arXiv subjects
Publications and source records attributed to T. Frederico.
The purpose of this work is to show that within a three-body description, the complete fusion process and the target excitation by the projectile can be taken into account by introducing a three-body optical potential in the formalism. We give a schematic description of such a potential and points to ways of testing the validity of the CDCC.
An approach for describing the electromagnetic form factors of hadrons, in the space- and time-like regions, within a constituent quark model on the light front is shortly illustrated and calculations for the pion case are reported. Three main ingredients enter our approach: i) the dressed photon vertex where a photon decays in a quark-antiquark pair, ii) the on-shell quark-hadron vertex functions in the valence sector, and iii) the non-valence component of the hadron state.
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.
The violation of the rotational symmetry constraint of the matrix elements of the plus component of the vector current, in the Drell-Yan frame, is studied using an analytical and covariant model of spin-1 composite particle. The contributions from pair terms or zero modes, if missed cause the violation of the rotational symmetry. We reanalyze the prescription by Grach and Kondratyuk to extract form factors which can eliminate theses contributions in particular models.
We show that a linear scaling between the weak decay constants of pseudoscalar and the vector mesons masses is supported by the available experimental data. The decay constant scale as $f_m/f_{pi}=M_V/M_ρ$ (f_m is decay constant and M_V vector meson ground state mass). This simple form is justified within a renormalized light-front QCD-inpired model for quark-antiquark bound states.
We investigate quark matter at finite temperature and finite chemical potential as an example for a relativistic many-particle quantum system. Special relativity is realized through the front form that allows for a Hamiltonian formulation of a statistical operator. Utilizing our previous results we generalize the present formulation of a relativistic thermal field theory to include a finite chemical potential. As an application we use the Nambu-Jona-Lasinio model to investigate the gap equation and chiral restoration.
The three-body recombination coefficient of a trapped ultracold atomic system, together with the corresponding two-body scattering length $a$, allow us to predict the energy $E_3$ of the shallow trimer bound state, using a universal scaling function. The production of dimers in Bose-Einstein condensates, from three-body recombination processes, in the regime of short magnetic pulses near a Feshbach resonance, is also studied in line with the experimental observation.
The Bethe-Salpeter equation for ground state of two fermions exchanging a gauge boson presents divergences. We used a prescription that allowed an apropriate prescription of the singularity in the boson propagator.
The experimental three-atom recombination coefficients of the atomic states $^{23}$Na$|F=1,m_F=-1>$, $^{87}$Rb$|F=1,m_F=-1>$ and $^{85}$Rb$|F=2,m_F=-2>$, together with the corresponding two-body scattering lengths, allow predictions of the trimer bound state energies for such systems in a trap. The recombination parameter is given as a function of the weakly bound trimer energies, which are in the interval $ 1 $, for a particular trap, is shown to be relatively small.
The projection of the four-dimensional two-body Bethe-Salpeter equation at the light-front hypersurface is discussed. Elimination of the relative time is performed using a quasi-potential expansion, which is shown to be equivalent to an infinite set of coupled equations for the Green's functions of the different light-front Fock-states.
A renormalized three-body model with zero-range potential is used to estimate the mean-square radii of three-body halo nuclei and molecular systems. The halo nuclei ($^6$He, $^{11}$Li, $^{14}$Be and $^{20}$C) are described as point-like inert cores and two neutrons. The molecular systems, with two helium atoms, are of the type $^4$He$_2-$X, where X$=^4$He, $^6$Li, $^7$Li, or $^{23}$Na. The estimations are compared with experimental data and realistic results.
The mean-square radii of the molecules $^4$He$_3$, $^4$He$_2-^6$Li, $^4$He$_2-^7$Li and $^4$He$_2-^{23}$Na are calculated using a three-body model with contact interactions. They are obtained from a universal scaling function calculated within a renormalized scheme for three particles interacting through pairwise Dirac-delta interaction. The root-mean-square distance between two atoms of mass $m_A$ in a triatomic molecule are estimated to be of de order of ${\cal C}\sqrt{\hbar^2/[m_A(E_3-E_2)]}$, where $E_2$ is the dimer and $E_3$ the trimer binding energies, and ${\cal C}$ is a constant (varying from $\sim 0.6$ to $\sim 1$) that depends on the ratio between $E_2$ and $E_3$. Considering previous estimates for the trimer energies, we also predict the sizes of Rubidium and Sodium trimers in atomic traps.
The results of a recent experiment with $^{85}$Rb Bose-Einstein condensates are analyzed within the mean-field approximation including dissipation due to three-body recombination. The intensity of the dissipative term is chosen from the three-body theory for large positive scattering lengths. The remaining number of condensed atoms in the experiment, obtained with applied magnetic field pulses, were used to adjust the intensity of the dissipative term. We found that the three-body recombination parameter depends on the pulse rise time; i.e., for longer rise times the values found become consistent with the three-body theory, while for shorter pulses this coefficient is found to be much larger. We interpret this finding as an indication of a coherent formation of dimers.
The neutron charge radius is studied within a light-front model with different spin coupling schemes and wave functions. The cancellation of the contributions from the Foldy term and Dirac form factor to the neutron charge form factor is verified for large nucleon sizes and it is independent of the detailed form of quark spin coupling and wave function. For the physical nucleon our results for the contribution of the Dirac form factor to the neutron radius are insensitive to the form of the wave function while they strongly depend on the quark spin coupling scheme.
The ground state masses and binding energies of the nucleon, $Λ^0$, $Λ^+_c$, $Λ^0_b$ are studied within a constituent quark QCD-inspired light-front model. The light-front Faddeev equations for the $Qqq$ composite spin 1/2 baryons, are derived and solved numerically. The experimental data for the masses are qualitatively described by a flavor independent effective interaction.
We investigate the stability of the relativistic three-boson system with a zero range force in the light front form. In particular we study the dependence of the system on an invariant cut-off. We discuss the conditions for the relativistic Thomas collapse. Finally, we fix the parameters of the model introducing a scale.
The frame dependence of the pair-term contribution to the electromagnetic form factor of the pion is studied within the Light Front approach. A symmetric ansatz for the pion Bethe-Salpeter amplitude with a pseudo scalar coupling of the constituent to the pion field is used. In this model, the pair term vanishes for the Drell-Yan condition, while it is dominant for momentum transfer along the light-front direction.
We address the issue of three-quark clusters in a medium of finite temperatures and densities. We use the light front approach and utilize a Green function formalism. We consider first the isolated case using a zero-range interaction and introducing an invariant cut-off and then medium effects. The Mott lines are shown for different values of the cut-off.