Nucleon and pion electromagnetic form factors in a light-front constituent quark model

Abstract

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...

Electroexcitation of nucleon resonances from CLAS data on single pion electroproduction

“…48 29.6 ± 0.8 ± 2.9 4.2 ± 2.5 ± 2.6 32.6 ± 0.9 ± 2.8 14.1 ± 2.4 ± 2.4 31.1 ± 0.9 ± 3.2 9.1 ± 2.5 ± 5.5 4. 16 19.3 ± 2.0 ± 4.0 10.8 ± 2.8 ± 4.7 23.1 ± 2.2 ± 4.9 17.5 ± 2.6 ± 5.6 21.2 ± 2.1 ± 4.9 14.1 ± 2.7 ± 6.1 4 90.6 ± 1.7 ± 1.4 −9.5 ± 1.9 ± 0.9 92.9 ± 1.6 ± 2.2 −15.9 ± 2.0 ± 2.2 91.8 ± 1.7 ± 2.1 −12.7 ± 2.0 ± 3.6 0.5 90.5 ± 1.9 ± 1.6 −10.8 ± 2.2 ± 1.2 91.7 ± 2.0 ± 2.7 −16.7 ± 2.4 ± 2.4 91.1 ± 2.0 ± 2.2 −13.8 ± 2.3 ± 3.5 0. 65 90.0 ± 1.7 ± 1.8 −12.9 ± 1.8 ± 1.0 91.6 ± 1.8 ± 3.3 −14.4 ± 1.9 ± 2.3 90.8 ± 1.8 ± 2.7 −13.6 ± 1.9 ± 1.8 0.…”

“…A consistent way to perform the relativistic treatment of the γ * N → N (N * ) transitions is to consider them in the light-front (LF) dynamics [9][10][11]. The relevant approaches were developed and used to describe the nucleon and its excited states [12][13][14][15][16][17][18][19]. However, much more effort is required to obtain a better understanding of what are the N and N * LF wave functions and what is their connection to the interquark forces and to the QCD confining mechanism.…”

Linking generalized parton distributions to constituent quark models

“…The SU(6) breaking effects which produce deviations from the value k p /k n = −1 come mainly from the Melosh rotations, with a small additional contribution from the quark wave functions in the case of the GBE model. The t dependence of F 1 and F 2 is rather smooth and in both models it does not reproduce the experimental results: a clear limitation due to the assumed pointlike structure of the quarks which can be solved by the introduction of specific quark form factors [33,41].…”

Baryons as relativistic three-quark bound states

“…Nucleon form factors have been calculated in light-front constituent-quark models [517][518][519][520][521][522], also including dressed quark form factors which effectively add higher Fock components to the wave functions [523][524][525][526]. Light-front quark model calculations are also available for nucleon resonances [21,527,528].…”