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E. Petitgirard

Publications and source records attributed to E. Petitgirard.

8 recordsLinked to original sources
arXiv1998

Infrared and light-cone limit in hot QED

In hot gauge theories a breakdown of the hard thermal loop expansion occurs for light-like external momenta or in the infrared region. In QED where a resummation of ladder diagrams is usually advocated, it is shown that long range magnetic interations involve a broader set of graphs. The consequence is a generalized compensation of the hard modes damping terms at leading order in the infrared limit and near the light-cone. The relevance of the so-called improved hard thermal loop resummation scheme is discussed.

hep-ph↗
arXiv1998

Hot QED beyond ladder graphs

At finite temperature a breakdown of the hard thermal loop expansion arises whenever external momenta are light-like or tend to very soft scales. A resummation of ladder graphs is important in these cases where the effects of infrared or light-cone singularities are enhanced. We show that in hot QED another class of diagrams is also relevant at leading order due to long range magnetic interactions and therefore recent studies about ladder expansions need to be corrected. A general cancellation of the hard modes damping effects still occurs near the light-cone or in the infrared region. The validity of an improved version of the hard thermal loop resummation scheme is discussed.

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↗
arXiv1993

Axion Emission from Red Giants and White Dwarfs

Using thermal field theory methods, we recalculate axion emission from dense plasmas. We study in particular the Primakoff and the bremsstrahlung processes. The Primakoff rate is significantly suppressed at high densities, when the electrons become relativistic. However, the bound on the axion-photon coupling, $G<10^{-10}$ GeV, is unaffected, as it is constrained by the evolution of HB stars, which have low densities. In contradistinction, the same relativistic effects enhance the bremsstrahlung processes. From the red giants and white dwarfs evolution, we obtain a conservative bound on the axion-electron coupling, $g_{ae} < 2\times 10^{-13}$.

hep-ph↗
arXiv1992

Photon Propagation in Dense Media

Using thermal field theory, we derive simple analytic expressions for the spectral density of photons in degenerate QED plasmas, without assuming the usual non or ultra-relativistic limit. We recover the standard results in both cases. Although very similar in ultra-relativistic plasmas, transverse and longitudinal excitations behave very differently as the electron Fermi momentum decreases.

hep-ph↗