Quark-gluon Interaction Research Articles

Quark self-energy in a truncated bag model: Study of high-order effects in perturbative QCD.

The mode sum in the field operators of the bag model is truncated for the purpose of studying high-order effects of QCD in a cavity. The quark self-energy is calculated to $O({\ensuremath{\alpha}}_{s}^{2})$. The quark self-energy due to just the quark-gluon interaction is determined to $O({\ensuremath{\alpha}}_{s}^{6})$ using a pattern recognition code which finds and calculates over 10 000 diagrams. This self-energy is then summed to all orders in the planar approximation, and insight regarding the nature of the ${N}_{c}^{\ensuremath{-}1}$ expansion is obtained.

On the checks of fundamental symmetries in atomic physics

The investigation of nuclear-spin-dependent P-odd effects in atoms is the source of important and reliable information on P-odd nuclear forces. The upper limits on the T-odd, P-odd quark-quark and quark-gluon interactions deduced from the searches for the neutron and atomic electric dipole moments are comparable. The upper limits on the T-odd, P-even electron-electron, electron-nucleon and nucleon-nucleon interactions, as well as on some T-odd β-decay constants, extracted from the bounds on the dipole moments are much better than those following from the corresponding direct experiments. Extremely strict upper limits on the T-odd, P-even photon-fermion interaction are deduced from high-energy experiments.

Baryon-Meson Couplings in the $q\bar{q}$ Pair-Creation Quark Model. I

Characteristic features of the baryon-meson coupling vertices are studied in terms of a quark model in which quark-antiquark pair-creation processes inherent in the quark-gluon interaction Lagrangian are explicitly incorporated. It is shown that the standard non-relativistic baryon-meson vertex functions are naturally derived from quark exchange diagrams in a way that approximate SU6 relations are apparently satisfied for the coupling constants. Under the assumption of the flavor SU3 symmetry, this formalism yields a simple analytic expression for the coupling constants of S-wave baryons, which contains only one parameter related to the overall strength. The model predictions are examined in conjunction with the available empirical information. The effect of flavor symmetry breaking is also discussed with respect to the pion decay widths of decuplet baryons. Due to the improper treatment of the chiral symmetry in constituent quark models, all the pion coupling constants are universally too small by factor 3 ∼6 in strength.

Baryon-Meson Couplings in the qq Pair-Creation Quark Model. I: Coupling Constants of S-Wave Baryons

Characteristic features of the baryon-meson coupling vertices are studied in terms of a quark model in which quark-antiquark pair-creation processes inherent in the quark-gluon interaction Lagrangian are explicitly incorporated. It is shown that the standard non-relativistic baryon-meson vertex functions are naturally derived from quark exchange diagrams in a way that approximate SU 6 relations are apparently satisfied for the coupling constants. Under the assumption of the flavor SU 3 symmetry, this formalism yields a simple analytic expression for the coupling constants of S'-wave baryons, which contains only- one parameter related to the overall strength. The model predictions are examined in conjunction with the available empirical information. The effect of flavor symmetry breaking is also discussed with respect to the pion decay widths of decuplet baryons

Quark-antiquark pair creation effects in the study of S = 0 and S = −1 baryons

Improved wavefunctions of non-strange and strange baryons are obtained by incorporating quark-antiquark excitations inherent in the quark-gluon interaction into the quark model. The working framework is the shell model where the antiquark is assigned an intrinsic parity quantum number to distinguish it from the quark. The parameters of the interaction are chosen to be consistent with the experimental data. It is found that the amplitudes of the 3 q( q q) and 3 q( q 2 q 2 ) components are not negligible and should be included in the calculation of the properties of the baryons. It is also found that the amplitude of the 3 q( q q) component for strange baryons is less than for nonstrange ones, indicating that the hyperon-nucleon(-hyperon) interaction is less attractive than the NN one.

Effect of temperature and quark-gluon interaction on strange matter

We have investigated the effect of temperature on strange quark matter by treating the quark-gluon interaction perturbatively to lowest order in the QCD coupling constant. We find that the quantities that depend on electron charge density get their leading contribution from the temperature dependent terms. In particular the quark charge density per baryon rises sharply with temperature and the neutrino emissivity increases by roughly two orders of magnitude when temperature dependent effects are taken into account.

Studies of the transverse spin-dependent structure function g2(x,Q2).

We study the transverse spin-dependent nucleon structure function ${g}_{2}(x, {Q}^{2})$ in leading order at large ${Q}^{2}$. After reviewing the operator-product-expansion analysis of ${g}_{2}(x, {Q}^{2})$ we calculate the twist-3 contributions which arise from quark-gluon interactions and quark masses in the bag model. The result confirms the presence of significant twist-3 effects in ${g}_{2}(x, {Q}^{2})$. We discuss two questions raised by our analysis: the validity of the Burkhardt-Cottingham sum rule and the relation to previous calculations of ${g}_{2}(x, {Q}^{2})$ in versions of the parton model.

Quark-Gluon Interactions at High Temperature and Large Distances

Continuum fourdimensional Quantum Chromodynamics (QCD) including quarks in the regime of high temperature and large distances (the HT regime) is studied to all perturbative orders. The imaginary time (τ) formalism is used. Then, as shown in previous works, QCD is described by a new generating functional ZHT, in which quark fields retain their dependences on τ, while gluon and ghost fields are τ-independent. The invariance of ZHT under BRST transformations in the HT regime is exhibited: it closed quark and electric gluon loops in the HT regime are obtained. The electric mass terms in ZHT. Infinite sets of non-abelian Ward identities for the closed quark and electric gluon loops in the HT regime are obtained. The electric mass terms in ZHT are shown to be infrared finite. We prove to all perturbative orders that one can regard as subdominant, and, hence, neglect consistently the contributions of :i) all closed electric gluon loops, ii) all closed quark loops with three or more vertices in diagrams having an even number of electric gluons (or none) in the external lines. In the HT regime, the axial anomalies are obtained: their expressions in terms of τ-independent gluon fields are similar to those for zero temperature. A non-trivial renormalization group (RG) equation in the HT regime, specifically due to the quark-gluon interaction, is presented. A positive beta function is obtained, and it is argued that interactions are not weak in that regime. The RG and the perturbative analysis to all orders appear to indicate that quarks and gluons may be confined in the HT regime (and, in particular in the Early Universe), due to the infrared divergent magnetic gluon sector. Other possibilities are also discussed.

CONFINEMENT AND QUARK STRUCTURE OF LIGHT HADRONS

We present a quark confinement model (QCM) for the description of the low energy physics of light hadrons (meson and baryons). The model is based on two hypotheses. First, the quark confinement is realized as averaging over some vacuum gluon fields which are believed to provide the confinement of any color objects. Second, hadrons are treated as collective colorless excitations of quark-gluon interactions. The description of strong, electromagnetic and weak interactions of mesons and baryons at the low energy is given from a unique point of view.

Strong nucleon and ?-isobar form factors in the quark-confinement model

The nucleon and the Δ-isobar are investigated as three-quark systems in the quark-confinement model (QCM). This model is based on two hypotheses. First, quark confinement is accomplished through averaging over some vacuum gluon fields which are assumed to provide the confinement of any colour states. Second, hadrons are treated as collective colourless excitations of quark-gluon interactions.

Nonperturbative approach to chiral phase transition in chromodynamics

A nonperturbative approach aimed at the localization of the QCD chiral phase transition atT, π≠0 is presented. We identify this transition with the dynamical quark mass peculiarity which results from the selfconsistent solution of the Schwinger-Dyson equation for the quark propagator. The specific model of the effective quark-gluon interaction, based both on the peculier interpolation for the running coupling constant and on the nonperturbative gluon magnetic and electric masses is exploited. The numerical estimates of the phase diagram are presented and it is shown that phase peculiarities are determined not only by the ultraviolet properties of QCD but also by its infrared structure. The obtained results are discussed, compared with other approaches and a possible interpretation is given.

Polarized electron-proton scattering at highQ 2

We show that polarized electron-proton scattering is an excellent tool to measure helicity changing amplitudes. The asymmetry of polarized protons measured with longitudinally polarized electrons determines the ratio of Pauli (F 2) to Dirac (F 1) form factors. For the leading Fock state the Pauli form factor originates only from helicity changing quarkgluon interactions which are zero for zero quark masses. Therefore at high momentum transfer the ratioF 2/F 1 depends essentially on the up and down quark masses.

RGM study of the NN interaction in an extended quark model

The extended quark model study of the NN interaction, in which the ( q q ) excitations inherent in the quark-gluon interaction are explicitly incorporated into the model space, is completed with the inclusion of the ( q q ) ( q q ) excitations generated by RPA-type terms of the color Breit interaction. The new coupling kernels connecting the dominant (3q)-(3q) components of the NN system to the (3 q)−(3 q)( q q )( q q ) components lead to potentials with the characteristics of conventional σ and δ meson exchange potentials and furnish the additional medium-range attraction needed to bind the deuteron. The full model is subjected to a quantitative test through a solution of the RGM equations in a coupled channel formalism. With one improvement of the model, to yield an Nπ tensor force with OPEP strength and long-range characteristics, this model leads to a prediction of the low-energy NN scattering data and deuteron bound state characteristics which is in semiquantitative agreement with the experimental data and is free of parameter adjustments.

The NN interaction in a quark model with quark-antiquark excitations

The ( q q ) and ( q q ) 2 excitations generated by off-shell terms of a Breit quark-gluon interaction are explicitly incorporated into the quark model of the NN system. Coupled channel RGM solutions lead to a semiquantitative zero adjustable-parameter fit of the NN scattering data.

NN tensor and LS potentials in a quark model with quark-antiquark excitations

A more detailed study is made of the NN interaction in a quark model in which the (qq̄) excitations inherent in the quark-gluon interaction are explicitly incorporated into the model space. A unified treatment for all types of exchange terms of the (3q)-(3q) to (3q)-(3q)(qq̄) coupling kernels is used to calculate the space parts of the full coupling kernels and their Wigner transforms in complete analytic form. The present investigation focuses on the noncentral parts of the NN interaction. The tensor force gains almost its full strength from coupling kernels of Nπ and Nρ type. If the Nπ contribution is adjusted to fit the experimental pion-nucleon coupling constant the predicted strength of the full tensor force is in reasonable agreement with that of conventional OBEP's over the range in which the tensor force can act. The LS force gains contributions from both the pure (3q)-(3q) and the coupling kernels, but the dominant contributions (about 60–65% of the triplet-odd LS potential in the 0.7–1.0 fm range), come from the coupling kernels and particularly from the Nω and Nρ components. The triplet-odd LS potential derived from the full quark-exchange kernel is in remarkably good agreement with the OBEP LS potential over the significant range. Both the tensor and LS potentials are approximated surprisingly well over their full range by the simple (qq̄) exchange terms of our model. The 3P RGM phase shifts are calculated to show that both tensor and LS forces of our quark model are in good agreement with the experimental facts.

Repulsive Core Strength of the Nucleon–Nucleon Interaction

The contributions to the repulsive core strength of the N–N force from both the quark-pion and quark-gluon interactions in the Chiral (or Cloudy) Bag Model are calculated. These contributions are about several hundred MeV, a great part of which comes from the quark-pion interactions. It is found that when self-energies due to both gluon and pion exchange are considered, the contribution from the self-energy due to pion exchange is more important than that from the direct and exchange energy of quark-pion interactions.

Formula omitted] excitations in a quark model of the NN interaction

The ( q q ) and ( q q ) 2 excitations generated by off-shell terms of the Fermi-Breit quark-gluon interaction are incorporated into a quark model of the NN system leading to a semiquantitative fit of the NN scattering data.

Meson exchange potentials in an NN quark model with quark-antiquark excitations

In a recent quark-model study of the NN interaction, we explicitly incorporated the qq̄ excitations inherent in the quark-gluon interaction lagrangian into the model space, and examined the contributions to the NN potential of particular (3 q)( q q ) components of the single-nucleon wave function, with specific baryon and meson quantum numbers, through the Wigner transforms of the resultant coupling terms of the quark exchange kernels. The present investigation focuses on the long-range parts of the potentials. It is shown that these arise almost entirely through two of the 25 possible types of exchange terms, and that these two correspond to the simple exchange of a q q pair between the two nucleons and contribute only through coupling terms for which the (3q) components have the quantum numbers of a nucleon and the ( q q ) component the color-singlet character of a real pseudoscalar or vector meson. The potentials arising from these simple color-singlet q q exchanges have been evaluated in explicit analytic form. In their dependence on nucleon ( σ 1, · σ 2) and ( τ 1 · τ 2) factors and the relative importance and signs of spin-spin, spin-independent central, LS, and tensor terms they have all the characteristics of conventional OBEP's. They are in remarkably good agreement with conventional OBEP's in the r ⩾ 1.2 fm range and have the same qualitative radial features over an even wider range, the one exception being the steep short-range repulsive rise of the spin-independent central term of the ω-exchange potential. The corresponding q q exchange potential instead changes sign to become attractive at r < 0.8 fm.

On the parametrization of hadron-nucleus multiplicity distributions

A new parametrization for the hadron-nucleus multiplicity distributions is proposed and compared with pion-nucleus and proton-nucleus data with the same values of parameters. We believe that these parameters have their origin in basic quark-gluon interactions. We further illustrate this by evaluating the value of these parameters with the help of QCD.

The NN interaction in a quark model with quark-antiquark excitations

Quark-antiquark excitations inherent in the quark-gluon interaction have been incorporated into a quark model of the nucleon to study the effects of such excitations on the NN interaction within the framework of the resonating group method. The three-quark (3q) components of a single nucleon are augmented by ( 3 q)( q q ) excitations with the 24 possible spin, isospin, color combinations for the energetically lowest p-wave relative motion function. Quark exchange kernels are then calculated for the two-nucleon system described by these improved nucleon internal functions; and these exchange kernels are converted to phase-shift-equivalent effective NN potentials by the Wigner-transform WKB technique. The off-shell qq̄ pair-creation terms are derived from the one-gluon exchange diagrams in the Breit approximation in analogy with the derivation of the qq and qq̄ potentials. The parameters of the interaction are chosen to be consistent with the experimental Δ-N mass difference, the nucleon-vector-meson coupling constants, and the nucleon magnetic moments. Within these constraints, the predicted amplitudes of the ( 3 q)( q q ) components of the nucleon internal functions have been shown to be insensitive to the precise values of the model parameters. In particular, they pass the crucial test of being insensitive to very large changes in the magnitude of the confinement potential constant which is a necessary ingredient of the model. The qq̄ excitations lead to the following effects in the S-wave NN potentials: (i) The repulsive core heights of the simple 3q-3q model are greatly reduced but retain their strong essentially linear energy dependence, with numerical values very similar to those of the short-range phenomenological terms of the Paris potential. (ii) The effective potentials have acquired an attractive part in the 0.8-1.5 fm range. However, this attraction is too weak to bind the deuteron or fit the extreme low-energy S-wave phase shifts.