Nucleon and pion electromagnetic form factors are evaluated in the spacelike region within a light-front constituent quark model, where eigenfunctions of a mass operator, reproducing a large set of hadron energy levels, are adopted and quark form factors are considered in the one-body current. The hadron form factors are sharply affected by the high momentum tail generated in the wave function by the one-gluon-exchange interaction. Useful information on the electromagnetic structure of light constituent quarks can be obtained from the comparison with nucleon and pion experimental data.The measurement of the electromagnetic (e.m.) form factors of hadrons represents a valuable tool for investigating in detail their internal structure. This fact has motivated a great deal of experimental and theoretical work, that will increase with the advent of new accelerator facilities, e.g. CEBAF, yielding unique information on the transition region from the non perturbative to the perturbative regime of QCD [1,2]. Though the fundamental theory of the strong interaction, QCD, should be applied for describing hadron structure, the practical difficulties to be faced in the nonperturbative regime have motivated the development of effective theories, e.g. constituent quark (CQ) models, that in turn could provide useful hints to model approximations to the "true" field theory [3]. Aim of this letter is to apply our approach [4,5], based on a relativistic CQ model, to the evaluation of the nucleon e.m. form factors in the spacelike region, keeping safe the good description already obtained for the pion form factor. Our model represents an extension of the one proposed in Refs. [6,7], where a relativistic treatement of light CQ's is achieved by adopting the light-front formalism [8] and gaussian wave functions are assumed for describing the pointlike CQ's inside the nucleon (see also [9]). In particular we have considered: i) hadron wave functions which are eigenvectors of a light-front mass operator, constructed from the effective qq and qq-interaction of Refs. [10,11], that reproduces a huge amount of energy levels; ii) the configuration mixing, due to the one-gluon-exchange (OGE) part of the effective interaction, leading to high momentum components and SU(6) breaking terms in the hadron wave function; iii) Dirac and Pauli form factors for the CQ's, as suggested by their quasi-particle nature (cf. [12]), summarizing the underlying degrees of freedom. The comparison of our calculations with the experimental data on nucleon and pion form factors will phenomenologically constrain the e.m. structure of the light CQ's.As known (cf.[8]), the light-front wave functions of hadrons are eigenvectors of a mass operator, e.g. M = M 0 + V, and of the non-interacting angular momentum operators j 2 and j n , where the vectorn = (0, 0, 1) defines the spin quantization axis. The operator M 0 is the free mass and the interaction term V is a Poincaré invariant. In this letter we briefly present the formalism for the nucleon only, since the r...

No abstract

No abstract

The effects of both kinematical and dynamical SU (6) breaking on the nucleon elastic form factors, G N E (Q 2 ) and G N M (Q 2 ), are investigated within the constituent quark model formulated on the light-front. The investigation is focused on G n E (Q 2 ) and the ratio G p M (Q 2 )/G n M (Q 2 ), which within the SU (6) symmetry are given by G n E (Q 2 ) = 0 andIt is shown that the kinematical SU (6) breaking caused by the Melosh rotations of the quark spins as well as the dynamical SU (6) breaking due to the mixed-symmetry component generated in the nucleon wave function by the spin-dependent terms of the quark-quark interaction, can affect bothis found to be qualitatively consistent with existing data, though only ≃ 65% of the experimental neutron charge radius can be explained without invoking effects from possible non-vanishing sizes of the constituent quarks and/or many-body currents. At the same time the predictions for the magnetic ratio G p M (Q 2 )/G n M (Q 2 ) turn out to be inconsistent with the data. It is however shown that the calculations of G N M (Q 2 ) based on different components of the one-body electromagnetic current lead to quite different results. In particular, the calculations based on the plus component are found to be plagued by spurious effects related to the loss of the rotational covariance in the light-front formalism. These unwanted effects can be avoided by considering the transverse y-component of the current. In this way our light-front predictions are found to be consistent with the data on both G n E (Q 2 ) and GFinally, it is shown that a suppression of the ratio G p E (Q 2 )/G p M (Q 2 ) with respect to the dipole-fit prediction can be expected in the constituent quark model provided the relativistic effects generated by the Melosh rotations of the constituent spins are taken into account.

A parameter-free evaluation of the $N - P_{11}(1440)$ electromagnetic transition form factors is performed within a light-front constituent quark model, using for the first time the eigenfunctions of a mass operator which generates a large amount of configuration mixing in baryon wave functions. A one-body electromagnetic current, which includes the phenomenological constituent quark form factors already determined from an analysis of pion and nucleon experimental data, is adopted. For $Q^2$ up to few $(GeV/c)^2$, at variance with the enhancement found in the elastic channel, the effect of configuration mixing results in a significant suppression of the calculated helicity amplitudes with respect to both relativistic and non-relativistic calculations, based on a simple gaussian-like ansatz for the baryon wave functions.Comment: revised version to appear in Phys. Lett.