arXiv subjects

W. de Paula

Publications and source records attributed to W. de Paula.

At least 37 records · Page 2Linked to original sources

arXiv2019

A unique equation of state for the universe evolution from AdS$_5$ space-time

We apply the Induced Matter Model to a five-dimensional metric. For the case with null cosmological constant, we obtain a solution able to describe the radiation-dominated era of the universe. The positive $Λ$ case yields a bounce cosmological model. In the negative five-dimensional cosmological constant case, the scale factor is obtained as $a(t)\sim\sqrt[]{\sinh t}$, which is able to describe not only the late-time cosmic acceleration but also the non-accelerated stages of the cosmic expansion in a continuous form. This solution together with the extra-dimensional scale factor solution yields the material content of the model to be remarkably related through an equation of state analogous to the renowned MIT bag model equation of state for quark matter $p=(ρ-4B)/3$. In our case, $ρ=ρ_m+B$, with $ρ_m$ being the energy density of relativistic and non-relativistic matter and $B=\ Λ /16π$ represents the bag energy constant, which plays the role of the dark energy in the four-dimensional universe, with $Λ$ being the cosmological constant of the AdS$_5$ space-time. Our model satisfactorily fits the observational data for the low redshift sample of the experimental measurement of the Hubble parameter, which resulted in $H_0=72.2^{+5.3}_{-5.5}$km s$^{-1}$ Mpc$^{-1}$.

gr-qc↗

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

Charged wormholes in f(R,T) extended theory of gravity

Wormholes are a solution for General Relativity field equations which characterize a passage or a tunnel that connects two different regions of space-time and is filled by some sort of exotic matter, that does not satisfy the energy conditions. On the other hand, it is known that in extended theories of gravity, the extra degrees of freedom once provided may allow the energy conditions to be obeyed and, consequently, the matter content of the wormhole to be non-exotic. In this work, we obtain, as a novelty in the literature, solutions for charged wormholes in the $f(R,T)$ extended theory of gravity. We show that the presence of charge in these objects may be a possibility to respect some stability conditions for their metric. Also, remarkably, the energy conditions are respected in the present approach.

gr-qc↗

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

Supersymmetry and Fermionic Modes in an Oscillon Background

The excitations referred to as oscillons are long-lived time-dependent field configurations which emerge dynamically from non-linear field theories. Such long-lived solutions are of interest in applications that include systems of Condensed Matter Physics, the Standard Model of Particle Physics, Lorentz-symmetry violating scenarios and Cosmology. In this work, we show how oscillons may be accommodated in a supersymmetric scenario. We adopt as our framework simple ($\mathcal{N}=1$) supersymmetry in $D=1+1$ dimensions. We focus on the bosonic sector with oscillon configurations and their (classical) effects on the corresponding fermionic modes, (supersymmetric) partners of the oscillons. The particular model we adopt to pursue our investigation displays cubic self-interactions in the scalar sector.

hep-th↗

arXiv2017

Configurational Entropy as a tool to select a physical Thick Brane Model

We analise braneworld scenarios via a configurational entropy (CE) formalism. Braneworld scenarios have drawn attention mainly due to the fact that they can explain the hierarchy problem and unify the fundamental forces through a symmetry breaking procedure. Those scenarios localize matter in a $(3+1)$ hypersurface, the brane, which is inserted in a higher dimensional space, the bulk. Novel analytical braneworld models, {in which the warp factor depends} on a free parameter $n$, were recently released in the literature. In this article we will provide a way to constrain this parameter through the relation between information and dynamics of a system described by the CE. We demonstrate that in some cases the CE is an important tool in order to provide the most probable physical system among all the possibilities. In addition, we show that the highest CE is correlated to a tachyonic sector of the configuration, where the solutions for the corresponding model are dynamically unstable.

hep-th↗

arXiv2017

Fermionic bound states in Minkowski-space: Light-cone singularities and structure

The Bethe-Salpeter equation for two-body bound system with spin $1/2$ constituent is addressed directly in the Minkowski space. In order to accomplish this aim we use the Nakanishi integral representation of the Bethe-Salpeter amplitude and exploit the formal tool represented by the exact projection onto the null-plane. This formal step allows one i) to deal with end-point singularities one meets and ii) to find stable results, up to strongly relativistic regimes, that settles in strongly bound systems. We apply this technique to obtain the numerical dependence of the binding energies upon the coupling constants and the light-front amplitudes for a fermion-fermion $0^+$ state with interaction kernels, in ladder approximation, corresponding to scalar-, pseudoscalar- and vector boson exchanges, respectively. After completing the numerical survey of the previous cases, we extend our approach to a quark-antiquark system in $0^-$ state, taking both constituent-fermion and exchanged boson masses, from lattice calculations. Interestingly, the calculated light-front amplitudes for such a mock pion show peculiar signatures of the spin degrees of freedom.

hep-ph↗

arXiv2017

Emerging 2D isospectral configurations for position-dependent mass quantum systems

In this work we construct a general class of exactly solvable non-relativistic bi-dimensional quantum systems with position-dependent masses (PDM). These systems are isospectral to a given system with constant mass. The case of a charged particle with a PDM interacting with an external magnetic field is included in the present investigation. We apply the approach in order to construct the SU(2) coherent states in some examples which are isospectral to the two-dimensional anisotropic harmonic oscillator, and discuss the impact of the introduction of special non-homogeneous external magnetic fields.

quant-ph↗

arXiv2017

Excited states of the Wick-Cutkosky model with the Nakanishi representation in the Light-Front framework

The Wick-Cutkosky model is investigated using the framework of the Nakanishi Perturbative Integral Representation projected in the Light-Front hyperplane for an s-wave amplitude. We developed a new technique based on sucessive integration by parts of the Nakanishi Representation which enabled us to transform one of the integrations into a sum. With a set of boundary conditions it was possible to guess the format of the solution, which was in fact a distribution. The eigenequations obtained were the same as the originals of Cutkosky \cite{CutPR54}. Finally, a numerical check confirmed that the final equation reproduce the same eigenvalues as the initial Bethe-Salpeter equation.

hep-ph↗

arXiv2017

Lorentz Violation and Topologically Trapped Charge Carriers in 2D Materials

The full spectrum of two-dimensional fermion states in a scalar soliton trap with a Lorentz breaking background is investigated in the context of the novel 2D materials, where the Lorentz symmetry should not be strictly valid. The field theoretical model with Lorentz breaking terms represents Dirac electrons in one valley and in a scalar field background. The Lorentz violation comes from the difference between the Dirac electron and scalar mode velocities, which should be expected when modelling the electronic and lattice excitations in 2D materials. We extend the analytical methods developed in the context of 1+1 field theories to explore the effect of the Lorentz symmetry breaking in the charge carrier density of 2D materials in the presence of a domain wall with a kink profile. The width and the depth of the trapping potential from the kink is controlled by the Lorentz violating term, which is reflected analytically in the band structure and properties of the trapped states. Our findings enlarge previous studies of the edge states obtained with domain wall and in strained graphene nanoribbon in a chiral gauge theory.

hep-th↗

arXiv2017

Signature of curved QFT effects on the optical properties of deformed graphene: Curved QFT effects on the optical properties of deformed graphene

The Dirac equation in curved space is used to study the optical transmittance of deformed graphene along a given direction. Our theoretical analysis of the available experimental data for the light transmittance suggests that the periodic ripple associated with the out-of-plane deformation observed in unstrained graphene explains the observations. Furthermore, the experimental uniaxial strained graphene for light transmittance show two features, namely the modification of the $\cos^2θ$ law and the decrease of the amplitude of the oscillations with the polarization angle $θ$, which can be well accommodated within the theoretical analysis used here and provide further evidence of the validity of using QFT in curved space to understand two dimensional materials.

cond-mat.mes-hall↗

arXiv2016

Advances in solving the two-fermion homogeneous Bethe-Salpeter equation in Minkowski space

Actual solutions of the Bethe-Salpeter equation for a two-fermion bound system are becoming available directly in Minkowski space, by virtue of a novel technique, based on the so-called Nakanishi integral representation of the Bethe-Salpeter amplitude and improved by expressing the relevant momenta through light-front components, i.e. $k^\pm=k^0 \pm k^3$. We solve a crucial problem that widens the applicability of the method to real situations by providing an analytically exact treatment of the singularities plaguing the two-fermion problem in Minkowski space, irrespective of the complexity of the irreducible Bethe-Salpeter kernel. This paves the way for feasible numerical investigations of relativistic composite systems, with any spin degrees of freedom. We present a thorough comparison with existing numerical results, evaluated in both Minkowski and Euclidean space, fully corroborating our analytical treatment, as well as fresh light-front amplitudes illustrating the potentiality of non perturbative calculations performed directly in Minkowski space.

hep-th↗

arXiv2016

Configurational entropy as a bounding of Gauss-Bonnet braneworld models

Configurational entropy has been revealed as a reliable method for constraining some parameters of a given model [Phys. Rev. D \textbf{92} (2015) 126005, Eur. Phys. J. C \textbf{76} (2016) 100]. In this letter we calculate the configurational entropy in Gauss-Bonnet braneworld models. Our results restrict the range of acceptability of the Gauss-Bonnet scalar values. In this way, the information theoretical measure in Gauss-Bonnet scenarios opens a new window to probe situations where the additional parameters, responsible for the Gauss-Bonnet sector, are arbitrary. We also show that such an approach is very important in applications that include p and Dp-branes and various superstring-motivated theories.

hep-th↗

arXiv2015

Quantum Dynamics in Noisy Backgrounds: from sampling to dissipation and fluctuations

We investigate the dynamics of a quantum system coupled linearly to Gaussian white noise using functional methods. By performing the integration over the noisy field in the evolution operator, we get an equivalent non-Hermitian Hamiltonian, which evolves the quantum state with a dissipative dynamics. We also show that if the integration over the noisy field is done for the time evolution of the density matrix, a gain contribution from the fluctuations, can be accessed in addition to the loss one from the non-hermitian Hamiltonian dynamics. We illustrate our study by computing analytically the effective non-Hermitian Hamiltonian, which we found to be the complex frequency harmonic oscillator, with a known evolution operator. It leads to space and time localisation, a common feature of noisy quantum systems in general applications.

quant-ph↗

arXiv2015

Two Dimensional Honeycomb Materials: random fields, dissipation and fluctuations

In this paper, we propose a method to describe the many-body problem of electrons in honeycomb materials via the introduction of random fields which are coupled to the electrons and have a Gaussian distribution. From a one-body approach to the problem, after integrating exactly the contribution of the random fields, one builds a non-hermitian and dissipative effective Hamiltonian with two-body interactions. Our approach introduces besides the usual average over the electron field a second average over the random fields. The interplay of two averages enables the definition of various types of Green's functions which allow the investigation of fluctuation-dissipation characteristics of the interactions that are a manifestation of the many-body problem. In the current work we study only the dissipative term, through the perturbative analysis of the dynamics associated the effective Hamiltonian generated by two different kinds of couplings. For the cases analysed, the eigenstates of the effective Hamiltonian are complex and, therefore, some of the states have a finite life time. Moreover, we also investigate, in the mean field approximation, the most general parity conserving coupling to the random fields and compute the width of charge carriers $Γ$ as a function of the Fermi energy $E_F$. The theoretical prediction for $Γ(E_F)$ is compared to the available experimental data for graphene. The good agreement between $Γ_{theo}$ and $Γ_{exp}$ suggests that description of the many-body problem associated to the electrons in honeycomb materials can indeed be done via the introduction of random fields.

cond-mat.mes-hall↗

arXiv2015

Quantum Field Theory Approach to the Optical Conductivity of Strained and Deformed Graphene

The computation of the optical conductivity of strained and deformed graphene is discussed within the framework of quantum field theory in curved spaces. The analytical solutions of the Dirac equation in an arbitrary static background geometry for one dimensional periodic deformations are computed, together with the corresponding Dirac propagator. Analytical expressions are given for the optical conductivity of strained and deformed graphene associated with both intra and interbrand transitions. The special case of small deformations is discussed and the result compared to the prediction of the tight-binding model.

cond-mat.mtrl-sci↗