J. P. B. C. de Melo

Pair term in the Electromagnetic Current within the Front-Form Dynamics: Spin-0 Case

Abstract The frame and scale dependence of the pair-term contribution to the electromagnetic form factor of a spin-zero composite system of two-fermions is studied within the Light Front. The form factor is evaluated from the plus-component of the current in the Breit frame, using for the first time a nonconstant, symmetric ansatz for the Bethe–Salpeter amplitude. The frame dependence is analyzed by allowing a nonvanishing plus component of the momentum transfer, while the dynamical scale is set by the masses of the constituents and by mass and size of the composite system. Axa0transverse momentum distribution, associated with the Bethe–Salpeter amplitude, is introduced which allows to define strongly and weakly relativistic systems. In particular, for strongly relativistic systems, the pair term vanishes for the Drell–Yan condition, while is dominant for momentum transfer along the light-front direction. For a weakly relativistic system, fitted to the deuteron scale, the pair term is negligible up to momentum transfers of 1 (GeV/c)2. Axa0comparison with results obtained within the Front-Form Hamiltonian dynamics with a fixed number of constituents is also presented.

Spacelike and timelike pion electromagnetic form factor and Fock state components within the light-front dynamics

The simultaneous investigation of the pion electromagnetic form factor in the space- and timelike regions within a light-front model allows one to address the issue of nonvalence components of the pion and photon wave functions. Our relativistic approach is based on a microscopic vector-meson-dominance model for the dressed vertex where a photon decays in a quark-antiquark pair, and on a simple parametrization for the emission or absorption of a pion by a quark. The results show an excellent agreement in the space like region up to -10 (GeV/c){sup 2}, while in timelike region the model produces reasonable results up to 10 (GeV/c){sup 2}.

Electromagnetic form factor of the pion in the space- and time-like regions within the front-form dynamics

Abstract The pion electromagnetic form factor is calculated in the space- and time-like regions from −10xa0(GeV/ c ) 2 up to 10xa0(GeV/ c ) 2 , within a front-form model. The dressed photon vertex where a photon decays in a quark–antiquark pair is depicted generalizing the vector meson dominance ansatz, by means of the vector meson vertex functions. An important feature of our model is the description of the on-mass-shell vertex functions in the valence sector, for the pion and the vector mesons, through the front-form wave functions obtained within a realistic quark model. The theoretical results show an excellent agreement with the data in the space-like region, while in the time-like region the description is quite encouraging.

Exciting flavored bound states

We study ground and radial excitations of flavor singlet and flavored pseudoscalar mesons within the framework of the rainbow-ladder truncation using an infrared massive and finite interaction in agreement with recent results for the gluon-dressing function from lattice QCD and Dyson-Schwinger equations. Whereas the ground-state masses and decay constants of the light mesons as well as charmonia are well described, we confirm previous observations that this truncation is inadequate to provide realistic predictions for the spectrum of excited and exotic states. Moreover, we find a complex conjugate pair of eigenvalues for the excited

Modeling electromagnetic form factors of light and heavy pseudoscalar mesons

D_{(s)}

Relativistic dynamics of Qqq systems

mesons, which indicates a non-Hermiticity of the interaction kernel in the case of heavy-light systems and the present truncation. Nevertheless, limiting ourselves to the leading contributions of the Bethe-Salpeter amplitudes, we find a reasonable description of the charmed ground states and their respective decay constants.

Electromagnetic Hadron Form Factors and Higher Fock Components

The electromagnetic form factors of light and heavy pseudoscalar mesons are calculated within two covariant constituent-quark models, viz., a light-front and a dispersion relation approach. We investigate the details and physical origins of the model dependence of various hadronic observables: the weak decay constant, the charge radius and the elastic electromagnetic form factor.

Kaon and Pion Electromagnetic Form Factor Ratios in the Light-Front

Ctr Aerosp Technol, Inst Tecnol Aeronaut, Dept Fis, BR-12228900 Sao Jose Dos Campos, Brazil

The Pion Electromagnetic Form Factor in a QCD-Inspired Model

Investigation of the spacelike and timelike electromagnetic form factors of hadrons, within a relativistic microscopical model characterized by a small set of hypothesis, could shed light on the components of hadron states beyond the valence one. Our relativistic approach has been successfully applied first to the pion and then the extension to the nucleon has been undertaken. The pion case is shortly reviewed as an illustrative example for introducing the main ingredients of our approach, and preliminary results for the nucleon in the spacelike range − 10 ( GeV / c ) 2 ≤ q 2 ≤ 0 are evaluated.

Light-Front model of transition form-factors in heavy meson decay

We have applied the light-front formalism to calculate the electromagnetic form factors for the pion and the kaon from two models at low and high energies in order to explore the differences between such models. We have also compared the results for the ratio F K ( Q 2 ) / F π ( Q 2 ) with the experimental data up to 10 [GeV/c]2 and we have observed that the theoretical results are in good concordance for low energies, but they are very different at higher energy scales.