192 research outputs found
Derivatives as an IR Regulator for Massless Fields
The free propagator for the scalar --theory is calculated
exactly up to the second derivative of a background field. Using this
propagator I compute the one--loop effective action, which then contains all
powers of the field but with at most two derivatives acting on each field. The
standard derivative expansion, which only has a finite number of derivatives in
each term, breaks down for small fields when the mass is zero, while the
expression obtained here has a well--defined expansion in . In this way
the resummation of derivatives cures the naive IR divergence. The extension to
finite temperature is also discussed.Comment: Late
Temperature Renormalization Group and Resummation
The temperature renormalization group equation (TRGE) is compared with a
diagrammatic expansion for the -theory. It is found that the
one-loop TRGE resums the leading powers of temperature for the effective mass.
A two-loop contribution to TRGE is required to do the leading resummation for
the coupling constant. It is also shown that the higher order TRGE resums
subleading powers of temperature.Comment: 17pp, LATEX and FEYNMAN, NORDITA 92/63
Dispersion relations from the Hard Thermal Loop effective action in a magnetic field
Dispersion relations for fermions at high temperature and in a background
magnetic field are calculated in two different ways. First from a
straightforward one-loop calculation where, in the weak field limit, we find an
expression closely related to the standard dispersion relations in the absence
of the magnetic field. Secondly, we derive the dispersion relations directly
from the Hard Thermal Loop effective action, which allows for an exact solution
(i.e. to all orders in the external field), up to the last numerical integrals.Comment: Latex+epsf with uuencoded ps-figure
Neutrino self-energy in a magnetized medium in arbitrary -gauge
We calculate the one-loop neutrino self-energy in a magnetized plasma to all
orders in the magnetic field. The calculation is done in a general gauge. We
obtain the dispersion relation and effective potential for neutrinos in a
CP-symmetric plasma under various conditions, and show that, while the
self-energy depends on the gauge parameter , the dispersion relation and
effective potential to leading order are independent of it.Comment: 13 pages, RevTeX, epsfig, axodra
QED effective action at finite temperature
The QED effective Lagrangian in the presence of an arbitrary constant
electromagnetic background field at finite temperature is derived in the
imaginary-time formalism to one-loop order. The boundary conditions in
imaginary time reduce the set of gauge transformations of the background field,
which allows for a further gauge invariant and puts restrictions on the choice
of gauge. The additional invariant enters the effective action by a topological
mechanism and can be identified with a chemical potential; it is furthermore
related to Debye screening. In concordance with the real-time formalism, we do
not find a thermal correction to Schwinger's pair-production formula. The
calculation is performed on a maximally Lorentz covariant and gauge invariant
stage.Comment: 9 pages, REVTeX, 1 figure, typos corrected, references added, final
version to appear in Phys. Rev.
Thermally induced photon splitting
We calculate thermal corrections to the non-linear QED effective action for
low-energy photon interactions in a background electromagnetic field. The
high-temperature expansion shows that at the vacuum contribution is
exactly cancelled to all orders in the external field except for a non-trivial
two-point function contribution. The high-temperature expansion derived reveals
a remarkable cancellation of infrared sensitive contributions. As a result
photon-splitting in the presence of a magnetic field is suppressed in the
presence of an electron-positron QED-plasma at very high temperatures. In a
cold and dense plasma a similar suppression takes place. At the same time
Compton scattering dominates for weak fields and the suppression is rarely
important in physical situations.Comment: 15 pages, 2 ps figures, Late
Light Cone Condition for a Thermalized QED Vacuum
Within the QED effective action approach, we study the propagation of
low-frequency light at finite temperature. Starting from a general effective
Lagrangian for slowly varying fields whose structure is solely dictated by
Lorentz covariance and gauge invariance, we derive the light cone condition for
light propagating in a thermalized QED vacuum. As an application, we calculate
the velocity shifts, i.e., refractive indices of the vacuum, induced by
thermalized fermions to one loop. We investigate various temperature domains
and also include a background magnetic field. While low-temperature effects to
one loop are exponentially damped by the electron mass, there exists a maximum
velocity shift of in the
intermediate-temperature domain .Comment: 9 pages, 3 figures, REVTeX, typos corrected, final version to appear
in Phys. Rev.
Atomic beam correlations and the quantum state of the micromaser
Correlation measurements on the states of two-level atoms having passed
through a micromaser at different times can be used to infer properties of the
quantum state of the radiation field in the cavity. Long(short) correlation
length in time is to some extent associated with super(sub)-Poissonian photon
statistics. The correlation length is also an indicator of a phase structure
much richer than what is revealed by the usual single-time observables, like
the atomic inversion or the Mandel quality factor. In realistic experimental
situations the correlations may extend over many times the decay time of the
cavity. Our assertions are verified by comparing theoretical calculations with
a high-precision Monte-Carlo simulation of the micromaser system.Comment: 4 pages, styles: aps, latex, times, epsf, More physical insight
added, title and figures changed, more references. The paper can be retrieved
as compressed file called elmfors.maser.ps.Z from
http://connect.nbi.dk/pub/lautrup/ or via anonymous ftp at
ftp://connect.nbi.dk/pub/lautrup
Thermal Fermionic Dispersion Relations in a Magnetic Field
The thermal self-energy of an electron in a static uniform magnetic field
is calculated to first order in the fine structure constant and to
all orders in . We use two methods, one based on the Furry picture and
another based on Schwinger's proper-time method. As external states we consider
relativistic Landau levels with special emphasis on the lowest Landau level. In
the high-temperature limit we derive self-consistent dispersion relations for
particle and hole excitations, showing the chiral asymmetry caused by the
external field. For weak fields, earlier results on the ground- state energy
and the anomalous magnetic moment are discussed and compared with the present
analysis. In the strong-field limit the appearance of a field-independent
imaginary part of the self-energy, related to Landau damping in the
plasma, is pointed out.Comment: Latex+FEYNMAN.tex. 5 figures and special files are submitted using
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