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.     

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.     

Low-energy dynamics in ultradegenerate QCD matter

We study the low-energy behavior of QCD Green functions in the limit that the baryon chemical potential is much larger than the QCD scale parameter LambdaQCD. We show that there is a systematic low-energy expansion in powers of (omega/m)(1/3), where omega is the energy and m is the screening scale. This expansion is valid even if the effective quark-gluon coupling g is not small. The expansion is purely perturbative in the magnetic regime |k| > k0. If the external momenta and energies satisfy |k| approximately k0, planar, Abelian ladder diagrams involving the full quark propagator have to be resummed but the corresponding Dyson-Schwinger equations are closed.     

Quark dynamics, thermal QCD, and the gravity dual

We perform a holographic renormalization of the supergravity action and compute the stress tensor of the dual gauge theory incorporating the logarithmic running of the gauge coupling. From the stress tensor we obtain the shear viscosity and the entropy of the medium at temperature T, and investigate the ratio η/s.     

Momentum loop dynamics and random surfaces in QCD

A new approach to QCD - momentum loop dynamics - is proposed. The basic quantity is the local momentum p_μ(s) of quarks propagating along the closed loop in a gluonic vacuum. Using the equations of QCD at N = ∞, we derive the stochastic motion of p_μ(s) in extra proper time H. We propose an explicit method for computer simulation, preserving all the symmetries of the continuum theory. An advantage of this approach is the genuine reduction of degrees of freedom. The problems are related with cancellations in numerical simulations.     

Soft and hard QCD dynamics in hadroproduction of charmonium

Both hard and soft QCD dynamics are important in charmonium production, as presented here through a next-to-leading order QCD matrix element calculation combined with the color evaporation model. The observed and distributions of in hadroproduction at fixed target and collider energies are reproduced. Quite similar results can also be obtained in a more phenomenologically useful Monte Carlo event generator where the perturbative production of pairs is instead obtained through leading order matrix elements and the parton shower approximation of the higher order processes. The soft dynamics may alternatively be described by the soft color interaction model, originally introduced in connection with rapidity gaps. We also discuss the relative rates of different charmonium states and introduce an improved model for mapping the continuous mass spectrum on the physical charmonium resonances Received: 29 November 2001 / Published online: 1 March 2002     

The QCD coherent state from asymptotic dynamics

The recently found coherent state operator for soft gluon emission is derived in a simple way from the Faddeev-Kulish approach of asymptotic dynamics. It is argued that this method can be extended to next subleading infrared singularities.     

Evidence for instanton-induced dynamics from lattice QCD

We perform a study of the nonperturbative dynamics of the light-quark sector of QCD, based on some recent results of lattice simulations with chiral fermions. We analyze some correlators that are designed to probe the Dirac structure of the quark-quark interaction at different scales. We show that, in the nonperturbative regime, such an interaction contains very large scalar and pseudoscalar components. We observe quantitative agreement between lattice QCD results and the predictions of the instanton liquid model. Moreover, we study how the quark-quark interaction is modified, when quark loops are suppressed. We observe a dramatic effect related to the loss of unitarity, which is naturally explained in the instanton picture. Such an effect cannot be explained in a Dyson-Schwinger equations (DSE) approach, if one assumes a vector quark-gluon coupling.     

Non-linear QCD dynamics and exclusive production in ep collisions

The exclusive processes in electron–proton (ep) interactions are an important tool to investigate the QCD dynamics at high energies as they are in general driven by the gluon content of proton which is strongly subject to parton saturation effects. In this paper we compute the cross sections for the exclusive vector meson production as well as the deeply virtual Compton scattering (DVCS) relying on the color dipole approach and considering the numerical solution of the Balitsky–Kovchegov equation including running coupling corrections. We show that the small-x evolution given by this evolution equation is able to describe the DESY-HERA data and is relevant for the physics of the exclusive observables in future electron–proton colliders and in photoproduction processes to be measured in coherent interactions at the LHC.     

Deconfinement and gluon plasma dynamics in improved holographic QCD

The thermodynamics of 5D dilaton gravity duals to confining gauge theories is analyzed. We show that they exhibit a first order Hawking-Page type phase transition. In the explicit background of improved holographic QCD of [U. Gursoy and E. Kiritsis, J. High Energy Phys. 02 (2008) 03210.1088/1126-6708/2008/02/032] [U. Gursoy, E. Kiritsis, and F. Nitti, J. High Energy Phys. 02 (2008) 01910.1088/1126-6708/2008/02/019], we find T_{c}=235 MeV. The temperature dependence of various thermodynamic quantities such as the pressure, entropy, and speed of sound is calculated. The results are in agreement with the corresponding lattice data.     

Chiral dynamics in bag-model QCD at low energy

The Goldberger-Treiman relation is studied in bagged QCD and related to the lowest order of chiral expansions of hadronic wave functions involving quark-antiquark admixtures. The latter contribute to meson-nuclear dynamics, e.g. the pion-nucleon absorption vertex and the neutron charge from factor.     

Nonperturbative dynamics in the chromomagnetic QCD vacuum above the deconfinement temperature

We investigate the residual chromomagnetic nonperturbative interaction between colour sources which survives above the deconfinement temperature. This interaction is demonstrated to be attractive enough to support bound states of quarks (and gluons). Although, for heavy quarks, such bound states appear to be very shallow and thus to dissociate easily, for light quarks (and gluons), the interaction is found to be strong enough to lead to quark (gluon) pair creation in the vacuum and thus to a complete rebuild of the vacuum state. The text was submitted by the authors in English.     

Asymptotic dynamics of QCD,coherent states,and the quark form factor

The method of asymptotic dynamics for large times developed by Kulish and Fadde'ev for QED is applied to QCD. We study the solution and calculate the on shell quark form factor in leading logarithmic order.     

Comment on the proper QCD string dynamics in a heavy-light system

The string correction to the interquark interaction at large distances is derived using the field theory approach to a heavy-light quark-antiquark system in the modified Fock-Schwinger gauge.     

Non-linear QCD dynamics in two-photon interactions at high energies

Perturbative QCD predicts that the growth of the gluon density at high energies should saturate, forming a Color Glass Condensate (CGC), which is described in mean field approximation by the Balitsky–Kovchegov (BK) equation. In this paper we study the γγ interactions at high energies and estimate the main observables which will be probed at future linear colliders using the color dipole picture. We discuss in detail the dipole–dipole cross section and propose a new relation between this quantity and the dipole scattering amplitude. The total γγ, γ ^? γ ^? cross sections and the real photon structure function $F_{2}^{\gamma }(x,Q^{2})$ are calculated using the recent solution of the BK equation with running coupling constant and the predictions are compared with those obtained using phenomenological models for the dipole–dipole cross section and scattering amplitude. We demonstrate that these models are able to describe the LEP data at high energies, but predict a very different behavior for the observables at higher energies. Therefore we conclude that the study of γγ interactions can be useful to constrain the QCD dynamics.     

Thermo-field dynamics and quark–hadron phase transition in QCD

Based on the topological structure of gauge theory, an effective dual version of QCD has been reviewed and analyzed for the phase structure and color confining properties of QCD by invoking the dynamical magnetic symmetry breaking. The multi-flux-tube configuration of condensed QCD vacuum has been explored and associated glueball masses and inter-quark potential have been derived. Thermal response of QCD vacuum has been analyzed using path-integral formalism alongwith the mean-field approach and associated thermodynamical potential is used to derive thermal form of glueball masses, monopole condensate, inter-quark potential and monopole density which then lead to an estimate of the critical temperature of QCD phase transition. During its thermal evolution, a smooth transition of hadronic system via a weakly bound QGP phase to the fully deconfined phase is established and the thermal evolution profiles of various parameters are shown to indicate a second-order deconfinement phase transition and the restoration of magnetic symmetry. Monopole density calculations have been shown to lead to gradual evaporation of magnetic condensate into thermal monopoles during QCD phase transition.