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T. Frederico

Publications and source records attributed to T. Frederico.

249 records · Page 14Linked to original sources
arXiv1998

Pairs in the light-front and covariance

The electromagnetic current of bound systems in the light-front is constructed in the Breit-Frame, in the limit of momentum transfer $q^+=(q^0+q^3)$ vanishing. In this limit, the pair creation term survives and it is responsible for the covariance of the current. The pair creation term is computed for the $j^+$ current of a spin one composite particle in the Breit-frame. The rotational symmetry of $j^+$ is violated if the pair term is not considered.

hep-ph↗
arXiv1997

Ward-Takahashi Identidy on the Ligth-Front

The Ward-Takahashi identity, reflecting local gauge invariance, is perturbatively verified for a boson model in light front field theory. A careful integration over the light front energy, corresponding to exactly taking into account pair terms, which are the contributions of the zero longitudinal momentum mode, is crucial to obtain this result. Furthermore, the one-loop boson form factors are calculated for arbitrary off-shell momenta.

hep-ph↗
arXiv1997

Pion Eletromagnetic Current in the Ligth-Cone Formalism

The electromagnetic form factor of the pion is calculated in a pseudoscalar field theoretical model which constituent quarks. We extract the form factor using the "+" component of the electromagnetic current in the light-cone formalism. For comparison, we also compute the form factor in the covariant framework and we obtain perfect agreement. It is shown that the pair terms do not contribute in this pseudoscalar model. This explains why a naive light-cone calculation, i.e., omitting pair terms from the onset, also yields the same results.

hep-ph↗
arXiv1997

Universal aspects of Efimov states and light halo nuclei

The parametric region in the plane defined by the ratios of the energies of the subsystems and the three body ground state, in which Efimov states can exist, is determined. We use a renormalizable model that guarantees the general validity of our results in the context of short range interactions. The experimental data for one and two-neutron separation energies, implies that among the halo-nuclei candidates, only C-20 has a possible Efimov state, with energy less than 14 KeV below the scattering threshold.

nucl-th↗
arXiv1997

Quark-antiquark correlation in the pion

The electromagnetic tensor for inclusive electron scattering off the pion, ($W^{μν}$), for momentum transfers such that $q^+=0$, ($q^+=q^0+q^3$) is shown to obey a sum-rule for the component $W^{++}$. From this sum-rule, one can define the quark-antiquark correlation function in the pion, which characterizes the transverse distance distribution between the quark and antiquark in the light-front pion wave-function. Within the realistic models of the relativistic pion wave function (including instanton vacuum inspired wave function) it is shown that the value of the two-quark correlation radius ($r_{q\bar q}$) is near twice the pion electromagnetic radius ($r_π$), where $r_π \approx 2/3$ fm. We also define the correlation length $l_{corr}$ where the two - particle correlation have an extremum. The estimation of $l_{corr}\approx 0.3 - 0.5$ fm is very close to estimations from instanton models of QCD vacuum. It is also shown that the above correlation is very sensitive to the pion light-front wave-function models.

hep-ph↗
arXiv1997

Application of Renormalization to Potential Scattering

A recently proposed renormalization scheme can be used to deal with nonrelativistic potential scattering exhibiting ultraviolet divergence in momentum space. A numerical application of this scheme is made in the case of potential scattering with $r^{-2}$ divergence for small r, common in molecular and nuclear physics, by the use of cut-offs in momentum and configuration spaces. The cut-off is finally removed in terms of a physical observable and model-independent result is obtained at low energies. The expected variation of the off-shell behavior of the t matrix arising from the renormalization scheme is also discussed.

hep-th↗
arXiv1997

Lattice Discretization in Quantum Scattering

The utility of lattice discretization technique is demonstrated for solving nonrelativistic quantum scattering problems and specially for the treatment of ultraviolet divergences in these problems with some potentials singular at the origin in two and three space dimensions. This shows that lattice discretization technique could be a useful tool for the numerical solution of scattering problems in general. The approach is illustrated in the case of the Dirac delta function potential.

hep-th↗
arXiv1997

^1S_0 pairing correlations in relativistic nuclear matter and the two-nucleon virtual state

We use the Gorkov formulation of the Dirac-Hartree-Fock-Bogoliubov approximation to nuclear pairing to study the ^1S_0 nucleon-nucleon correlations in nuclear matter. We find the short-range correlations of the ^1S_0 pairing fields to be almost identical to those of the two-nucleon virtual state. We obtain mutually consistent results for the pairing fields, using several different sets of effective interaction parameters, when we demand that each of these sets places the virtual-state pole at its physical location.

nucl-th↗
arXiv1997

Scaling Law for Baryon Coupling to its Current and its possible applications

The baryon- coupling to its current ($λ_{B}$), in conventional QCD sum rule calculations (QCDSR), is shown to scale as the cubic power of the baryon mass, $M_B$. Some theoretical justification for it comes from a simple light-cone model and also general scaling arguments for QCD. But more importantly, taken as a phenomenological ansatz for the present, this may find very good use in current explorations of possible applications of QCDSR to baryon physics both at temperature $T = 0$, $T \ne 0$ and/or density $ρ= 0$, $ρ\ne 0$.

hep-ph↗
arXiv1996

How reliable is the mean-field Nuclear Matter description for supporting Chiral Effective Lagrangians?

The link between non-linear chiral effective Lagrangians and the Walecka model description of bulk nuclear matter [1] is questioned. This fact is by itself due to the Mean Field Approximation (MFA) which in nuclear mater makes the picture of a nucleon-nucleon interaction based on scalar(vector) meson exchange, equivalent to the description of a nuclear matter based on attractive and repulsive contact interactions. We present a linear chiral model where this link between the Walecka model and an underlying to chiral symmetry realization still holds, due to MFA.

nucl-th↗
arXiv1994

Model for asymptotic $D$-state parameters of light nuclei: Application to $^4He$

A simple method for calculating the asymptotic $D$ state observables for light nuclei is suggested. The method exploits the dominant clusters of the light nuclei. The method is applied to calculate the $^4He$ asymptotic $D$ to $S$ normalization ratio $ρ^α$ and the closely related $D$ state parameter $D_2^α$. The study predicts a correlation between $D_2^α$ and $B_α$, and between $ρ^α$ and $B_α$, where $B_α$ is the binding energy of $^4He$. The present study yields $ρ^α\simeq -0.14$ and $D_2^α\simeq -0.12$ fm$^2$ consistent with the correct experimental $η^D$ and the binding energies of the deuteron, triton, and the $α$ particle, where $η^d$ is the deuteron $D$ state to $S$ state normalization ratio.

nucl-th↗
arXiv1993

Relativistic three-particle dynamical equations: I. Theoretical development

Starting from the two-particle Bethe-Salpeter equation in the ladder approximation and integrating over the time component of momentum, we rederive three dimensional scattering integral equations satisfying constraints of relativistic unitarity and covariance, first derived by Weinberg and by Blankenbecler and Sugar. These two-particle equations are shown to be related by a transformation of variables. Hence we show how to perform and relate identical dynamical calculation using these two equations. Similarly, starting from the Bethe-Salpeter-Faddeev equation for the three-particle system and integrating over the time component of momentum, we derive several three dimensional three-particle scattering equations satisfying constraints of relativistic unitarity and covariance. We relate two of these three-particle equations by a transformation of variables as in the two-particle case. The three-particle equations we derive are very practical and suitable for performing relativistic scattering calculations.

nucl-th↗
arXiv1993

Shape-Independent Expansion for the $^3S_1-^3D_1$ Mixing Parameter

A low-energy shape-independent expansion is suggested for the function $[tan(2ε_{BB})/(2k^2)]$, where $ε_{BB}$ is the Blatt-Biedenharn mixing parameter for the $^3S_1 -$ $^3D_1$ channel. This expansion allows an evaluation of the mixing parameter $ε_{BB}$ from a knowledge of the deuteron asymptotic $D$ to $S$ ratio, pion mass and other low-energy observables, such as the scattering lengths, deuteron binding etc., of the nucleon-nucleon system. We demonstrate that the correct long range behavior of the tensor potential is essential for a realistic reproduction of $ε_{BB}$.

nucl-th↗
arXiv1993

Relativistic three-particle scattering equations

We derive a set of relativistic three-particle scattering equations in the three-particle c.m. frame employing a relativistic three-particle propagator suggested long ago by Ahmadzadeh and Tjon in the c.m. frame of a two-particle subsystem. We make the coordinate transformation of this propagator from the c.m. frame of the two-particle subsystem to the three-particle c.m. frame. We also point out that some numerical applications of the Ahmadzadeh and Tjon propagator to the three-nucleon problem use unnecessary nonrelativistic approximations which do not simplify the computational task, but violate constraints of relativistic unitarity and/or covariance.

nucl-th↗