65 research outputs found
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,
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, .Comment: 17 pp, 3 PS figures, CERN-TH-7044/9
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.Comment: 12pp (3 PS figures available upon request), ENSLAPP-A-412/9
Two-loop Compton and annihilation processes in thermal QCD
We calculate the Compton and annihilation production of a soft static lepton
pair in a quark-gluon plasma in the two-loop approximation. We work in the
context of the effective perturbative expansion based on the resummation of
hard thermal loops. Double counting is avoided by subtracting appropriate
counterterms. It is found that the two-loop diagrams give contributions of the
same order as the one-loop diagram. Furthermore, these contributions are
necessary to obtain agreement with the naive perturbative expansion in the
limit of vanishing thermal masses.Comment: Latex, 24 pages, postscript figures included with the package
graphic
Non-Analytic Vertex Renormalization of a Bose Gas at Finite Temperature
We derive the flow equations for the symmetry unbroken phase of a dilute
3-dimensional Bose gas. We point out that the flow equation for the interaction
contains parts which are non-analytic at the origin of the frequency-momentum
space. We examine the way this non-analyticity affects the fixed point of the
system of the flow equations and shifts the value of the critical exponent for
the correlation length closer to the experimental result in comparison with
previous work where the non-analyticity was neglected. Finally, we emphasize
the purely thermal nature of this non-analytic behaviour comparing our approach
to a previous work where non-analyticity was studied in the context of
renormalization at zero temperature.Comment: 21 pages, 4 figure
Collective fermionic excitations in systems with a large chemical potential
We study fermionic excitations in a cold ultrarelativistic plasma. We
construct explicitly the quantum states associated with the two branches which
develop in the excitation spectrum as the chemical potential is raised. The
collective nature of the long wavelength excitations is clearly exhibited.
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