Feynman Schwinger Research Articles

Feynman’s propagator in Schwinger’s picture of Quantum Mechanics

A novel derivation of Feynman’s sum-over-histories construction of the quantum propagator using the groupoidal description of Schwinger picture of Quantum Mechanics is presented. It is shown that such construction corresponds to the GNS representation of a natural family of states called Dirac–Feynman–Schwinger (DFS) states. Such states are obtained from a q-Lagrangian function [Formula: see text] on the groupoid of configurations of the system. The groupoid of histories of the system is constructed and the q-Lagrangian [Formula: see text] allows us to define a DFS state on the algebra of the groupoid. The particular instance of the groupoid of pairs of a Riemannian manifold serves to illustrate Feynman’s original derivation of the propagator for a point particle described by a classical Lagrangian L.

Non-perturbative Methods in Relativistic Field Theory

This talk reviews relativistic methods used to compute bound and low energy scattering states in field theory, with emphasis on approaches that John Tjon and I discussed (and argued about) together. I compare the Bethe–Salpeter and Covariant Spectator equations, show some applications, and then report on some of the things we have learned from the beautiful Feynman–Schwinger technique for calculating the exact sum of all ladder and crossed ladder diagrams in field theory.

Nonperturbative dynamics of scalar field theories through the Feynman-Schwinger representation

In this paper we present a summary of results obtained for scalar field theories using the Feynman-Schwinger (FSR) approach. Specifically, scalar QED and chi^2phi theories are considered. The motivation behind the applications discussed in this paper is to use the FSR method as a rigorous tool for testing the quality of commonly used approximations in field theory. Exact calculations in a quenched theory are presented for one-, two-, and three-body bound states. Results obtained indicate that some of the commonly used approximations, such as Bethe-Salpeter ladder summation for bound states and the rainbow summation for one body problems, produce significantly different results from those obtained from the FSR approach. We find that more accurate results can be obtained using other, simpler, approximation schemes.

The Feynman–Schwinger Representation in QCD

The proper time path integral representation is derived explicitly for Green's functions in QCD. After an introductory analysis of perturbative properties, the total gluonic field is separated in a rigorous way into a nonperturbative background and valence gluon part. For nonperturbative contributions the background perturbation theory is used systematically, yielding two types of expansions, illustrated by direct physical applications. As an application, we discuss the collinear singularities in the Feynman–Schwinger representation formalism. Moreover, the generalization to nonzero temperature is made and expressions for partition functions in perturbation theory and nonperturbative background are explicitly written down.

Feynman–Schwinger representation method for bound states

In nuclear and particle physics one is often faced with problems where perturbation theory is not applicable. An example of this is the description of bound states. Therefore, an exact solution of field theory to all orders is an unavoidable and interesting problem. Path integrals provide a framework for exact solutions in field theory. In this talk I will present an economical method of evaluating path integrals using the Feynman-Schwinger representation (FSR).

Nonperturbative evaluation of the few-body states for scalar [formula omitted] interaction

A knowledge of nonpertubative propagators is often needed when the standard perturbative methods are not applicable. An example of this is the bound state problem in field theory. While a nonperturbative result is valuable by itself, it is also an important guide for those who work on developing phenomenological models for the nonperturbative problem. The Feynman–Schwinger representation approach provides a convenient framework for calculating nonperturbative propagators. In this paper we provide an algorithm for computing 1-, 2-, and 3-body bound states with the inclusion of all self energies, vertex corrections, ladder and crossed ladder exchanges. The calculation is done in the quenched approximation by ignoring the matter loops. We provide simulation results for 1-, 2- and 3-body states.

Classical radiation reaction off-shell corrections to the covariant Lorentz force

We show that the problem of radiation reaction may be formulated in a space of five dimensions, with five corresponding gauge fields in the framework of the classical version of a fully gauge covariant form of the Stueckelberg–Feynman–Schwinger covariant mechanics (the zero mode fields of the 0,1,2,3 components correspond to the Maxwell fields). The particles and fields are not confined to their mass shells. We show that in the mass-shell limit, the generalized Lorentz force obtained by means of the retarded Green's functions for the five-dimensional field equations provides the classical Abraham–Lorentz–Dirac radiation reaction terms (with renormalized mass and charge). We also obtain general coupled equations for the orbit and the off-shell dynamical mass during the evolution as well as an autonomous nonlinear equation of third order for the off-shell mass. The theory does not admit radiation if the particle does not move off-shell. The structure of the equations implies that the mass-shell deviation is bounded when the external field is removed.

Polyakov's spin factor and new algorithms for efficient perturbative computations in QCD

Polyakov's spin factor enters as a weight in the path-integral description of particle-like modes propagating in Euclidean space–times, accounting for particle spin. The Fock–Feynman–Schwinger path integral applied to QCD accommodates Polyakov's spin factor in a natural manner while, at the same time, it identifies Wilson line (loop) operators as sole agents of interaction dynamics among matter and gauge field quanta. A direct application of such a separation between spin content and dynamics is the emergence of master expressions for the perturbative series involving either open or closed fermionic lines which provide new, comprehensive approaches to perturbative QCD.

Radiation Reaction of the Classical Off-Shell Relativistic Charged Particle

It has been shown by Gupta and Padmanabhan that the radiation reaction force of the Abraham–Lorentz–Dirac equation can be obtained by a coordinate transformation from the inertial frame of an accelerating charged particle to that of the laboratory. We show that the problem may be formulated in a flat space of five dimensions, with five corresponding gauge fields in the framework of the classical version of a fully gauge covariant form of the Stueckelberg–Feynman–Schwinger covariant mechanics (the zero mode fields of the 0, 1, 2, 3 components correspond to the Maxwell fields). Without additional constraints, the particles and fields are not confined to their mass shells. We show that in the mass-shell limit, the generalized Lorentz force obtained by means of the retarded Green's functions for the five dimensional field equations provides the classical Abraham–Lorentz–Dirac radiation reaction terms (with renormalized mass and charge). We also obtain general coupled equations for the orbit and the off-shell dynamical mass during the evolution as well as an autonomous non-linear equation of third order for the off-shell mass. The theory does not admit radiation if the particle does not move off-shell. The structure of the equations implies that mass-shell deviation is bounded when the external field is removed.

Feynman-Schwinger technique in field theories

In these lectures we introduce the Feynman-Schwinger representation method for solving nonperturbative problems in field theory. As an introduction we first give a brief overview of integral equations and path integral methods for solving nonperturbative problems. Then we discuss the Feynman-Schwinger (FSR) representation method with applications to scalar interactions. The FSR approach is a continuum path integral integral approach in terms of covariant trajectories of particles. Using the exact results provided by the FSR approach we test the reliability of commonly used approximations for nonperturbative summation of interactions for few body systems.

High-momentum asymptotics from the Fock–Feynman–Schwinger path integral

The asymptotics of n-point Green's function at large external momenta is obtained in the exponentiated from using the Fock–Feynman–Schwinger representation for propagators in the external field. The method is applied to gauge theories such as QCD and QED, and the Sudakov form-factor is calculated as an example in QED and meson form-factor in QCD. Nonperturbative contributions can be conveniently included, as it is demonstrated in the example of the confinement correction to the form-factor.

Some exact solutions of reduced scalar Yukawa theory

The scalar Yukawa model, in which a complex scalar field, ϕ, interacts via a real scalar field, χ, is reduced by using covariant Green functions. It is shown that exact few-particle eigenstates of the truncated QFT Hamiltonian can be obtained in the Feshbach–Villars formulation if an unorthodox "empty" vacuum state is used. Analytic solutions for the two-body case are obtained for massless chion exchange in 3+1 dimensions and for massive chion exchange in 1+1 dimensions. Comparison is made to ladder Bethe–Salpeter, Feynman–Schwinger, and quasipotential results for massive chion exchange in 3+1. Equations for the three-body case are also obtained. PACS Nos.: 11.10.Ef, 11.10.Qr, and 03.70.+k

A quantum field theoretical approach to plasmas

By making use of the Feynman–Schwinger formalism of quantum electrodynamics, the classical dispersion relation for electrostatic and electromagnetic waves in a homogeneous plasma is derived. It is then argued that such techniques can be helpful in dealing with quasi-linear and nonlinear problems in plasmas.