Search results for "PROPAGATOR"

The master two-loop two-point function. The general case

1991

Abstract We present a new calculation of the two-loop two-point function. Avoiding standard techniques such as Feynman parametrization and Wick rotation we end up with a simple double integral representation valid for arbitrary mass-cases. Numerical and analytical checks confirm our result.

2014

We present a unified framework for the description of the interaction of fast electrons with complex nanostructures based on the Green dyadic method. We show that the computation of a generalized field propagator yields the electron energy losses and cathodoluminescence of nano-objects of arbitrary morphologies embedded in complex dielectric media. Spectra and maps for both penetrating and non-penetrating electron trajectories are provided. This numerical approach can be extended to describe complex experiments involving fast electrons and optically excited nanostructures.

Field propagator of a dressed junction: Fluorescence lifetime calculations in a confined geometry

1997

The study of the fluorescence phenomenon by near-field optical techniques requires one to describe precisely the spontaneous emission change occurring when the fluorescing particle is placed in a complex optical environment. For this purpose, the field susceptibility (also called the field propagator) of a planar junction formed by a cavity bounded by two semi-infinite bodies with arbitrary optical constant is derived within the framework of linear-response theory. The field propagator associated with the junction is then modified in a self-consistent manner to account for the presence of any arbitrary object inside the junction. As a first illustration the alteration of the fluorescence li…

The spiked harmonic oscillatorV(r)=r 2+λr −4 as a challenge to perturbation theory

1991

The standard weak- and strong-coupling perturbation series are interpreted as extreme special cases of expansions obtainable within the framework of Rayleigh-Schroedinger perturbation theory with non-diagonal propagators and unspecified zero-order energies. The formalism of the latter type is then tested by our strongly singular example. It proves suitable for applications in the domain of virtually arbitrary couplings. A few related technicalities and especially the quadruple problem of convergence are also discussed.

Mechanisms in theC12(γ,pn)and(γ,pp)Reactions

1998

${}^{12}\mathrm{C}(\ensuremath{\gamma},pn)$ and $(\ensuremath{\gamma},pp)$ cross sections have been measured in kinematics which emphasize direct $2N$ knockout for ${E}_{\ensuremath{\gamma}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}120--400\mathrm{MeV}$ and ${\ensuremath{\theta}}_{p}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}23\ifmmode^\circ\else\textdegree\fi{}--152\ifmmode^\circ\else\textdegree\fi{}$. The cross sections for both reactions peak strongly in the $\ensuremath{\Delta}$-resonance region. Theoretical calculations that account for the medium dependence of the $\ensuremath{\Delta}$ propagator predict a similar photon energy dependence to the data and describe the …

Schwinger mechanism in linear covariant gauges

2016

In this work we explore the applicability of a special gluon mass generating mechanism in the context of the linear covariant gauges. In particular, the implementation of the Schwinger mechanism in pure Yang-Mills theories hinges crucially on the inclusion of massless bound-state excitations in the fundamental nonperturbative vertices of the theory. The dynamical formation of such excitations is controlled by a homogeneous linear Bethe-Salpeter equation, whose nontrivial solutions have been studied only in the Landau gauge. Here, the form of this integral equation is derived for general values of the gauge-fixing parameter, under a number of simplifying assumptions that reduce the degree of…

Abelian projection and studies of gauge-variant quantities in lattice QCD without gauge fixing

1996

We suggest a new (dynamical) Abelian projection of the lattice QCD. It contains no gauge condition imposed on gauge fields so that Gribov copying is avoided. Configurations of gauge fields that turn into monopoles in the Abelian projection can be classified in a gauge invariant way. In the continuum limit, the theory respects the Lorentz invariance. A similar dynamical reduction of the gauge symmetry is proposed for studies of gauge-variant correlators (like a gluon propagator) in lattice QCD. Though the procedure is harder for numerical simulations, it is free of gauge fixing artifacts, like the Gribov horizon and copies.

Gauge-independent approach to resonant transition amplitudes

1996

We present a new gauge-independent approach to resonant transition amplitudes with nonconserved external currents, based on the pinch technique method. In the context of $2\to 2$ and $2\to 3$ scattering processes, we show explicitly that the analytic results derived respect $U(1)_{em}$ gauge symmetry and do not depend on the choice of the $SU(2)_L$ gauge fixing. Our analytic approach treats, on equal footing, fermionic as well as bosonic contributions to the resummed gauge boson propagators, does not contain any residual space-like threshold terms, shows the correct high-energy unitarity behaviour, admits renormalization, and satisfies a number of other required properties, including the op…

Yang-Mills two-point functions in linear covariant gauges

2015

In this work we use two different but complementary approaches in order to study the ghost propagator of a pure SU(3) Yang-Mills theory quantized in the linear covariant gauges, focusing on its dependence on the gauge-fixing parameter $\xi$ in the deep infrared. In particular, we first solve the Schwinger-Dyson equation that governs the dynamics of the ghost propagator, using a set of simplifying approximations, and under the crucial assumption that the gluon propagators for $\xi>0$ are infrared finite, as is the case in the Landau gauge $(\xi=0)$. Then we appeal to the Nielsen identities, and express the derivative of the ghost propagator with respect to $\xi$ in terms of certain auxiliary…

On the Physical Propagators of QED

1993

The true variables in QED are the transverse photon components and Dirac's physical electron, constructed out of the fermionic field and the longitudinal components of the photon. We calculate the propagators in terms of these variables to one loop and demonstrate their gauge invariance. The physical electron propagator is shown not to suffer from infrared divergences in any gauge. In general, all physical Green's functions are gauge invariant and infrared-finite.