Hadronic Decays of N and \Delta Resonances in a Chiral Quark Model

\pi and \eta decay modes of light baryon resonances are investigated within a chiral quark model whose hyperfine interaction is based on Goldstone-boson exchange. For the decay mechanism a modified version of the 3P0 model is employed. Our primary aim is to provide a further test of the recently proposed Goldstone-boson-exchange constituent quark model. We compare the predictions for \pi and \eta decay widths with experiment and also with results from a traditional one-gluon-exchange constituent quark model. The differences between nonrelativistic and semirelativistic versions of the constituent quark models are outlined. We also discuss the sensitivity of the results on the parametrization of the meson wave function entering the 3P0 model.Comment: 17 pages, 6 eps figure

Chiral symmetry patterns of excited mesons with the Coulomb-like linear confinement

The spectrum of $\bar q q$ mesons in a model where the only interaction is a linear Coulomb-like instantaneous confining potential is studied. The single-quark Green function as well as the dynamical chiral symmetry breaking are obtained from the Schwinger-Dyson (gap) equation. Given the dressed quark propagator, a complete spectrum of "usual" mesons is obtained from the Bethe-Salpeter equation. The spectrum exhibits restoration of chiral and $U(1)_A$ symmetries at large spins and/or radial excitations. This property is demonstrated both analytically and numerically. At large spins and/or radial excitations higher degree of degeneracy is observed, namely all states with the given spin fall into reducible representations $[(0,1/2) \oplus (1/2,0)] \times [(0,1/2) \oplus (1/2,0)]$ that combine all possible chiral multiplets with the given $J$ and $n$. The structure of the meson wave functions as well as the form of the angular and radial Regge trajectories are investigated.Comment: 1. Order of references has been changed and one reference has been added; 2. A short discussion of nonrelativistic and semirelativistic quark models has been added in the conclusion part on referee's request. To appear in Phys. Rev.

Extended Goldstone-Boson-Exchange Constituent Quark Model

We present an extension of the Goldstone-boson-exchange constituent quark model including additional interactions beyond the ones used hitherto. For the hyperfine interaction between the constituent quarks we assume pseudoscalar, vector, and scalar meson exchanges and consider all relevant force components produced by these types of exchanges. The resulting model, which corresponds to a relativistic Poincare-invariant Hamiltonian (or equivalently mass operator), provides a unified framework for a covariant description of all light and strange baryons. The ground states and resonances up to an excitation energy of about 2 GeV are reproduced in fair agreement with phenomenology, with the exception of the first excitations above the Lambda and Xi ground states.Comment: 11 pages, 4 figures, 3 tables; substantial revisions, additional author, bibliography extended and update

Covariant baryon charge radii and magnetic moments in a chiral constituent quark model

The charge radii and magnetic moments of all the light and strange baryons are investigated within the framework of a constituent quark model based on Goldstone-boson-exchange dynamics. Following the point-form approach to relativistic quantum mechanics, the calculations are performed in a manifestly covariant manner. Relativistic (boost) effects have a sizeable influence on the results. The direct predictions of the constituent quark model are found to fall remarkably close to the available experimental data.Comment: 6 pages, 4 table

Electromagnetic nucleon form factors in instant and point form

We present a study of the electromagnetic structure of the nucleons with constituent quark models in the framework of relativistic quantum mechanics. In particular, we address the construction of spectator-model currents in the instant and point forms. Corresponding results for the elastic nucleon electromagnetic form factors as well as charge radii and magnetic moments are presented. We also compare results obtained by different realistic nucleon wave functions stemming from alternative constituent quark models. Finally, we discuss the theoretical uncertainties that reside in the construction of spectator-model transition operators.Comment: 13 pages, 9 figures, updated and extended version for publicatio

Nucleon electroweak form factors in a meson-cloud model

The meson-cloud model of the nucleon consisting of a system of three valence quarks surrounded by a meson cloud is applied to study the electroweak structure of the proton and neutron. The electroweak nucleon form factors are calculated within a light-front approach, by obtaining an overall good description of the experimental data. Charge densities as a function of the transverse distance with respect to the direction of the three-momentum transfer are also discussed.Comment: Prepared for Proceedings of NSTAR2007, Workshop on the physics of excited nucleons, Bonn (Germany), 5-8 September 200

Covariant calculation of mesonic baryon decays

We present covariant predictions for pi and eta decay modes of N and Delta resonances from relativistic constituent-quark models based on one-gluon-exchange and Goldstone-boson-exchange dynamics. The results are calculated within the point-form approach to Poincare-invariant relativistic quantum mechanics applying a spectator-model decay operator. The direct predictions of the constituent-quark models for covariant pi and eta decay widths show a behaviour completely different from previous ones calculated in nonrelativistic or so-called semirelativistic approaches. It is found that the present theoretical results agree with experiment only in a few cases but otherwise always remain smaller than the experimental data (as compiled by the Particle Data Group). Possible reasons for this behaviour are discussed with regard to the quality of both the quark-model wave functions and the mesonic decay operator.Comment: 10 pages, 2 figures, accepted for publication in Phys. Rev.

Form factors in RQM approaches: constraints from space-time translations

Different relativistic quantum mechanics approaches have recently been used to calculate properties of various systems, form factors in particular. It is known that predictions, which most often rely on a single-particle current approximation, can lead to predictions with a very large range. It was shown that accounting for constraints related to space-time translations could considerably reduce this range. It is shown here that predictions can be made identical for a large range of cases. These ones include the following approaches: instant form, front form, and "point-form" in arbitrary momentum configurations and a dispersion-relation approach which can be considered as the approach which the other ones should converge to. This important result supposes both an implementation of the above constraints and an appropriate single-particle-like current. The change of variables that allows one to establish the equivalence of the approaches is given. Some points are illustrated with numerical results for the ground state of a system consisting of scalar particles.Comment: 37 pages, 7 figures; further comments in ps 16 and 19; further references; modified presentation of some formulas; corrected misprint

A unified meson-baryon potential

We study the spectra of mesons and baryons, composed of light quarks, in the framework of a semirelativistic potential model including instanton induced forces. We show how a simple modification of the instanton interaction in the baryon sector allows a good description of the meson and the baryon spectra using an interaction characterized by a unique set of parameters.Comment: 7 figure

Electromagnetic Form Factors of the Nucleon in a Relativistic Quark Pair Creation Model

We study the effects of the | qqq q\bar{q} > component of the hadronic wave function on the description of the electromagnetic structure of the nucleon. Starting with a qqq baryonic wave function which describes the baryonic and mesonic low energy spectrum, the extra q\bar{q} pair is generated through a relativistic version of the 3P_0 model. It is shown that this model leads to a renormalization of the quark mass that allows one to construct a conserved electromagnetic current. We conclude that these dynamical relativistic corrections play an important role in reproducing the Q2 dependence of the electromagnetic form factors at low Q^2.Comment: 15 pages, 3 figures. Minor change