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T. Frederico

Publications and source records attributed to T. Frederico.

At least 55 records · Page 3Linked to original sources
arXiv2019

Pion off-shell electromagnetic form factors: data extraction and model analysis

We investigate the pion electromagnetic half off-shell form factors, which parametrize the matrix element of the charged pion electromagnetic current with one leg off-mass-shell and the other leg on-mass-shell, using an exactly solvable manifestly covariant model of a $(3+1)$ dimensional fermion field theory. The model provides a 3D imaging of the two off-shell pion form factors $F_1$ and $F_2$ as a function of $(Q^2,t)$, which are related to each other satisfying the Ward-Takahashi identity. The normalization of the renormalized charge form factor $F_1$ is fixed by $F_1(Q^2=0, t=m^2_π)=1$ while the other form factor $F_2$ vanishes, i.e. $F_2(Q^2, t=m^2_π)=0$ for any value of $Q^2$ due to the time-reversal invariance of the strong interaction. We define the new form factor $g(Q^2,t)=F_2(Q^2,t)/(t-m^2_π)$ and find that $g(Q^2,t)$ can be measurable in the on-mass-shell limit. In particular, $g(Q^2=0, t=m^2_π)$ is related with the pion charge radius. We also compare our form factors with the data extracted from the pion electroproduction reaction for both the off-shell region ($t<0$) and the on-shell limit ($t \rightarrow m_π^2$).

hep-ph↗
arXiv2019

Creating Oscillons and Oscillating Kinks in Two Scalar Field Theories

Oscillons are time-dependent, localized in space, extremely long-lived states in nonlinear scalar-field models, while kinks are topological solitons in one spatial dimension. In the present work, we show new classes of oscillons and oscillating kinks in a system of two nonlinearly coupled scalar fields in $1 + 1$ spatiotemporal dimensions. The solutions contain a control parameter, the variation of which produces oscillons and kinks with a flat-top shape. The model finds applications to condensed matter, cosmology, and high-energy physics.

hep-th↗
arXiv2019

Solving the Bethe-Salpeter Equation in Minkowski Space for a Fermion-Scalar system

The ladder Bethe-Salpeter Equation of a bound (1/2)+ system, composed by a fermion and a scalar boson, is solved in Minkowski space, for the first time. The formal tools are the same already successfully adopted for two-scalar and two-fermion systems, namely the Nakanishi integral representation of the Bethe-Salpeter amplitude and the light-front projection of the fulfilled equation. Numerical results are presented and discussed for two interaction kernels: i) a massive scalar exchange and ii) a massive vector exchange, illustrating both the correlation between binding energies and the interaction coupling constants, as well as the valence content of the interacting state, through the valence probabilities and the light-front momentum distributions. In the case of the scalar exchange, an interesting side effect, to be ascribed to the repulsion generated by the small components of the Dirac spinor, is pointed out, while for the vector exchange the manifestation of the helicity conservation opens new interesting questions to be addressed within a fully non-perturbative framework, as well as the onset of a scale-invariant regime.

hep-ph↗
arXiv2019

Interwoven limit cycles in the spectra of mass imbalanced many-boson system

The independence between few-body scales beyond the van der Waals universality is demonstrated for the extreme mass-imbalanced case of a specific many-boson system. This finding generalizes the scaling properties of universal tetramers to a broader class of heterogeneous few-boson systems. We assume two heavy atoms interacting with $(N-2)-$lighter ones at the unitary limit, using a particular case where no interactions are active between identical particles, by investigating the interwoven spectra of this many-body system for an arbitrary number of light bosons. A large mass-ratio between the particles allows us to treat this $N-$body system analytically, by solving an effective inverse-squared long-range interaction which is stablished for the two heavy bosons. For a cluster with $N-2$ light bosons ($N\ge 4$), we discuss the implications of the corresponding long-range potentials associated with different subsystem thresholds, implying in independent interwoven limit cycles for the correlation between the energies of excited $N-$body system. Our study with extreme mass-imbalanced few-boson bound states provides a fundamental understanding of the scaling behavior of their interwoven spectra. The novel insights enlarge the well-known Efimov physics paradigm and show the existence of different limit cycles, which could be probed by new experiments.

quant-ph↗
arXiv2019

Solving the three-body bound-state Bethe-Salpeter equation in Minkowski space

The scalar three-body Bethe-Salpeter equation, with zero-range interaction, is solved in Minkowski space by direct integration of the four-dimensional integral equation. The singularities appearing in the propagators are treated properly by standard analytical and numerical methods, without relying on any ansatz or assumption. The results for the binding energies and transverse amplitudes are compared with the results computed in Euclidean space. A fair agreement between the calculations is found.

hep-ph↗
arXiv2019

The Quark-Gluon Vertex and the QCD Infrared Dynamics

The Dyson-Schwinger quark equation is solved for the quark-gluon vertex using the most recent lattice data available in the Landau gauge for the quark, gluon and ghost propagators, the full set of longitudinal tensor structures in the Ball-Chiu vertex, taking into account a recently derived normalisation for a quark-ghost kernel form factors and the gluon contribution for the tree level quark-gluon vertex identified on a recent study of the lattice soft gluon limit. A solution for the inverse problem is computed after the Tikhonov linear regularisation of the integral equation, that implies solving a modified Dyson-Schwinger equation. We get longitudinal form factors that are strongly enhanced at the infrared region, deviate significantly from the tree level results for quark and gluon momentum below 2 GeV and at higher momentum approach their perturbative values. The computed quark-gluon vertex favours kinematical configurations where the quark momentum $p$ and the gluon momentum $q$ are small and parallel. Further, the quark-gluon vertex is dominated by the form factors associated to the tree level vertex $γ_μ$ and to the operator $2 \, p_μ+ q_μ$. The higher rank tensor structures provide small contributions to the vertex.

hep-ph↗
arXiv2019

The Bethe-Salpeter approach to bound states: from Euclidean to Minkowski space

The challenge to obtain from the Euclidean Bethe--Salpeter amplitude the amplitude in Minkowski is solved by resorting to un-Wick rotating the Euclidean homogeneous integral equation. The results obtained with this new practical method for the amputated Bethe--Salpeter amplitude for a two-boson bound state reveals a rich analytic structure of this amplitude, which can be traced back to the Minkowski space Bethe--Salpeter equation using the Nakanishi integral representation. The method can be extended to small rotation angles bringing the Euclidean solution closer to the Minkowski one and could allow in principle the extraction of the longitudinal parton density functions and momentum distribution amplitude, for example.

hep-ph↗
arXiv2018

Efimov effect in a $D$-dimensional Born-Oppenheimer approach

We study a three-body system, formed by two identical heavy bosons and a light particle, in the Born-Oppenheimer approximation for an arbitrary dimension $D$. We restrict $D$ to the interval $2\,<\,D\,<\,4$, and derive the heavy-heavy $D$-dimensional effective potential proportional to $1/R^2$ ($R$ is the relative distance between the heavy particles), which is responsible for the Efimov effect. We found that the Efimov states disappear once the critical strength of the heavy-heavy effective potential $1/R^2$ approaches the limit $-(D-2)^2/4$. We obtained the scaling function for the $^{133}$Cs-$^{133}$Cs-$^6$Li system as the limit cycle of the correlation between the energies of two consecutive Efimov states as a function of $D$ and the heavy-light binding energy $E^{D}_2$. In addition, we found that the energy of the $(N+1)^{\rm th}$ excited state reaches the two-body continuum independently of the dimension $D$ when $\sqrt{E^{D}_2/E_3^{(N)}}=0.89$, where $E_3^{(N)}$ is the $N^{\rm th}$ excited three-body binding energy.

physics.atom-ph↗
arXiv2018

Nuclear processes in Astrophysics: Recent progress

The origin of the elements is a fascinating question that scientists have been trying to answer for the last seven decades. The formation of light elements in the primordial universe and heavier elements in astrophysical sources occurs through nuclear reactions. We can say that nuclear processes are responsible for the production of energy and synthesis of elements in the various astrophysical sites. Thus, nuclear reactions have a determining role in the existence and evolution of several astrophysical environments, from the Sun to the spectacular explosions of supernovae. Nuclear astrophysics attempts to address the most basic and important questions of our existence and future. There are still many issues that are unresolved such as, how stars and our Galaxy have formed and how they evolve, how and where are the heaviest elements made, what is the abundance of nuclei in the universe and what is the nucleosynthesis output of the various production processes and why the amount of lithium-7 observed is less than predicted. In this paper, we review our current understanding of the different astrophysical nuclear processes leading to the formation of chemical elements and pay particular attention to the formation of heavy elements occurring during high-energy astrophysical events. Thanks to the recent multi-messenger observation of a binary neutron star merger, which also confirmed production of heavy elements, explosive scenarios such as short gamma-ray bursts and the following kilonovae are now strongly supported as nucleosynthesis sites.

astro-ph.SR↗
arXiv2018

Scattering of cold $^4$He on $^4$He$-^{6,7}$Li and $^4$He$-^{23}$Na molecules

We predict $s-$wave elastic cross-sections $σ$ for low-energy atom-molecule collisions with kinetic energies up to 40 mK, for the $^4$He collision with weakly bound diatomic molecules formed by $^4$He with $^7$Li, $^6$Li and $^{23}$Na. Our scattering calculations are performed by using diatomic and triatomic molecular binding energies obtained from several available realistic models as input in a renormalized zero-range model, as well as a finite-range one-term separable potential in order to quantify the relevance of range corrections to our predictions. Of particular relevance for possible experimental realization, we show the occurrence of a zero in $σ$ for the collision of cold $^4$He on $^4$He$-^{23}$Na molecule below 20 mK. Also our results for the elastic collision $^4$He on $^4$He$-^{6,7}$Li molecules suggest that $σ$ varies considerably for the realistic models studied. As the chosen molecules are weakly bound and the scattering energies are very low, our results are interpreted on the light of the Efimov physics, which explains the model independent and robustness of our predictions, despite some sensitivity on the potential range.

physics.atom-ph↗
arXiv2018

Four-body eikonal approach to three-body halo nuclei scattering

Halo nuclei projectiles can undergo into a fragmentation process when scattered by a nucleus target. The corresponding core from exotic nuclei is usually observed while the others fragments of the reaction are not. We use the recently proposed theory by [Phys. Lett. B767, 53 (2017)] with a four-body description of the inclusive breakup reaction where the projectile is 20C described like a two-neutron halo nucleus. The momentum of both neutrons are integrated out giving a generic description of the core angular distribution. In this preliminary study we perform an analysis of the inclusive inelastic breakup cross-section using the eikonal approximation for the distorted wave function of the projectile. A study of the inclusive inelastic cross-sections of 20C from the collision with different targets are presented.

nucl-th↗
arXiv2018

Oscillons in $ϕ^6$-theories: Possible occurrence in MHD

In this work, we report on the possibility of occurrence of oscillon configurations in the fourth state of matter. Oscillons are extremely long-lived, time-periodic, spatially-localised scalar field structures. Starting from a scalar field theory in 1+1 space-time dimensions, we find out that small-amplitude oscillons can be obtained in the framework of a $ϕ^6$ self-interacting potential. A connection between our results and ideal MHD theory is established. Perspectives for a development of the present work are pointed out.

hep-th↗
arXiv2018

Exploring the Quark-Gluon Vertex with Slavnov-Taylor Identities and Lattice Simulations

The soft gluon limit of the longitudinal part of the quark-gluon vertex is studied by resorting to non-perturbative approaches to Quantum Chromodynamics (QCD). Based on a Slavnov-Taylor identity (STI), the longitudinal form factors is expressed in terms of the quark-ghost kernel, the quark self energy and the quark wave function. An exact relation between the non-vanishing longitudinal form factors is derived for the soft gluon limit and explored to understand the behaviour of the vertex. Within a Ball-Chiu vertex, the form factor $λ_1$ was analysed using recent lattice simulations for full QCD for the soft gluon limit. The lattice data shows that the gluon propagator resumes the momentum dependence of such component of the vertex. This connection is understood via a fully dressed one-loop Bethe-Salpeter equation. The behaviour of the remaining longitudinal form factors $λ_2(p^2)$ and $λ_3(p^2)$ is investigated combining both the information of lattice simulations and the derived relations based on the STI.

hep-ph↗
arXiv2018

Efimov effect in $D$ spatial dimensions in $AAB$ systems

The existence of the Efimov effect is drastically affected by the dimensionality of the space in which the system is embedded. The effective spatial dimension containing an atomic cloud can be continuously modified by compressing it in one or two directions. In the present article we determine for a general $AAB$ system formed by two identical bosons $A$ and a third particle $B$ in the two-body unitary limit, the dimensionsality $D$ for which the Efimov effect can exist for different values of the mass ratio $\mathpzc{A}=m_B/m_A$. In addition, we provide a prediction for the Efimov discrete scaling factor, ${\rm exp}\,(π/s)$, as a function of a wide range of values of $\mathpzc{A}$ and $D$, which can be tested in experiments that can be realized with currently available technology.

physics.atom-ph↗
arXiv2018

In-Medium $K^+$ Electromagnetic Form Factor with a Symmetric Vertex in a Light Front Approach

Using the light-front kaon wave function based on a Bethe-Salpeter amplitude model for the quark-antiquark bound state, we study the Electromagnetic Form Factor (EMFF) of the kaon in nuclear medium within the framework of light-front field theory. The kaon model we adopt is well constrained by previous and recent studies to explain its properties in vacuum. The in-medium kaon EMFF is evaluated for the + component of the electromagnetic current, $J^+$, in the Breit frame. In order to consistently incorporate the constituent up and antistrange quarks of the kaon immersed in symmetric nuclear matter, we use the Quark-Meson Coupling (QMC) model, which has been widely applied to various hadronic and nuclear phenomena in a nuclear medium with success. We predict the in-medium modification of the kaon EMFF in symmetric nuclear matter. It is found that, after a fine tuning of the regulator mass, i.e. $m_R = 0.600$ GeV, the model is suitable to fit the available experimental data in vaccum within the theoretical uncertainties, and based on this we predict the in-medium modification of the EMFF.

hep-ph↗
arXiv2018

Equation for the Nakanishi weight function using the inverse Stieltjes transform

The bound state Bethe-Salpeter amplitude was expressed by Nakanishi in terms of a smooth weight function g. By using the generalized Stieltjes transform, we derive an integral equation for the Nakanishi function g for a bound state case. It has the standard form g= Vg, where V is a two-dimensional integral operator. The prescription for obtaining the kernel V starting with the kernel K of the Bethe-Salpeter equation is given.

hep-ph↗
arXiv2018

Spin-1 particles and perturbative QCD

Due to the angular condition in the light-front dynamics (LFD), the extraction of the electromagnetic form factors for spin-1 particles can be uniquely determined taking into account implicitly non-valence and/or the zero-mode contributions to the matrix elements of the electromagnetic current. No matter which matrix elements of the electromagnetic current is used to extract the electromagnetic form factors, the same unique result is obtained. As physical observables, the electromagnetic form factors obtained from matrix elements of the current in LFD must be equal to those obtained in the instant form calculations. Recently, the Babar collaboration \cite{Aubert2008} has analyzed the reaction $e^+ e^-\rightarrow ρ^+ + ρ^-$ at $\sqrt{s}=10.58 GeV$ to measure the cross section as well as the ratios of the helicity amplitudes $F_{λ'λ}$. We present our recent analysis of the Babar data for the rho meson considering the angular condition in LFD to put a stringent test on the onset of asymptotic perturbative QCD and predict the energy regime where the subleading contributions are still considerable.

hep-ph↗