[en] The pion electromagnetic form factor is calculated in the space- and time-like regions from -10 (GeV/c)² up to 10 (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

[en] In this work the problem related to pairs terms contribution of the electromagnetic current for the kaon in a light-front model is investigated. The electromagnetic form factor is extracted for both components of the electromagnetic current J⁺ and J^-. In the case of the J⁺ component of the electromagnetic current, the pair contribution is zero. For the J^- component of the electromagnetic current in the light-front formalism, the pair terms contribution is different from zero. The breaking of the rotational symmetry is explicitly showed due the pair terms in the light front formalism. In this model, the free parameters are the quark mass and the regulator mass. Numerical calculation is performed and the dependence of the parameters in this model is studied. (author)

[en] The pion electromagnetic form factor is calculated with a light-front quark model. The 'plus' and 'minus' components of the electromagnetic current are used to calculate the electromagnetic form factor in the Breit frame with two models for the q q-bar vertex. The light-front constituent quark model describes very well the hadronic wave functions for pseudo-scalar and vector particles. Symmetry problems arising in the light-front approach are solved by the pole dislocation method. The results are compared with new experimental data and with other quark models. (author)

[en] The pion electromagnetic form factor is calculated with a light-front quark model. The 'plus' and 'minus' components of the electromagnetic current are used to calculate the electromagnetic form factor in the the Breit frame with two models for the qq-bar vertex. The light-front constituent quark model describes very well the hadronic wave functions for pseudo-scalar and vector particles. Symmetry problems arising in the light-front approcah are solved by the pole dislocation method. The results are compared with new experimental data and with other quark models

[en] The electromagnetic form factors of a composite vector particle within the light-front formulation of the Mandelstam formula is investigated. In order to extract the form factors from the matrix elements of the plus component of the current in the Drell–Yan frame, where the momentum transfer is chosen such that $q^{+}=q^0+q^3=0$, one has in principle the freedom to choose between different linear combinations of matrix elements of the current operator. The different prescriptions to calculate the electromagnetic form factors, $G_0,G_1$ and $G_2$, i.e.; charge form factor, magnetic and quadrupole respectively. If the covariance is respected, all prescriptions give the same results; misfortune, is not the situation; the light-front approach produce different results, which depend of the prescriptions as utilized to extract the electromagnetic form factors in the case of the spin-1 particles. The main differences of the prescriptions appear because of the light-front matrix elements of the electromagnetic current are contaminated by the zero-modes contributions to the same with the plus component of the matrix elements of the electromagnetic current. However, the Inna Grach prescription is immune to the zero-modes contributions to the electromagnetic current, then the electromagnetic form factors extracted with that prescriptions do not have zero-modes contribution and give the same result compared with the instant form quantum field theory. Another's prescriptions with the light-front approach are contaminated by the zero-modes contributions to the matrix elements of the electromagnetic current with the plus component of the current. With some relations between the electromagnetic matrix elements of the electromagnetic current $J_{ji}^{+}$, as demonstrated analytical here, it was possible to calculate the electromagnetic form factors for spin-1 particles without zero-modes or non-valence contributions.

[en] The constituent quark ρ-meson electromagnetic from-factors are calculated, with covariant and null-plane approaches within the same model. The null-plane formalism produces the breakdown of the rotational symmetry for the one-body current operator, which is investigated by comparing the numerical results in both approaches. This allows to choose the appropriate null-plane prescription, among the several ones, to evaluate the ρ-meson form factors. (author)

[en] Full text: Considering phenomenological wave-functions in the light-cone, we obtain the neutral pion (π⁰) decay and the electromagnetic transition form-factor. The form-factor is obtained from the one-loop quark-diagrams projected on the null plane. By studying different models for the π⁰ → γ^* γ process, it is found out a strong model sensitivity of the π⁰ width. This result suggests that such observable should be used as an important constraint to the model wave function. The relativistic approach to the wave-function based only on constituents quarks is possible in the light-cone due to the suppression of pair creation process. This property arises from the particular choice of the light-cone coordinates. Also the center of mass coordinate is easily separated. In specific processes involving a bound-state, the internal loop-momentum is integrated first in the light-cone energy, then the wave-function of the bound-state appears naturally. This procedure is the essence of the diagrammatic approach that was applied to obtain the weak decay constant and electromagnetic form-factor of the charged pion. In this reference, it was used one-loop dia- grams, the triangle diagram for the form-factor and the bubble diagram which expresses the Partial Conservation of the Axial Current (PCAC). The integration over the light-cone energy in the triangle diagram is per- formed and the asymptotic wave-function of the bound quark-antiquark pair is replaced by phenomenological pion wave-functions. We use three distinct model wave-functions: the Gaussian; the hydrogen-atom; and the wave-function model. This last model has the two characteristics that one believes belongs to Quantum Chromodynamics, i.e., confinement and short distance one gluon exchange. The Gaussian model has only the property of confinement and the Hydrogen model mimics the one gluon exchange at short distances. It is observed that the neutral pion radius presents a correlation with the quark mass. The width is also strongly dependent on the mass even for f_π fixed. The consistence of the theoretical width and the experimental value is reached together with the radii. The asymptotic value of the transition form-factor is approximately stable against the mass variation for fixed f_π In summary, we explore numerically the consequences of three different wave-functions for the neutral pion radius, decay width and asymptotic value of the transition form-factor. The decay width is an important source of information of the wave-function. It is independent to some extend of f_π which determines it in the limit of constant pion vertex.(Author)

[en] Properties of ρ-meson in symmetric nuclear matter are investigated in a light-front constituent quark model (LFCQM), using the in-medium inputs calculated by the quark–meson coupling (QMC) model. The LFCQM used in this study was already applied for the studies of the electromagnetic properties of ρ-meson in vacuum, namely, the charge $G_0$, magnetic $G_1$, and quadrupole $G_2$ form factors, electromagnetic charge radius, and electromagnetic decay constant. Using the two different density dependence of the regulator mass in medium, we predict that the charge radius, and quadrupole moment are enhanced as increasing the nuclear matter density, while the magnetic moment is slightly quenched. Furthermore, we predict the value $Q_{\mathrm{zero}}^2$, which crosses zero of the charge form factor, $G_0(Q_{\mathrm{zero}}^2)=0$ ($Q^2=-q^2>0$ with q being the four-momentum transfer), decreases as increasing the nuclear matter density by the two different density dependence of the regulator mass. On the other hand, for the electromagnetic decay constant of the ρ-meson, the two different density dependence of the regulator mass predict the opposite density dependence. Namely, as increasing the nuclear matter density, the naive treatment with the density independent regulator mass as in the vacuum, predicts the increase of the decay constant, while the other that assumes the same density dependence of the regulator mass as that of the in-medium constituent quark mass, predicts the decrease of the decay constant. Thus, although the other physical quantities are predicted to have similar density dependence by the two different density dependence of the regulator mass applied, the density dependence of the ρ-meson electromagnetic decay constant is predicted to have opposite density dependence, and the facts suggest that the in-medium ρ-meson decay constant needs to be investigated further in the future.

[en] The process π⁰ → γ ^* γ is studied using different model wave-function in the null-plane. The π ⁰-decay and the electromagnetic transition form-factor is calculated. We then predict the electromagnetic radius of the pion and compare with the experiments value. Is found that the width has a strong model dependence and should be used as an important constraint to the model wave function. (author)

[en] The electromagnetic current of spin-1 composite particles does not transform properly under rotations if only the valence contribution is considered in the light-front model. In particular, the plus component of the current, evaluated only for the valence component of the wave function, in the Drell-Yan frame violates rotational symmetry, which does not allow a unique calculation of the electromagnetic form-factors. The prescription suggested by Grach and Kondratyuk [Sov. J. Nucl. Phys. 38, 198 (1984)] to extract the form factors from the plus component of the current, eliminates contributions from pair diagrams or zero modes, which if not evaluated properly cause the violation of the rotational symmetry. We address this problem in an analytical and covariant model of a spin-1 composite particle. (author)