215 research outputs found
Self-duality, helicity and background field loopology
I show that helicity plays an important role in the development of rules for
computing higher loop effective Lagrangians. Specifically, the two-loop
Heisenberg-Euler effective Lagrangian in quantum electrodynamics is remarkably
simple when the background field has definite helicity (i.e., is self-dual).
Furthermore, the two-loop answer can be derived essentially algebraically, and
is naturally expressed in terms of one-loop quantities. This represents a
generalization of the familiar ``integration-by-parts'' rules for manipulating
diagrams involving free propagators to the more complicated case where the
propagators are those for scalars or spinors in the presence of a background
field.Comment: 12 pages; 1 figure; Plenary talk at QCD2004, Minnesot
QED Effective Actions in Inhomogeneous Backgrounds: Summing the Derivative Expansion
The QED effective action encodes nonlinear interactions due to quantum vacuum
polarization effects. While much is known for the special case of electrons in
a constant electromagnetic field (the Euler-Heisenberg case), much less is
known for inhomogeneous backgrounds. Such backgrounds are more relevant to
experimental situations. One way to treat inhomogeneous backgrounds is the
"derivative expansion", in which one formally expands around the soluble
constant-field case. In this talk I use some recent exactly soluble
inhomogeneous backgrounds to perform precision tests on the derivative
expansion, to learn in what sense it converges or diverges. A closely related
question is to find the exponential correction to Schwinger's pair-production
formula for a constant electric field, when the electric background is
inhomogeneous.Comment: 8 pages, talk at QED2000, Trieste (October 2000
Heisenberg-Euler Effective Lagrangians : Basics and Extensions
I present a pedagogical review of Heisenberg-Euler effective Lagrangians,
beginning with the original work of Heisenberg and Euler, and Weisskopf, for
the one loop effective action of quantum electrodynamics in a constant
electromagnetic background field, and then summarizing some of the important
applications and generalizations to inhomogeneous background fields, nonabelian
backgrounds, and higher loop effective Lagrangians.Comment: 82 pages; to appear in Ian Kogan Memorial Collection, ``From Fields
to Strings: Circumnavigating Theoretical Physics'
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