A prediction for the q t distribution in weak intermediate boson production is also given.

Specific features of heavy quark production: Local parton-hadron duality approach to heavy particle spectra.

A perturbative QCD formula for inclusive energy spectra of heavy quarks from heavy quark initiated jets, which takes into account collinear and/or soft logarithms in all orders, the exact first order result, and two-loop effects, is applied to distributions of heavy flavored hadrons in the framework of the local parton-hadron duality concept. {copyright} {ital 1995 The American Physical Society.}

Hard semi-inclusive processes in QCD

Abstract We present formulae resulting from a complete perturbative analysis of differential cross sections of (i) massive lepton pair production in hadron collisions, (ii) leptoproduction of a particle from the current fragmentation region, (iii) production of two particles from opposite jets in e+e− annihilation into hadrons. A new effective double logarithmic quark form factor is derived, which manifests itself in these processes. Experimental investigation of two-particle angle correlations in e+e− annihilation provides a possibility to measure directly this form factor and thus to make a nice test of QCD.

Hump-backed QCD plateau in hadron spectra

New data on inclusive energy distribution of hadrons from quark jets ine+e− annihilation are shown to agree with the quantitative predictions of perturbative QCD, supporting the hypothesis of local parton-hadron duality.

On specific QCD properties of heavy quark fragmentation ('dead cone')

It is the restriction on the phase space of emitting gluons connected with the kinematics of a heavy quark Q=c, b, . . . which determines the difference of the QCD jet produced by Q from that of ordinary light (practically massless) quarks q=u, d, s. The authors consider the Q-quark as relativistic (EQ>>MQ, Theta 0 identical to MQ/EQ<<1) and taking care of logarithmic effects ignoring small (power) corrections of the order of Theta 0.

Particle spectra in light and heavy quark jets

The application of the analytical perturbative technique to the description of inclusive particle spectra in QCD jets is reviewed. The authors discuss characteristic properties of the jets generated by heavy quarks and list the prospects for future studies of coherent parton cascades.

INCLUSIVE PARTICLE SPECTRA FROM QCD CASCADES

New results in applications of the analytical perturbative approach to the description of particle distributions in QCD jets are presented. We study inclusive energy spectra of partons from QCD cascades truncated at different values of the cutoff parameter Q0, which can be used to model the energy distributions of massive hadrons in the framework of the local parton-hadron duality hypothesis. Results of straightforward numerical calculations of parton distributions which solve the MLLA evolution equations are compared to approximate formulas representing the spectra in terms of a few analytically calculated shape parameters.

Phenomenology of the particle spectra in QCD jets in a modified leading logarithmic approximation

Inclusive energy spectra of hadrons produced ine+e− annihilation are compared to predictions of the model independent perturbative approach based on the modified leading logarithmic approximation for description of QCD parton cascades and the local parton-hadron duality concept.