Nonperturbative corrections of quark and gluon condensates to QCD inspired quark potentials

We adopt the Lorentz gauge to derive the non-local two-gluon vacuum expectation value (VEV) with translational invariance. By means of the obtained non-local two-gluon VEV, the leading nonperturbative QCD corrections to one gluon exchange quark-quark, quark-antiquark and pair-excitation potentials are given by employing non-vanishing vacuum condensates of quarks and gluons to modify the free gluon propagator. The linear, cubic and Yukawa-type terms in quark-quark potential appear automatically. In the pair-excitation potential with , the linear, square and cubic terms arise from the nonzero quark and gluon condensates. Received: 22 June 1998 / Revised version: 10 August 1998 / Published online: 29 October 1998     

Static baryon properties in a relativistic quark model with centre-of-mass correction

The static properties such as magnetic moments, charge radii and axial vector coupling constant ratios of the quark core of baryons in the nucleon octet have been calculated in an independent-quark model based on the Dirac equation with equally mixed scalar-vector potential in linear form in the current quark mass limit. The results obtained with appropriate corrections due to centre-of-mass motion are in reasonable agreement with experimental data. The magnetic moments of the quark core of baryons in the charmed andb-flavoured sectors have also been calculated with this model and the overall predictions so obtained compare very well with other model predictions.     

A relativized quark model for radiative baryon transitions

In this paper we investigate the electromagnetic form factors of baryons and their resonances using the framework of a relativized constituent quark model. Beyond the usual single-quark transition ansatz, we incorporate relativistic corrections which are welldetermined by the intrinsic interaction and confinement forces between the quarks. Furthermore we separate off for the compound three-quark system the relativistic center-of-mass motion by an approximately Lorentz-invariant approach. In this way for the first time recoil effects could be explicity studied. Using the harmonic oscillator wavefunctions with the configuration mixing as derived in the Isgur-Karl model, after restoring gauge invariance our relativized interaction hamiltonian can be used to calculate the transversely and longitudinally polarized photon transition form factors of the baryons.     

Shielding of quark charge and baryon magnetic moment

By introducing the concept of effective charge of a quark in the baryon, we obtain a good fit to the magnetic moments within the constituent quark model.     

Single quark transition analysis of baryon resonance photocouplings

Resonance photocouplings of Barbour and Crawford are analysed within the context of the Melosh transformation. These photocouplings are the first set that favour a particular relative sign of the P-wave and F-wave pionic decays of the $\text{[}\text{56}\text{, L = 2]}$ multiplet. This conclusion rests primarily on the photocouplings of the P13⁺(1790) resonance. The favoured sign disagrees with that found in an analysis of πN → πΔ which relies on the contribution of the F15(1680) resonance.     

Three-flavor quark mass dependence of baryon spectra in holographic QCD

We introduce the strange quark mass to the Sakai–Sugimoto model of holographic QCD. We compute mass shifts in the spectra of three-flavor baryons at the leading order in perturbation in quark masses. Comparison with experimental data shows an agreement only qualitatively.     

Semi-leptonic charm baryon decays in the relativistic spectator quark model

We calculate the exclusive semi-leptonic charm baryon decays of the lowest lying charm baryon states into the ground state strangeness baryons using the covariant spectator quark model approach. We present results on rates,q ²- andE _l -spectra as well as on the angular decay distribution in the cascade decay \(\Omega _c \to \Omega ( \to \Xi \pi ,\Lambda {\rm K})\) .     

Some comments on quark masses and baryon magnetic moments

The suggestion made by Lipkin regarding the quark masses and magnetic moments of baryons is examined in the context of the variable pressure bag model. We find that the exact agreement obtained by Lipkin between theory and experiment in the case ofμ(Λ) cannot be considered accidental, contrary to the scepticism expressed by Minami.     

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.     

Nonperturbative contribution to the quark form factor within the instanton model

The nonperturbative effects in the quark form factor are considered in the Wilson loop formalism, within the framework of the instanton liquid model. For the integration path corresponding to this form factor, the explicit expression for the vacuum expectation value of the Wilson operator is found to the leading order. It is shown that the instantons produce the power-like corrections to the perturbative result, which are comparable in magnitude with the perturbative part at the scale of order of the inverse average instanton size. It is demonstrated that the instanton contributions to the quark form factor are exponentiated to high orders in the small instanton density parameter.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.38.-t Quantum chromodynamics - 11.15.Tk Other nonperturbative techniques     

Nonperturbative gluoproduction versus perturbative QCD fusion models for heavy quark production

Technicolor (hypercolor, etc.) theories of the strong and electro-weak interactions predict the existence of stable charged particles with masses greater than 10³ GeV/c². We point out that hydrogenic bound states of such stable particles and electrons may be detectable in current spectral data due to the mass dependence of hydrogenic energy levels.     

Photon production from quark gluon plasma at finite baryon density

The photon yield from a baryon-rich quark gluon plasma (QGP) at SPS energy has been estimated. In the QGP phase, rate of photon production is evaluated up to two-loop level. In the hadron phase, dominant contribution from π,ρ, ω mesons has been considered. The evolution of the plasma has been studied with appropriate equation of state in both QGP and hadron phase for a baryon-rich system. At SPS energy, the total photon yield is found to increase marginally in the presence of baryon density.     

Nonperturbative dynamics of noncommutative gauge theory

We present a nonperturbative lattice formulation of noncommutative Yang–Mills theories in arbitrary even dimension. We show that lattice regularization of a noncommutative field theory requires finite lattice volume which automatically provides both an ultraviolet and an infrared cutoff. We demonstrate explicitly Morita equivalence of commutative U(p) gauge theory with p·n_f flavours of fundamental matter fields on a lattice of size L with twisted boundary conditions and noncommutative U(1) gauge theory with n_f species of matter on a lattice of size p·L with single-valued fields. We discuss the relation with twisted large N reduced models and construct observables in noncommutative gauge theory with matter.     

Weak electric and magnetic form factors for semileptonic baryon decays in a relativistic quark model

Weak electric and magnetic form factors for semileptonic baryon decays are calculated in a relativistic quark model based on the Dirac equation with the independent-quark confining potential of the formV _q(r)=1/2(1+γ ⁰)(a ² r+V ₀). The values obtained for (g ₂/g ₁) are not very much different from the nonrelativistic results of Donoghue and Holstein. The values of (g ₁/f ₁) extracted from our model calculations of (f ₂/f ₁) in the Cabibbo limit compare well with the experimental values. The values of (f ₂/f ₁) for various semileptonic transitions are also estimated incorporating phenomenologically the effect of nonzerog ₂ in the ratio (g ₁/f ₁). It is found that the SU(3)-symmetry breaking does not generate significant departures in (f ₂/f ₁) values from the corresponding Cabibbo predictions.