arXiv subjects

Bernard Silvestre-Brac

Publications and source records attributed to Bernard Silvestre-Brac.

14 recordsLinked to original sources

arXiv2012

The auxiliary field method in quantum mechanics

The auxiliary field method is a new technique to obtain closed formulae for the solutions of eigenequations in quantum mechanics. The idea is to replace a Hamiltonian $H$ for which analytical solutions are not known by another one $\tilde H$, including one or more auxiliary fields. For instance, a potential $V(r)$ not solvable is replaced by another one $P(r)$ more familiar, or a semirelativistic kinetic part is replaced by an equivalent nonrelativistic one. The approximation comes from the replacement of the auxiliary fields by pure real constants. The approximant solutions for $H$, eigenvalues and eigenfunctions, are then obtained by the solutions of $\tilde H$ in which the auxiliary parameters are eliminated by an extremization procedure for the eigenenergies. If $H=T(\bm p)+V(r)$ and if $P(r)$ is a power law, the approximate eigenvalues can be written $T(p_0)+V(r_0)$, where the mean impulsion $p_0$ is a function of the mean distance $r_0$ and where $r_0$ is determined by an equation which is linked to the generalized virial theorem. The general properties of the method are studied and the connections with the envelope theory presented. This method is first applied to nonrelativistic and semirelativistic two-body systems, with a great variety of potentials. Closed formulae are produced for energies, eigenstates, various observables and critical constants, with sometimes a very good accuracy. The method is then used to solve nonrelativistic and semirelativistic many-body systems with one-body and two-body interactions. For such cases, analytical solutions can only be obtained for systems of identical particles, but several systems of interest for atomic and hadronic physics are studied. General results concerning the many-body critical constants are presented, as well as duality relations existing between approximate and exact eigenvalues.

quant-ph↗

arXiv2011

Duality relations in the auxiliary field method

The eigenenergies $ε^{(N)}(m;\{n_i,l_i\})$ of a system of $N$ identical particles with a mass $m$ are functions of the various radial quantum numbers $n_i$ and orbital quantum numbers $l_i$. Approximations $E^{(N)}(m;Q)$ of these eigenenergies, depending on a principal quantum number $Q(\{n_i,l_i\})$, can be obtained in the framework of the auxiliary field method. We demonstrate the existence of numerous exact duality relations linking quantities $E^{(N)}(m;Q)$ and $E^{(p)}(m';Q')$ for various forms of the potentials (independent of $m$ and $N$) and for both nonrelativistic and semirelativistic kinematics. As the approximations computed with the auxiliary field method can be very close to the exact results, we show with several examples that these duality relations still hold, with sometimes a good accuracy, for the exact eigenenergies $ε^{(N)}(m;\{n_i,l_i\})$.

quant-ph↗

arXiv2010

Eigenstates with the auxiliary field method

The auxiliary field method is a powerful technique to obtain approximate closed-form energy formulas for eigenequations in quantum mechanics. Very good results can be obtained for Schrödinger and semirelativistic Hamiltonians with various potentials, even in the case of many-body problems. This method can also provide approximate eigenstates in terms of well known wavefunctions, for instance harmonic oscillator or hydrogen-like states, but with a characteristic size which depends on quantum numbers. In this paper, we consider two-body Schrödinger equations with linear, logarithmic and exponential potentials and show that analytical approximations of the corresponding eigenstates can be obtained with the auxiliary field method, with a very good accuracy in some cases.

quant-ph↗

arXiv2010

The quantum N-body problem and the auxiliary field method

Approximate analytical energy formulas for N-body relativistic Hamiltonians with one- and two-body interactions are obtained within the framework of the auxiliary field method. This method has already been proved to be a powerful technique in the case of two-body problems. A general procedure is given and applied to various Hamiltonians of interest, in atomic and hadronic physics in particular. A test of formulas is performed for baryons described as a three-quark system.

math-ph↗

arXiv2009

Semirelativistic Hamiltonians and the auxiliary field method

Approximate analytical closed energy formulas for semirelativistic Hamiltonians of the form $σ\sqrt{\bm p^{2}+m^2}+V(r)$ are obtained within the framework of the auxiliary field method. This method, which is equivalent to the envelope theory, has been recently proposed as a powerful tool to get approximate analytical solutions of the Schrödinger equation. Various shapes for the potential $V(r)$ are investigated: power-law, funnel, square root, and Yukawa. A comparison with the exact results is discussed in detail.

quant-ph↗

arXiv2009

Auxiliary field method for the square root potential

Using the auxiliary field method, we give an analytical expression for the eigenenergies of a system composed of two non-relativistic particles interacting via a potential of type $\sqrt{a^2 r^2 + b}$. This situation is usual in the case of hybrid mesons in which the quark-antiquark pair evolves in an excited gluonic field. Asymptotic expressions are proposed and the approximate results are compared to the exact ones. It is shown that the accuracy is excellent.

math-ph↗

arXiv2008

Auxiliary field method and analytical solutions of the Schrödinger equation with exponential potentials

The auxiliary field method is a new and efficient way to compute approximate analytical eigenenergies and eigenvectors of the Schrödinger equation. This method has already been successfully applied to the case of central potentials of power-law and logarithmic forms. In the present work, we show that the Schrödinger equation with exponential potentials of the form $-αr^λ\exp(-βr)$ can also be analytically solved by using the auxiliary field method. Formulae giving the critical heights and the energy levels of these potentials are presented. Special attention is drawn on the Yukawa potential and the pure exponential one.

quant-ph↗

arXiv2008

Extensions of the auxiliary field method to solve Schrödinger equations

It has recently been shown that the auxiliary field method is an interesting tool to compute approximate analytical solutions of the Schrödinger equation. This technique can generate the spectrum associated with an arbitrary potential $V(r)$ starting from the analytically known spectrum of a particular potential $P(r)$. In the present work, general important properties of the auxiliary field method are proved, such as scaling laws and independence of the results on the choice of $P(r)$. The method is extended in order to find accurate analytical energy formulae for radial potentials of the form $a P(r)+V(r)$, and several explicit examples are studied. Connections existing between the perturbation theory and the auxiliary field method are also discussed.

quant-ph↗

arXiv2007

Spin dependent operators in correlated gaussian bases

In their textbook, Suzuki and Varga [Y. Suzuki and K. Varga, {\em Stochastic Variational Approach to Quantum-Mechanical Few-Body Problems} (Springer, Berlin, 1998)] present the stochastic variational method with the correlated Gaussian basis in a very exhaustive way. The matrix elements for central potentials are put under a pleasant form but the elements for spin dependent operators, when treated, are given as very cumbersome expressions. In this paper, we find a lot of new formulae for those elements. Their expressions are given in terms of the same geometrical functions that appear in the case of central potentials. These functions get therefore a universal status; this property is very useful for numerical applications.

physics.comp-ph↗

arXiv2007

The few-body problem in terms of correlated gaussians

In their textbook, Suzuki and Varga [Y. Suzuki and K. Varga, {\em Stochastic Variational Approach to Quantum-Mechanical Few-Body Problems} (Springer, Berlin, 1998)] present the stochastic variational method in a very exhaustive way. In this framework, the so-called correlated gaussian bases are often employed. General formulae for the matrix elements of various operators can be found in the textbook. However the Fourier transform of correlated gaussians and their application to the management of a relativistic kinetic energy operator are missing and cannot be found in the literature. In this paper we present these interesting formulae. We give also a derivation for new formulations concerning central potentials; the corresponding formulae are more efficient numerically than those presented in the textbook.

hep-ph↗

arXiv2004

Universality of Regge and vibrational trajectories in a semiclassical model

The orbital and radial excitations of light-light mesons are studied in the framework of the dominantly orbital state description. The equation of motion is characterized by a relativistic kinematics supplemented by the usual funnel potential with a mixed scalar and vector confinement. The influence of finite quark masses and potential parameters on Regge and vibrational trajectories is discussed. The case of heavy-light mesons is also presented.

hep-ph↗

arXiv2003

Auxiliary fields and hadron dynamics

The relations existing between the auxiliary field (einbein field) formalism and the spinless Salpeter equation are studied in the case of two particles with the same mass, interacting via a confining potential. The problem of non-orthogonality for radial excited states in the auxiliary field formalism is discussed and found to be non-crucial. It is shown that the classical equations of motion of the rotating string model, derived from the QCD lagrangian, reduce exactly to the classical equations of motion of the phenomenological semirelativistic flux tube model, provided all auxiliary fields are eliminated correctly from the rotating string hamiltonian.

hep-ph↗

arXiv2003

The baryonic Y-shape confining potential energy and its approximants

We discuss the validity of replacing the complicated three-body confinement operator of the Y string junction type by three kinds of approximation which are numerically much simpler to handle: a one-body operator with the junction point at the centre of mass, a two-body operator corresponding to half the perimeter of the triangle formed by the three particles, and the average of both. Two different approaches for testing the quality of the approximations are proposed: a geometrical treatment based on the comparison of the potential energy strengths for the various inter quark distances, and a dynamical treatment based on the comparison of the corresponding effective string tensions using a hyperspherical approach. Both procedures give very similar results. It is shown how to simulate the genuine string junction operator by the approximations proposed above. Exact three-body calculations are presented in order to compare quantitatively the various approximations and to confirm our analysis.

hep-ph↗

arXiv1998

Isospin Mass Splittings of Baryons in Potential Models

We discuss the isospin-breaking mass differences among baryons, with particular attention in the charm sector to the $Σ_c^{+}-Σ_c^0$, $Σ_c^{++}-Σ_c^0$, and $Ξ_c^+-Ξ_c^0$ splittings. Simple potential models cannot accommodate the trend of the available data on charmed baryons. More precise measurements would offer the possibility of testing how well potential models describe the non-perturbative limit of QCD.

hep-ph↗