A comparison between relativistic and semi-relativistic treatment in the diquark–quark model (original) (raw)

Nucleon properties in the covariant quark-diquark model

The European Physical Journal A, 2000

In the covariant quark-diquark model the effective Bethe-Salpeter (BS) equations for the nucleon and the ∆ are solved including scalar and axialvector diquark correlations. Their quark substructure is effectively taken into account in both, the interaction kernel of the BS equations and the currents employed to calculate nucleon observables. Electromagnetic current conservation is maintained. The electric form factors of proton and neutron match the data. Their magnetic moments improve considerably by including axialvector diquarks and photon induced scalar-axialvector transitions. The isoscalar magnetic moment can be reproduced, the isovector contribution is about 15% too small. The ratio µ GE/GM and the axial and strong couplings gA, gπNN , provide an upper bound on the relative importance of axialvector diquarks confirming that scalar diquarks nevertheless describe the dominant 2-quark correlations inside nucleons. PACS. 11.10.St (Bound states; Bethe-Salpeter equations) -12.39.Ki (Relativistic quark model) -12.40.Yx (Hadron mass models and calculations) -13.40.Em (Electric and magnetic moments) -13.40.Gp (Electromagnetic form factors) -13.75.Gx (Pion-baryon interactions) -14.20.Dh (Protons and neutrons)

Nucleon form factors in a covariant diquark-quark model

Nuclear Physics A, 1997

In a model where constituent quarks and diquarks interact through quark exchange the Bethe-Salpeter equation in ladder approximation for the nucleon is solved. Quark and diquark confinement is effectively parametrized by choosing appropriately modified propagators. The coupling to external currents is implemented via nontrivial vertex functions for quarks and diquarks to ensure gauge invariance at the constituent level. Nucleon matrix elements are evaluated in a generalised impulse approximation, and electromagnetic, pionic and axial form factors are calculated.

Current conservation in the covariant quark-diquark model of the nucleon

2000

The description of baryons as fully relativistic bound states of quark and glue reduces to an effective Bethe-Salpeter equation with quark-exchange interaction when irreducible 3-quark interactions are neglected and separable 2-quark (diquark) correlations are assumed. This covariant quark-diquark model of baryons is studied with the inclusion of the quark substructure of the diquark correlations. In order to maintain electromagnetic current conservation it is then necessary to go beyond the impulse approximation. A conserved current is obtained by including the coupling of the photon to the exchanged quark and direct "seagull" couplings to the diquark structure. Adopting a simple dynamical model of constituent quarks and exploring various parametrisations of scalar diquark correlations, the nucleon Bethe-Salpeter equation is solved and the proton and neutron electromagnetic form factors are calculated numerically. The resulting magnetic moments are still about 50% too small, the improvements necessary to remedy this are discussed. The results obtained in this framework provide an excellent description of the electric form factors (and charge radii) of the proton, up to a photon momentum transfer of 3.5GeV 2 , and the neutron.

Nucleon electromagnetic and axial form factors in point-form relativistic quantum mechanics

2002

Results for the proton and neutron electric and magnetic form factors as well as the nucleon axial form factor are presented for constituent quark models, based on either one-gluon-exchange and Goldstone-boson-exchange dynamics. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. The only input to the calculations is the nucleon wave function of the corresponding constituent quark model. A comparison is given to results of the instanton-induced constituent quark model treated with the Bethe-Salpeter equation.

A relativistic study of the nucleon form factors

European Physical Journal A

We perform a calculation of the relativistic corrections to the electromagnetic elastic form factors of the nucleon obtained with various Constituent Quark Models. With respect to the non relativistic calculations a substantial improvement is obtained up to Q 2 ≃ 2(GeV /c) 2 .

Quark confinement potential and the quark-diquark model for nucleons

Annals of Physics, 1977

The quark contining two-step potential used earlier for mesons is now applied to nucleons. By using the Coulomb part of this potential for the quark-quark pair we investigate the predictions of the quark-diquark (QDQ) model for the nucleons. It is found that this kind of composite model, consistent for both the proton and the neutron, not only accounts for the recent data on the nucleon form factors but also provides a deeper understanding of the already available results in such a model. Further, it is noticed that the present QDQ model, accounting satisfactorily for various properties of the nucleon, offers an alternative way to investigate the ground state of deuteron and the nature of nucleon-nucleon force in terms of the pre-Yukawa picture of chemical-type forces. The results obtained in this alternative approach are found to be in agreement with those obtained by other conventional calculations.

Nucleon electromagnetic form factors in a relativistic quark-pair-creation model

Nuclear Physics A, 2001

We study the effects of the |qqq qq component of the hadronic wave function on the description of the electromagnetic structure of the nucleon. Starting with a 3q baryonic wave function which describes the baryonic and mesonic low energy spectrum, the extra qq pair is generated through a relativistic version of the 3 P 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 Q 2 dependence of the electromagnetic form factors at low Q 2 .

Electromagnetic form factors of nucleons in a light-cone diquark model

Physical Review C, 2002

We investigate the electromagnetic form factors of nucleons within a simple relativistic quark spectator-diquark model using the light-cone formalism. Melosh rotations are applied to both quark and vector diquark. It is shown that the difference between vector and scalar spectator diquarks reproduces the right electric form factor of neutrons, and both the form factors G E Q 2 and G M Q 2 of the proton and neutron agree with experimental data well up to Q 2 = 2 GeV 2 in this simple model.

Relativistic study of nucleon electroweak properties in a constituent-quark model

The European Physical Journal A - Hadrons and Nuclei, 2003

We discuss the predictions of the Goldstone-boson-exchange constituent-quark model for the proton and neutron electric and magnetic form factors as well as the nucleon axial and induced pseudoscalar form factors. The results are calculated in a covariant framework using the point-form approach to relativistic quantum mechanics. The only input into the calculations is the nucleon wave function from the constituent-quark model. A remarkably consistent picture, with all aspects of the electroweak nucleon structure close to existing experimental data, is obtained.