Coherent effects in the perturbative QCD parton jets

The influence of coherence in a soft parton bremsstrahlung on hadron spectra in jets is discussed. We present a simple functional method for calculation of different inclusive parton distributions in the “plateau” region. Inclusive parton cross sections, spectra, multiplicities, energy and angular correlations, etc. are derived in the Double Logarithmic approximation.     

Perturbation theory and the parton model in QCD

We prove that for any process which admits a parton-model interpretation, the naive parton model can be modified to include the effects of QCD interactions to all orders in perturbation theory. This requires that the mass singularities in quark and gluon inclusive cross sections factor into universal functions which renormalize the naive parton model distribution and decay functions. We prove that this factorization takes place for all leading and non-leading logs and thus check consistency of the parton model to all orders in perturbation theory.     

A model for parton showers in QCD

A Monte Carlo model for the development of parton jets in QCD is described. Explicit low-order calculations are supplemented by leading logarithmic approximations for higher orders.     

Nuclear effects at large transverse momentum in a QCD parton model

Production of single mesons and opposite side pairs of mesons in hadron-nucleus and nucleusnucleus collisions are calculated in the framework of the QCD parton model, with the assumption that quarks and gluons undergo multiple scattering in the nucleus. A comparison with all available data is made. It is also shown that the anomalous nuclear enhancement, especially its dependence on the flavour, can be quantitatively described within the model. Furthermore, we predict no enhancement for symmetric pairs as it seems to be indicated by the data. Predictions for largep _T production in α?α scattering at ISR are made.     

Global QCD analysis of parton structure of the nucleon: CTEQ5 parton distributions

An up-to-date global QCD analysis of high energy lepton-hadron and hadron-hadron interactions is performed to better determine the gluon and quark parton distributions in the nucleon. Improved experimental data on inclusive jet production, in conjunction with precise deep inelastic scattering data, place good constraints on the gluon over a wide range of x; while new data on asymmetries in Drell-Yan processes contribute to better determine the d/u ratio. Comparisons with results of other recent global analyses are made, and the differences are described. Open issues and the general problem of determining the uncertainties of parton distributions are discussed. Received: 7 April 1999 / Published online: 21 December 1999     

Heavy quark production in parton model and in QCD

The cross sections of heavy quark production inpp collision and for their photo- and electroproduction are calculated in the framework of QCD. The virtual nature of the interacting gluons as well as their transverse motion and different polarizations are taken into account. The obtained cross sections exhibit more rapid growth with the initial energy than the parton model predictions and thep _T distributions of produced heavy quarks are more smooth.     

The QCD generalized parton model and a next-to-leading approximation of the gluon density

Using recent neutrino data, we calculate in the framework of the generalized parton model the modification of the gluon density caused by next-to-leading logarithmic effects. We find that they are of the same order of magnitude as the experimental uncertainties, and that the gluon density tends to be more peaked towards small x.     

Evolution of the truncated Mellin moments of the parton distributions in QCD analysis

We review evolution equations for the truncated Mellin moments of the parton distributions and some their applications in QCD analysis. The main finding of the presented approach is that the nth truncated moment of the parton distribution obeys also the DGLAP equation but with a rescaled splitting function P′(z) = z ⁿ P(z). This allows one to avoid the problem of dealing with the experimentally unexplored Bjorken-x region. The evolution equations for truncated moments are universal—they are valid in each order of perturbation expansion and can be useful additional tool in analysis of unpolarized as well as polarized nucleon structure functions.     

Massive quark polarization in QCD subprocesses

It is well known that single polarization asymmetries vanish in QCD at momenta that are high relative to quark masses, due to chirality conservation. However, the particular form of those asymmetries for non-zero masses has not been explored. We have calculated the fourth order contributions to massive quark polarization in quark-quark and quark-gluon scattering. The dependence on the non-abelian character of the theory is emphasized and a comparison is made with the Mott scatterng limit. Unusual momentum dependence and large polarizations are noted for some regions of parameter space. the relation to polarized hyperon production is explored.     

Nonequilibrium parton dynamics in the strongly interacting QGP

The dynamics of partons, hadrons and strings in relativistic nucleus-nucleus collisions is analyzed within the novel Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for partons (DQPM) matched to reproduce recent lattice-QCD results—including the partonic equation of state—in thermodynamic equilibrium. The transition from partonic to hadronic degrees of freedom is described by covariant transition rates for the fusion of quark-antiquark pairs or three quarks (antiquarks), respectively, obeying flavor current-conservation, color neutrality as well as energymomentum conservation. Since the dynamical quarks and antiquarks become very massive close to the phase transition, the formed resonant ‘pre-hadronic’ color-dipole states (q [`(q)]bar q or qqq) are of high invariant mass, too, and sequentially decay to the groundstate meson and baryon octets increasing the total entropy. When applying the PHSD approach to Pb + Pb colllisions at 158 A GeV we find a significant effect of the partonic phase on the production of multi-strange antibaryons due to a slightly enhanced s [`(q)]bar q pair production from massive time-like gluon decay and a larger formation of antibaryons in the hadronization process.     

Moments of the heavy-quark parton distribution function from QCD sum rules

The moments of the heavy-quark parton distribution functions in a heavy pseudoscalar meson are calculated from QCD sum rules. Expanding these sum rules in the inverse heavy-quark mass we obtain the heavy-mass limits of the moments. Comparison with the finite-mass results reveals that while the heavy-mass expansion works reasonably well for the b quark, one has to take into account terms of higher order than (1/m _c )² for the c quark. This result can provide a quantitative assessment of c- and b-quark fragmentation models based on the heavy-quark mass limit. The text was submitted by the authors in English.     

String dynamics in QCD

Recent works of the authors on string interpretation of the Wilson loop operators in QCD are reviewed in a self-contained fashion. Although most of the results have already appeared in print, some new materials is presented on renormalization of the Wilson loop operator and on the use of light-cone expansion to derive a linear string-like equation in light-cone formalism.     

A recurrence relation for the polarization in elastic p-p scattering in the framework of a parton model

Recent studies have shown that it is possible to account for the general features of large angle elastic proton-proton differential cross sections in a model where each constituent of one proton scatters from at least one constituent of the other proton in such a way that all constituents remain near their mass shell. Each of the parton-parton scatters is assumed to occur at approximately the same angle, so that the outgoing partons can easily recombine to form the two final state protons. In the present article we show that such a model, with additional assumptions regarding the scattering mechanisms at small |t|, leads to a non-trivial recurrence relation which is consistent with the structure recently reported in the high energy p-p polarization and fixed angle differential cross section.     

Connection formulas between the different QCD orders in application to the valence parton distribution functions

Formulas directly connecting parton distribution functions (PDFs) at the leading (LO) and next to leading (NLO) QCD orders are applied with respect to both unpolarized and polarized valence PDFs. It is shown that the connection formulas allow without any restriction on the allowed Q ² range for the analyzed data to produce improved LO results on valence PDFs, which strongly differ from the standard parametrizations on these quantities, and which could be obtained within the standard approach only by using the data produced at very high Q ² values (that is hardly possible in reality).     

Nonperturbative dynamics in the color-magnetic QCD vacuum

In the deconfinement phase of QCD, quarks and gluons interact with the dense stochastic color-magnetic vacuum. We consider the dynamics of quarks in this deconfinement phase using the field correlators method and derive an effective nonperturbative interquark potential, in addition to the usual perturbative short-range interaction. We find the resulting angular-momentum-dependent interaction to be attractive enough to maintain bound states and, for light quarks (and gluons), to cause emission of quark and gluon pairs. Possible consequences for the strong-interacting quark-gluon plasma are briefly discussed. The text was submitted by the authors in English.     

Braves and the dynamics of QCD

A brane configuration is described that is relevant to understanding the dynamics of N = 1 supersymmetric Yang-Mills theory. Confinement and spontaneous breaking of a discrete chiral symmetry can be understood as consequences of the topology of the brane. Because of the symmetry breaking, there can be domain walls separating different vacua; the QCD string can end on such a domain wall. The model in which these properties can be understood semiclassically does not coincide with supersymmetric Yang-Mills theory but is evidently in the same universality class.