arXiv subjects

R. Kobes

Publications and source records attributed to R. Kobes.

32 records · Page 2Linked to original sources

arXiv2000

Spontaneous Scale Symmetry Breaking in 2+1-Dimensional QED at Both Zero and Finite Temperature

A complete analysis of dynamical scale symmetry breaking in 2+1-dimensional QED at both zero and finite temperature is presented by looking at solutions to the Schwinger-Dyson equation. In different kinetic energy regimes we use various numerical and analytic techniques (including an expansion in large flavour number). It is confirmed that, contrary to the case of 3+1 dimensions, there is no dynamical scale symmetry breaking at zero temperature, despite the fact that chiral symmetry breaking can occur dynamically. At finite temperature, such breaking of scale symmetry may take place.

hep-ph↗

arXiv2000

Shear viscosity in $ϕ^4$ theory from an extended ladder resummation

We study shear viscosity in weakly coupled hot $ϕ^4$ theory using the CTP formalism . We show that the viscosity can be obtained as the integral of a three-point function. Non-perturbative corrections to the bare one-loop result can be obtained by solving a decoupled Schwinger-Dyson type integral equation for this vertex. This integral equation represents the resummation of an infinite series of ladder diagrams which contribute to the leading order result. It can be shown that this integral equation has exactly the same form as the Boltzmann equation. We show that the integral equation for the viscosity can be reexpressed by writing the vertex as a combination of polarization tensors. An expression for this polarization tensor can be obtained by solving another Schwinger-Dyson type integral equation. This procedure results in an expression for the viscosity that represents a non-perturbative resummation of contributions to the viscosity which includes certain non-ladder graphs, as well as the usual ladders. We discuss the motivation for this resummation. We show that these resummations can also be obtained by writing the viscosity as an integral equation involving a single four-point function. Finally, we show that when the viscosity is expressed in terms of a four-point function, it is possible to further extend the set of graphs included in the resummation by treating vertex and propagator corrections self-consistently. We discuss the significance of such a self-consistent resummation and show that the integral equation contains cancellations between vertex and propagator corrections.

hep-ph↗

arXiv1999

Four-Point Spectral Functions and Ward Identities in Hot QED

We derive spectral representations for the different components of the 4-point function at finite temperature in the real time formalism in terms of five real spectral densities. We explicitly calculate all these functions in QED in the hard thermal loop approximation. The Ward identities obeyed by the 1-loop 3- and 4-point functions in real time and their spectral functions are derived. We compare our results with those derived previously in the imaginary-time formalism for retarded functions in hot QCD, and we discuss the generalization of our results to non-equilibrium situations.

hep-ph↗

arXiv1999

Approach to Equilibrium in the Micromaser

We examine the approach to equilibrium of the micromaser. Analytic methods are first used to show that for large times (i.e. many atoms) the convergence is governed by the next to leading eigenvalue of the corresponding discrete evolution matrix. The model is then studied numerically. The numerical results confirm the phase structure expected from analytic approximation methods and agree for large times with the analysis of Elmfors et al in terms of the continuous master equation. For short times, however, we see evidence for interesting new structure not previously reported in the literature.

physics.atom-ph↗

arXiv1999

A New Formulation of a 1+1 Dimensional Field Theory Constrained to a Box

We consider a 1+1 dimensional field theory constrained to a finite box of length L. Traditionally, calculations in a box are done by replacing the integrals over the spatial momenta by discrete sums and then evaluating sums and doing analytic continuations. We show that it is also possible to do such calculations using an analogy to finite temperature field theory. We develop a formalism that is similar to the closed time path formulation of finite temperature field theory. Our technique can be used to calculate spatially retarded green functions, without evaluating sums or doing analytic continuations. We calculate the self energy in a simple scalar theory as an example.

hep-th↗

arXiv1998

Bremsstrahlung and photon production in thermal QCD

In this paper, we extend the study of bremsstrahlung photon production in a quark-gluon plasma to the cases of soft static photons and hard real photons. The general framework of this study is the effective perturbative expansion based on the resummation of hard thermal loops. Despite the fact that bremsstrahlung only comes at two loops, we find that in both cases it generates contributions of the same order of magnitude as those already calculated by several other groups at one loop. Furthermore, a new process contained in the two-loop diagrams dominate the emission of a very hard real photon. In all cases, the rate of real or virtual photon production in the plasma is appreciably increased compared to the one-loop predictions.

hep-ph↗

arXiv1997

The general cancellation of ladder graphs at finite temperature

In some cases, an important example being at finite temperature, extreme infrared, collinear, or light-cone behaviour may cause the usual loop expansion to break down. For some of these cases higher order ladder graphs can become important. In an earlier paper it was shown that, given a particular relation between a vertex and a self-energy function, the resummation of the ladder graphs simplifies significantly when other types of graphs are included in a consistent effective expansion. In this paper we show that this assumed relation is valid for a large class of vertex and self-energy functions at finite temperature.

hep-ph↗

arXiv1997

Cancellation of ladder graphs in an effective expansion

A resummation of ladder graphs is important in cases where infrared, collinear, or light-cone singularities render the loop expansion invalid, especially at high temperature where these effects are often enhanced. It has been noted in some recent examples of this resummation that the ladder graphs are canceled by other types of terms. In this note we show that this cancellation is quite general, and for the most part algebraic.

hep-ph↗

arXiv1996

Breakdown of the Hard Thermal Loop expansion near the light-cone

We discuss the bremsstrahlung production of soft real and virtual photons in a quark-gluon plasma at thermal equilibrium beyond the Hard Thermal Loop (HTL) resummation. The physics is controlled by the ratio $Q^2/q_0^2$ of the virtuality to the energy. When $Q^2/q_0^2$ is much smaller than $g^2$, where $g$ is the strong coupling constant, the emission rate is enhanced by a factor $1/g^2$ over the HTL results due to light-cone singularities and the bremsstrahlung is induced by scattering of the quark via both transverse and longitudinal soft gluon exchanges. When $Q^2/q_0^2$ increases, the enhancement factor is given by $q_0^2/Q^2$. When this ratio is near unity, the bremsstrahlung contribution is of the same order as the rate predicted by the HTL resummation. In that case, the bremsstrahlung is induced by both soft and hard gluon exchanges.

hep-ph↗

arXiv1996

Enhanced photon production rate on the light--cone

Recent studies of the high temperature soft photon production rate on the light--cone using Braaten--Pisarski resummation techniques have found collinear divergences present. We show that there exist a class of terms outside the Braaten--Pisarski framework which, although also divergent, dominate over these previously considered terms. The divergences in these new terms may be alleviated by application of a recently developed resummation scheme for processes sensitive to the light--cone.

hep-ph↗

arXiv1995

Finite size effects in metallic superlattice systems

Clean metallic superlattice systems composed of alternating layers of superconducting and normal materials are considered, particularly aspects of the proximity effect as it affects the critical temperature. A simple model is used to address the question of when a finite--sized system theoretically approximates well a true infinite superlattice. The methods used in the analysis afford some tests of the approximation used that the pair amplitude of the Cooper pairs is constant over a superconducting region. We also use these methods to construct a model of a single superconducting layer which intends to incorporate a more realistic form of the pair amplitude than a simple constant.

cond-mat↗

arXiv1994

Parameterization Invariance and the Resolution of the Unitary Gauge Puzzle

We examine the calculation of the critical temperature $T_c$ for the restoration of a spontaneously broken symmetry. Motivated by a set of recently developed gauge dependence identities, we give a gauge and parameterization independent definition of this temperature in terms of the physical mass of the scalar particle as determined by the propagator pole. As an explicit illustration, we consider the Abelian Higgs model in the unitary gauge, where the usual definition of the critical temperature based upon the effective potential leads to an erroneous result. We show how the gauge invariant definition reproduces the correct result as found in the ``renormalizable'' parameterizations.

hep-ph↗

arXiv1994

Transformations of real-time finite-temperature Feynman rules

We consider transformations of the $2\times2$ propagator matrix in real-time finite-temperature field theory, resulting in transformed $n$--point functions. As special cases of such a transformation we examine the Keldysh basis, the retarded/advanced $RA$ basis, and a Feynman-like $F\bar F$ basis, which differ in this context as to how ``economically'' certain constraints on the original propagator matrix elements are implemented. We also obtain the relation between some of these real-time functions and certain analytic continuations of the imaginary-time functions. Finally, we compare some aspects of these bases which arise in practical calculations.

hep-ph↗

arXiv1994

On the Damping Rate of a Fast Fermion in Hot QED

The self-consistent determination of the damping rate of a fast moving fermion in a hot QED plasma is reexamined. We argue how a detailed investigation of the analytic properties of the retarded fermion Green's function motivated by the cutting rules at finite temperature may resolve ambiguities related to the proper definition of the mass-shell condition.

hep-ph↗