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

Heavy-Quark Mass and Heavy-Meson Decay Constants from QCD Sum Rules

We present a sum-rule extraction of heavy-meson decay constants from the two-point correlator of heavy-light pseudoscalar currents. Our main concern is to control the uncertainties of the decay constants, induced by both input QCD parameters and limited accuracy of the sum-rule method. Gaining this control is possible by applying our novel procedure for the extraction of hadron observables utilizing Borel-parameter-depending dual thresholds. For the charmed mesons, we obtain f_D = 206.2 \pm 7.3 (OPE) \pm 5.1 (syst) MeV and f_{D_s} = 245.3 \pm 15.7 (OPE) \pm 4.5 (syst) MeV. For the beauty mesons, the decay constants prove to be extremely sensitive to the exact value of the b-quark MS-bar mass m_b(m_b). Matching our sum-rule prediction for f_B to the lattice findings, the very accurate b-mass value m_b(m_b) = 4.245 \pm 0.025 GeV is found, which yields f_B = 193.4 \pm 12.3 (OPE) \pm 4.3 (syst) MeV and f_{B_s} = 232.5 \pm 18.6 (OPE) \pm 2.4 (syst) MeV.Comment: talk presented at "Quark Confinement and the Hadron Spectrum IX - QCHS IX" (30 August - 3 September 2010, Madrid, Spain), to appear in the proceedings; version 2: minor, purely cosmetic changes, 1 reference adde