Quark-gluon Degrees Of Freedom Research Articles

Impact of fragment formation on shear viscosity in the nuclear liquid-gas phase transition region

The dimensionless ratio of shear viscosity to entropy density $\ensuremath{\eta}/s$ typically has a minimum at a phase transition as observed in many atomic and molecular systems. This quantity has been under intense study in high-energy heavy-ion collisions as a sign of a QCD phase transition or rapid crossover between hadronic and quark-gluon degrees of freedom. The authors perform numerical simulations of matter at lower density and temperature associated with a minimum in $\ensuremath{\eta}/s$ which may shed light on the nuclear liquid-gas phase transition and its connection to the QCD transition.

J-PARC Hadron Physics and Future Possibilities on Color Transparency

The Japan Proton Accelerator Research Complex (J-PARC) is a hadron-accelerator facility that aims to provide secondary beams of kaons, pions, neutrinos, muons, and others together with the primary proton beam for investigating a wide range of science projects. High-energy hadron physics can be studied by using high-momentum beams of unseparated hadrons, which are essentially pions, and also primary protons. In this report, possible experiments are explained on color transparency and generalized parton distributions (GPDs). These projects are complementary to lepton-scattering experiments at Jefferson Laboratory (JLab), COMPASS/AMBER, and future electron-ion colliders. Thank to hadron-beam energies up to 30 GeV, J-PARC is a unique facility to investigate the transition region from the hadron degrees of freedom to the quark-gluon degrees of freedom. It is suitable for finding mechanisms of the olor transparency. Such color-transparency studies are also valuable for clarifying the factorization of hadron production processes in extracting the GPDs from actual measurements. These studies will lead to the understanding of basic high-energy hadron interactions in nuclear medium and to clarifications on the origins of hadron spins, masses, and internal pressure mechanisms.

NICA-MPD: Azimuthal and femtoscopic particle correlations

The discussion is focused on the study of the fundamental symmetries ($\mathcal{P/CP}$) of QCD and geometry of the particle source. The combination of correlators corresponding to the absolute asymmetry of distribution of electrically charged particles with respect to the reaction plane in heavy ion collisions is studied. A significant decrease of the absolute asymmetry is observed in the intermediate energy range which can be considered as indication of possible transition to predominance of hadronic states over quark-gluon degrees of freedom in the mixed phase created in heavy ion collisions at intermediate energies. For the investigation of the energy evolution of the geometric properties of the particle source the use of femtoscopic radii scaled on the averaged radius of colliding ions is suggested. This approach allows the expansion of the set of interaction types, in particular, on the collisions of non-symmetrical ion beams which can be studied within the framework of common treatment. There is no sharp changing of femtoscopic parameter values with increasing of the initial energy. The suggestions are made for future advancement of these studies on NICA-MPD.

Photoproduction of Vector Meson ϕ off Deuteron in QCD Inspired Model

Based on the quark-gluon contents of nucleon and strongly believing that the force mediators, Pomeron and its counterpart in the conventional approach of Regge theory, for high energy diffractive process would be the tensor glueball and Odderon respectively, we discuss photo-production of vector meson ϕ off the deuteron at energy less than 3 GeV in the QCD inspired model in which the quark gluon degrees of freedom and glueball, Odderon exchange are taken into account. A calculation is performed for γ + D → ϕ + D, and the theoretical predictions of the differential cross section dσγD /dt, are presented and compared with available experimental data. Our QCD inspired model reproduces data quite well in the whole range of the experimental measurements up to |t| ≅ 0.4 GeV. Our results can be used to extract γn → ϕn data, which cannot be measured in experiment.

Diffractive upsilon photo-production off the proton in the QCD inspired model

Based on the generalized vector meson dominance model in QCD, we study the photoproduction of vector meson ϒ off the proton by use of the QCD-inspired model in which the contributions from quark-quark, gluon-gluon and quark-gluon interference term to observable are taken into consideration. Calculations are performed for total cross section σ tot, differential cross section dσ/dt, ratio of the real part to imaginary part of forward scattering amplitude ρ and nuclear slop parameter function β. We analyze the individual contributions from quark gluon degrees of freedom and the QCD Odderon to the total cross section σ tot (s), differential cross section dσ/dt, ratio of the real part to imaginary part of the forward scattering amplitude ρ, and nuclear slop parameter function β. The mediators of interactions between projectiles (the quark and antiquark pair fluctuated from the real photon) and the proton target (three quark system) are the tensor Glueball and Odderon instead of using the usual Pomeron exchange. The theoretical predictions for σ tot(s) are consistent with the experimental data within error bars of the data. The data for dσ/dt, β and ρ are urgently needed. The gluon-gluon interaction makes a significant contribution to the observables while the Odderon contribution is negligibly small. Therefore, we may conclude that it is impossible to find the QCD Odderon in the γ+p → ϒ+p process as suggested before.

Evidence for hadronic deconfinement in collisions at 1.8 TeV

We have measured deconfined hadronic volumes, $4.4 < V < 13.0$ fm$^{3}$, produced by a one dimensional (1D) expansion. These volumes are directly proportional to the charged particle pseudorapidity densities $6.75 < dN_{c}/d\eta < 20.2$. The hadronization temperature is $T = 179.5 \pm 5$ (syst) MeV. Using Bjorken's 1D model,the hadronization energy density is $\epsilon_{F} = 1.10 \pm 0.26$ (stat) GeV/fm$^{3}$ corresponding to an excitation of $24.8 \pm 6.2$ (stat) quark-gluon degrees of freedom.

What can we learn from antihyperon-hyperon production?

Exclusive antihyperon-hyperon production provides a test for models of strangeness production and final state interactions. Of primary interest is whether the reaction mechanism is better described in terms of meson-baryon or quark-gluon degrees of freedom. The sensitivity of these reactions to final state interactions should provide information on the YY states, which are not directly accessible by scattering experiments. Good agreement with existing data can be found for both quark and meson exchange models, but recently measured target depolarization data in ΛΛ production should distinguish between them. These measurements may also test models proposed to explain the negative polarization of strange quarks in the proton.

Field-theoretical description of electromagnetic Δ resonance production and determination of the magnetic moment of the Δ+ (1232) resonance by the ep → e′ N′ π′ γ′ and γp → N′ π′ γ′ reactions

The ep → e′ N′ π′ γ′ and γp → N′ π′ γ′ reactions in the Δ resonance region are investigated in the framework of the relativistic field-theoretical spectral decomposition method for the γ ∗N → γ′π′N′ transition amplitude. This approach allows us to express the amplitude through three-dimensional diagrams with on-mass shell intermediate states. The Δ resonance is constructed as intermediate cluster state | Ψ Δ 〉 with real bare mass M Δ ∼ 1232 MeV and with a complex propagator of Breit-Wigner shape for the P 33 π N amplitude. Thus, the usual off-mass shell ambiguities in spin 3 2 effective Lagrangians do not affect the present description. Using a unified treatment of nucleons, mesons, and Δ's as bound systems of quarks according to the Haag-Nishijima-Zimmermann field-theoretical approach, it is shown that quark-gluon degrees of freedom only change the rule for constructing the form factors in three-dimensional diagrams for obtaining the γ ∗N → γ′π′N′ transition amplitude. However, the structure of these-dimensional diagrams remains the same as in formulations with only meson and nucleon degrees of freedom. A simple redefinition of the γ ∗N → γ′π′N′ amplitude which insures gauge invariance in every order of perturbation theory with an arbitrary choice of phenomenological vertices is considered. We calculate the leading diagram of this amplitude in the Δ resonance region. It is demonstrated that the differential cross section of the ep → e′ N′ π′ γ′ and γp → N′ π′ γ′ reactions is sensitive to the value of the magnetic moment of the Δ + resonance. Thus, the measurement of the differential cross section for ep → e′ N′ π′ γ′ and γp → N′ π′ γ′ reactions can be used for determining the magnetic dipole moment of Δ +.

Virtual meson cloud of the nucleon and deep inelastic lepton scattering.

We address the question of whether the nucleon{close_quote}s nonperturbative antiquark sea can be attributed entirely to its virtual meson cloud and, in essence, whether there exists a smooth transition between hadronic and quark-gluon degrees of freedom. We take into account contributions from {pi} and {ital K} mesons and compare them with the nucleon{close_quote}s antiquark distributions which serve as nonperturbative inputs to the QCD evolution equations. While the agreement of our calculations with data-based parametrizations is satisfactory and scale independent for the flavor-breaking share of the nucleon{close_quote}s antiquark sea, the flavor singlet component is quite poorly described. We find that large meson loop momenta, {parallel}{ital k}{parallel}{approx_equal}0.8 GeV, dominate in the calculation of the nucleon{close_quote}s sea quark distributions in the meson cloud model. This value is close to the scale where perturbative QCD evolution starts, which further indicates the limitations of the applicability of the physical picture of a meson cloud around the nucleon in high-energy processes. {copyright} {ital 1996 The American Physical Society.}

Screening of static quantum-electrodynamic electric fields in hot quantum chromodynamics

The screening length for static quantum-electrodynamic electric fields in hot quantum chromodynamics (QCD) is computed using hadronic degrees of freedom at low temperature and quark-gluon degrees of freedom at high temperature. The high temperature result is equivalent to computing the photon self-energy to two-loop order in QCD plus summation of the leading infrared divergent diagrams at three-loop and higher order. Comparison is made with the screening length extracted from singlet and nonsinglet susceptibilities as measured in two-flavor lattice QCD.

Screening of static QED electric fields in hot QCD.

The screening length for static QED electric fields in hot QCD is computed using hadronic degrees of freedom at low temperature and quark-gluon degrees of freedom at high temperature. The high-temperature result is equivalent to computing the photon self-energy to two-loop order in QCD plus summing the leading infrared-divergent diagrams at three-loop and higher order. A comparison is made to the screening length extracted from singlet and nonsinglet susceptibilities as measured in two-flavor lattice QCD.

Relativistic two-nucleon wavefunctions

Following our description of the two-nucleon system via i) a meson-baryon picture at long distances ii) and quarkgluon degrees of freedom at short distances (Ruhr-Pot), we derive relativistic two-nucleon equations and give solutions for the deuteron. Differences to the Gross-equations are discussed.

The EMC effect in3He and4He due to a three-body gluonic force

We consider a new force, due to quarkgluon degrees of freedom, which effectively removes some of the momentum fraction from the valence quarks in the simplest tightly bound nuclei,3He and4He. This leads to a structure function per nucleon for3He and4He which deviates from that of a free nucleon.

Exploring quark-gluon degrees of freedom in nuclei

Lepton-nucleus scattering and ultra-relativistic heavy ion collisions provide complementary means to study the interface of nuclear physics and QCD.This talk focusses primarily on the physics of ultra-relativistic heavy ion collisions, and how they may produce deconfined, fully chirally symmetric phases of nuclear matter in which quark-gluon degrees of freedom may be studied over extended distance scales.

Equations for on-shell partial-wave amplitudes in the boundary condition model

The author considers a method that makes it possible to take into account the quark-gluon degrees of freedom in the system in the framework of the quark bag model and the meson degrees of freedom on the basis of fairly general requirements of S-matrix theory. It is shown that the proposed equations contain the potential scattering model as the limiting case. The interaction of the hadrons with the meson fields is determined by model-free characteristics and can be taken into account by the methods of quantum field theory. The proposed method makes it possible to exploit fully the principle of the dominant contribution of the singularities of the scattering amplitude with respect to the momentum transfer nearest the physical region.

The meson and quark-gluon degrees of freedom in NN-interaction at low and intermediate energies

A set of nonsingular integral equations has been formulated to calculate the partial amplitudes of hadron-hadron scattering at low and intermediate energies making allowance for the meson and quark-gluon degrees of freedom. The results are presented of the calculations of the np effective range parameters and of the np scattering 1S 0- and 3S 1-phase shifts at incident nucleon energies ranging from 1 to 1050 MeV.