arXiv subjects

Alexandre G. M. Schmidt

Publications and source records attributed to Alexandre G. M. Schmidt.

At least 19 recordsLinked to original sources

arXiv2026

Neutral scalar particle in a charged wormhole background

We investigate the dynamics of a neutral scalar particle in a charged traversable wormhole background, considering harmonic and modified harmonic oscillator couplings. By solving the Klein--Gordon equation, the radial problem is reduced to a generalized Heun equation, whose polynomial truncation yields the bound-state energy spectrum and quantized oscillator frequencies. Although the particle is electrically neutral, the energy spectrum depends indirectly on the wormhole charge through the geometric deformation of spacetime. We also analyze the dependence of the oscillator frequency on the angular momentum, throat parameter, electric charge, and the additional interaction parameter $V_{0}$. The appropriate limits recover the corresponding results for the uncharged wormhole and the standard harmonic oscillator.

gr-qc↗

arXiv2026

Dyonic rotating cosmological black hole surrounded by quintessence

In this work, we constructed a new metric describing a rotating cosmological black hole with electric and magnetic charges in the presence of quintessence dark energy, where the cosmological constant and quintessence are associated with external matter. Starting from a Schwarzschild-type metric and considering all energy contributions in the function $f(r)$, we introduced rotation through the Newman--Janis algorithm using a unique and general complexification rule, avoiding possible ambiguities. Assuming that the metric satisfies the Einstein field equations with matter, we calculate the event horizon condition, the total stress--energy tensor $T_{μν}$, and the Kretschmann and Ricci scalars. We also analyze the influence of the cosmological constant $Λ$ and quintessence parameter $α$ on the event horizon and ergosphere. In the limit $α\rightarrow 0$, the scalar curvature differs from the vacuum result, $R \neq -4Λ$, while the singularity region remains unchanged. Finally, we investigate the angular and rotational velocities of a test particle, as well as the energy conditions, through the energy density and pressures required for the existence of this black hole solution.

gr-qc↗

arXiv2026

Exact solution of the Klein-Gordon equation in a Kiselev black hole background

In this work, we investigate exactly the dynamics of a relativistic spinless particle influenced by a static and uncharged black hole surrounded by a quintessence-like anisotropic fluid or Kiselev black hole. Considering two quintessence-like models as examples, we calculate the radial wave function and determine, in both cases, the quasispectrum of energy, as well as the Hawking radiation and temperature. We find that the stronger the influence of quintessence-like anisotropic fluid, the smaller the radiation observed outside the event horizon. Furthermore, the temperatures obtained in both cases are directly influenced by the quintessence-like anisotropic fluid, and we recover the values obtained in other contexts, such as those derived using the surface gravity. Finally, in the absence of quintessence, we recover in both cases the Hawking temperature $T_{H} = 1/(8πk_{B} M)$ corresponding to the Schwarzschild black hole.

gr-qc↗

arXiv2026

Perturbative aspects of analogue FLRW spacetime Jellium models

We study electro-acoustic perturbative modes in homogeneous, isotropic and expanding -- dubbed as Friedmann-Lemaitre-Robertson-Walker (FLRW) -- Jellium models, thereby mimicking density perturbations in analogous Newtonian cosmological expansions. We present both novel analytic solutions for linear perturbations in specific analogue cosmological expansions and full numerical evaluations that characterize the temporal evolution of the electro-acoustic modes capturing their full dynamical behavior across the linear and the nonlinear regimes. For both the case of pressure supported evolution or modes sourced by nonadiabatic contributions, we also characterize the temporal evolution of such perturbations by introducing their scale dependent particle number fluctuation power spectrum which can act as a tool to connect theory and experiments. The dependence of the latter on the physical parameters of the model is demonstrated in detail.

gr-qc↗

arXiv2025

Schrödinger equation is $\mathcal{R}$-separable in toroidal coordinates

We present, for the first time, exact solutions for the Schrödinger equation in Moon and Spencer's toroidal coordinates, and in the electromagnetic toroidal--poloidal coordinate systems. Curiously, both systems present a fractional angular momentum, because of the torus's hole. We achieve these novel solutions using the irregular $\mathcal{R}$-separation of variables, an unexplored approach in Physics, which results in a wavefunction with fractional angular momentum eigenvalues. Numerous solutions for the Schrödinger equation in a variety of external potentials are shown, including an external magnetic field. A plane-wave expansion and a Green function are also presented, setting the stage for future progress in this area.

quant-ph↗

arXiv2024

Analogue black string in a quantum harmonic oscillator

For a scalar particle without self-interaction or backreaction from the space-time background, the dynamics are governed by the Klein-Gordon equation. In this work, we write the exact solution of this equation in the background of a chargeless, static black string in terms of the biconfluent Heun function. In this curious system, we are able to explore what happens if we have negative values for the masses. The eigenvalue problem provides complex energy values for the particle, which may indicate the presence of quasinormal modes. We show a simple quantum system that can imitate the particle in the black string background, whose solutions are also applications of the biconfluent Heun function.

hep-th↗

arXiv2020

Mapping the charge-dyon system into the position-dependent effective mass background via Pauli equation

This work aims to reproduce a quantum system composed of a charged spin - $1/2$ fermion interacting with a dyon with an opposite electrical charge (charge-dyon system), utilizing a position-dependent effective mass (PDM) background in the non-relativistic regime via the PDM free Pauli equation. To investigate whether there is a PDM quantum system with the same physics (analogous model) that a charge-dyon system (target system), we resort to the PDM free Pauli equation itself. We proceed with replacing the exact bi-spinor of the target system into this equation, obtaining an uncoupled system of non-linear partial differential equations for the mass distribution $M$. We were able to solve them numerically for $M$ considering a radial dependence only, i.e., $M=M(r)$, fixing $θ_0$, and considering specific values of $μ$ and $m$ satisfying a certain condition. We present the solutions graphically, and from them, we determine the respective effective potentials, which actually represent our analogous models. We study the mapping for eigenvalues starting from the minimal value $j = μ- 1/2$.

quant-ph↗

arXiv2004

$ε$-Expansion for non-planar double-boxes

We present calculations for non-planar double-box with four massless/massive external/internal legs/propagators. The results are expressed for arbitrary exponents of propagators and dimension in terms of Lauricella's hypergeometric functions of three variables and hypergeometric-like multiple series.

hep-ph↗

arXiv2002

Green functions for generalized point interactions in 1D: A scattering approach

Recently, general point interactions in one dimension has been used to model a large number of different phenomena in quantum mechanics. Such potentials, however, requires some sort of regularization to lead to meaningful results. The usual ways to do so rely on technicalities which may hide important physical aspects of the problem. In this work we present a new method to calculate the exact Green functions for general point interactions in 1D. Our approach differs from previous ones because it is based only on physical quantities, namely, the scattering coefficients, $R$ and $T$, to construct $G$. Renormalization or particular mathematical prescriptions are not invoked. The simple formulation of the method makes it easy to extend to more general contexts, such as for lattices of $N$ general point interactions; on a line; on a half-line; under periodic boundary conditions; and confined in a box.

quant-ph↗

arXiv2002

General Formula for N-point One-loop Feynman Integrals

The negative dimensional integration method (NDIM) is a technique where several difficulties concerning loop integration can be overcome. From usual covariant gauges to complicated Coulomb gauge integrals, and even the trickiest light-cone integrals one can apply the methodology of NDIM. In this work we show how to construct a general formula -- we mean arbitrary exponents of propagators, off-shell external momenta and distinct massive propagators -- for one-loop scalar integrals, for {\it covariant} gauges, and apply it to one through six-point loop integrals. We present detailed analysis of pentagon and hexagon scalar integrals for massive/massless internal particles being external momenta on or off mass shell.

hep-ph↗

arXiv2000

Light-cone gauge integrals: Prescriptionlessness at two loops

The only calculations performed beyond one-loop level in the light-cone gauge make use of the Mandelstam-Leibbrandt (ML) prescription in order to circumvent the notorious gauge dependent poles. Recently we have shown that in the context of negative dimensional integration method (NDIM) such prescription can be altogether abandoned, at least in one-loop order calculations. We extend our approach, now studying two-loop integrals pertaining to two-point functions. While previous works on the subject present only divergent parts for the integrals, we show that our prescriptionless method gives the same results for them, besides finite parts for arbitrary exponents of propagators.

hep-th↗

arXiv2000

First results for the Coulomb gauge integrals using NDIM

The Coulomb gauge has at least two advantadges over other gauge choices in that bound states between quarks and studies of confinement are easier to understand in this gauge. However, perturbative calculations, namely Feynman loop integrations are not well-defined (there are the so-called energy integrals) even within the context of dimensional regularization. Leibbrandt and Williams proposed a possible cure to such a problem by splitting the space-time dimension into $D=ω+ρ$, i.e., introducing a specific one parameter $ρ$ to regulate the energy integrals. The aim of our work is to apply negative dimensional integration method (NDIM) to the Coulomb gauge integrals using the recipe of split-dimension parameters and present complete results -- finite and divergent parts -- to the one and two-loop level for arbitrary exponents of propagators and dimension.

hep-th↗

arXiv2000

Loop integrals in three outstanding gauges: Feynman, Light-cone and Coulomb

We apply negative dimensional integration method (NDIM) to three outstanding gauges: Feynman, light-cone and Coulomb gauges. Our aim is to show that NDIM is a very suitable technique to deal with loop integrals, being them originated from any gauge choice. In Feynman gauge we perform scalar two-loop four-point massless integrals; in the light-cone gauge we calculate scalar two-loop integrals contributing for two-point functions without any kind of prescriptions, since NDIM can abandon such devices -- this calculation is the first test of our prescriptionless method beyond one-loop order; finally, for the Coulomb gauge we consider a four propagator massless loop integral, in the split dimensional regularization context.

hep-th↗

arXiv1999

Solutions for a massless off-shell two-loop three-point vertex

Negative dimensional integration method (NDIM) seems to be a very promising technique for evaluating massless and/or massive Feynman diagrams. It is unique in the sense that the method simultaneously gives solutions in different regions of external momenta. Moreover, it is a technique whereby the difficulties associated with performing parametric integrals --- the standard approach --- are transferred to a simpler solving of a system of linear algebraic equations. Employing this method, we calculate a massless two-loop three point vertex with all the external legs off-shell. Then NDIM approach allows us to obtain twenty-one distinct new power series representations for the integral in question. In order to verify the correctness of our results, we consider five particular cases where either two of the external legs are put on-shell, or one of them amputated or one exponent of the propagators is set to zero, and compare our results thus obtained with the ones calculated with standard methods in positive dimension.

hep-th↗

arXiv1999

Prescriptionless light-cone integrals

Perturbative quantum gauge field theory seen within the perspective of physical gauge choices such as the light-cone entails the emergence of troublesome poles of the type $(k\cdot n)^{-α}$ in the Feynman integrals, and these come from the boson field propagator, where $α= 1,2,...$ and $n^μ$ is the external arbitrary four-vector that defines the gauge proper. This becomes an additional hurdle to overcome in the computation of Feynman diagrams, since any graph containing internal boson lines will inevitably produce integrands with denominators bearing the characteristic gauge-fixing factor. How one deals with them has been the subject of research for over decades, and several prescriptions have been suggested and tried in the course of time, with failures and successes. However, a more recent development in this front which applies the negative dimensional technique to compute light-cone Feynman integrals shows that we can altogether dispense with prescriptions to perform the calculations. An additional bonus comes attached to this new technique in that not only it renders the light-cone prescriptionless, but by the very nature of it, can also dispense with decomposition formulas or partial fractioning tricks used in the standard approach to separate pole products of the type $(k\cdot n)^{-α}[(k-p)\cdot n]^{-β}$, $(β= 1,2,...)$. In this work we demonstrate how all this can be done.

hep-th↗

arXiv1997

An easy way to solve two-loop vertex integrals

Negative dimensional integration is a step further dimensional regularization ideas. In this approach, based on the principle of analytic continuation, Feynman integrals are polynomial ones and for this reason very simple to handle, contrary to the usual parametric ones. The result of the integral worked out in $D<0$ must be analytically continued again --- of course --- to real physical world, $D>0$, and this step presents no difficulties. We consider four two-loop three-point vertex diagrams with arbitrary exponents of propagators and dimension. These original results give the correct well-known particular cases where the exponents of propagators are equal to unity.

hep-th↗

arXiv1997

Negative Dimensional Integration for Massive Four Point Functions--II: New Solutions

In this sequel calculation of the one-loop Feynman integral pertaining to a massive box diagram contributing to the photon-photon scattering amplitude in quantum electrodynamics, we present the six solutions as yet unknown in the literature. These six new solutions arise quite naturally in the context of negative dimensional integration approach, revealing a promising technique to handle Feynman integrals.

hep-th↗

arXiv1997

Two-loop self-energy diagrams worked out with NDIM

In this work we calculate two two-loop massless Feynman integrals pertaining to self-energy diagrams using NDIM (Negative Dimensional Integration Method). We show that the answer we get is 36-fold degenerate. We then consider special cases of exponents for propagators and the outcoming results compared with known ones obtained via traditional methods.

hep-th↗