Generalized Parton Distributions in the Light-Front Constituent Quark Model

The Generalized Parton Distributions (GPDs) of the nucleon are analyzed within the relativistic constituent quark model formulated on the light-front. It is shown that the matrix elements of the plus component of the one-body vector current are plagued by spurious effects related to the dependence on the hyperplane where the nucleon wave function is defined in terms of its constituents. The physical GPDs can be extracted only from the matrix elements of a transverse component of the one-body current. The loss of the polinomiality property is then related to the neglect of the pair creation process for non-vanishing values of the skewness. The need of implementing effective many-body currents corresponding to the Z-graph is stressed and a possible approach to achieve such a goal is proposed.Comment: latex 8 pages; one reference corrected; to appear in the Proc. of the Int'l Conf. on "The Physics of Excited Nucleons" (NSTAR '04), Grenoble (France), March 24-027, 200

Relativistic quark models

The application of relativistic constituent quark models to the evaluation of the electromagnetic properties of the nucleon and its resonances is addressed. The role of the pair creation process in the Feynmann triangle diagram is discussed and the importance both of choosing the light-front formalism and of using a Breit frame where the plus component of the four-momentum transfer is vanishing, is stressed. The nucleon elastic form factors are calculated free of spurious effects related to the loss of rotational covariance. The effect of a finite constituent size is considered and the parameters describing the constituent form factors are fixed using only the nucleon data up to Q**2 ~ 1 (GeV/c)**2; a constituent charge radius of ~ 0.45 fm is obtained in this way. Above Q**2 ~ 1 (GeV/c)**2 our parameter-free predictions are in good agreement with the experimental data, showing that soft physics may be dominant up to Q**2 ~ 10 (GeV/c)**2. Our new results for the longitudinal and transverse helicity amplitudes of the N - P_{11}(1440) transition are presented.Comment: to appear in the Procedings of the Int'l Conf. on "The Physics of Excited Nucleons" (N* 2001), Mainz, March 200