3,195 research outputs found
A General Expression for Symmetry Factors of Feynman Diagrams
The calculation of the symmetry factor corresponding to a given Feynman
diagram is well known to be a tedious problem. We have derived a simple formula
for these symmetry factors. Our formula works for any diagram in scalar theory
( and interactions), spinor QED, scalar QED, or QCD.Comment: RevTex 11 pages with 10 figure
Spontaneous Symmetry Breaking for Scalar QED with Non-minimal Chern-Simons Coupling
We investigate the two-loop effective potential for both minimally and
non-minimally coupled Maxwell-Chern-Simons theories. The non-minimal gauge
interaction represents the magnetic moment interaction between a charged scalar
and the electromagnetic field. In a previous paper we have shown that the two
loop effective potential for this model is renormalizable with an appropriate
choice of the non-minimal coupling constant. We carry out a detailed analysis
of the spontaneous symmetry breaking induced by radiative corrections. As long
as the renormalization point for all couplings is chosen to be the true minimum
of the effective potential, both models predict the presence of spontaneous
symmetry breaking. Two loop corrections are small compared to the one loop
result, and thus the symmetry breaking is perturbatively stable.Comment: Revtex 25 pages, 9 figure
Results from the 4PI Effective Action in 2- and 3-dimensions
We consider a symmetric scalar theory with quartic coupling and solve the
equations of motion from the 4PI effective action in 2- and 3-dimensions using
an iterative numerical lattice method. For coupling less than 10 (in
dimensionless units) good convergence is obtained in less than 10 iterations.
We use lattice size up to 16 in 2-dimensions and 10 in 3-dimensions and
demonstrate the convergence of the results with increasing lattice size. The
self-consistent solutions for the 2-point and 4-point functions agree well with
the perturbative ones when the coupling is small and deviate when the coupling
is large.Comment: 14 pages, 11 figures; v5: added numerical calculations in 3D; version
accepted for publication in EPJ
Temperature dependent anisotropy of the penetration depth and coherence length in MgB$_2
We report measurements of the temperature dependent anisotropies
( and ) of both the London penetration depth
and the upper critical field of MgB. Data for
was obtained from measurements of
and on a single crystal sample using a tunnel diode
oscillator technique. was
deduced from field dependent specific heat measurements on the same sample.
and have opposite temperature dependencies, but
close to tend to a common value (). These results are in good agreement with theories
accounting for the two gap nature of MgBComment: 4 pages with figures (New version
Covariant approach to equilibration in effective field theories
The equilibration of two coupled reservoirs is studied using a Green function
approach which is suitable for future development with the closed time path
method. The problem is solved in two parameterizations, in order to demonstrate
the non-trivial issues of parameterization in both the intermediate steps and
the interpretation of physical quantities. We use a covariant approach to find
self-consistent solutions for the statistical distributions as functions of
time. We show that by formally introducing covariant connections, one can
rescale a slowly varying non-equilibrium theory so that it appears to be an
equilibrium one, for the purposes of calculation. We emphasize the importance
of properly tracking variable redefinitions in order to correctly interpret
physical quantities.Comment: 11 pages, Late
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