430 research outputs found
Resummation in hot field theories
We consider a scalar theory at finite temperature in the 2PI resummation
scheme, including phi^3 and phi^4 interactions. Already at the one loop level
in this scheme, we have to deal with a non local approximation. We carry out
the renormalization and obtain finite equations for the propagator. Within this
model we can explore the effect of non local contributions to the self-energy
in the evaluation of thermodynamic quantities.Comment: 5 pages, 3 figures, contribution to the conference Strong and
Electroweak Matter (SEWM2002), Heidelberg, Germany, October 2-5, 200
Renormalization and gauge symmetry for 2PI effective actions
We explore a method to recover symmetry identities in the 2PI formalism. It
is based on non-perturbative approximations to the 1PI effective action. We
discuss renormalization questions raised by this technique.Comment: 5 pages, 3 figures. To appear in the conference proceedings of Strong
Electroweak Matter 2004 (SEWM04), Helsinki, Finland. 16-19 Jun 200
Non-perturbative renormalization of Phi-derivable approximations in theories with multiple fields
We provide a renormalization procedure for Phi-derivable approximations in
theories coupling different types of fields. We illustrate our approach on a
scalar phi^4 theory coupled to fermions via a Yukawa-like interaction. The
non-perturbative renormalization amounts to fixing the scalar coupling via a
set of nested Bethe-Salpeter equations coupling fermions to scalars.Comment: 4 pages, poster contribution at the Workshop on Quark Gluon Plasma
Thermalization, Vienna, August 10-12, 2005 Minor changes: added reference and
definitio
Renormalization out of equilibrium in a superrenormalizable theory
We discuss the renormalization of the initial value problem in Nonequilibrium
Quantum Field Theory within a simple, yet instructive, example and show how to
obtain a renormalized time evolution for the two-point functions of a scalar
field and its conjugate momentum at all times. The scheme we propose is
applicable to systems that are initially far from equilibrium and compatible
with non-secular approximation schemes which capture thermalization. It is
based on Kadanoff-Baym equations for non-Gaussian initial states, complemented
by usual vacuum counterterms. We explicitly demonstrate how various
cutoff-dependent effects peculiar to nonequilibrium systems, including
time-dependent divergences or initial-time singularities, are avoided by taking
an initial non-Gaussian three-point vacuum correlation into account.Comment: 5 pages, 2 figure
A critical look at the role of the bare parameters in the renormalization of Phi-derivable approximations
We revisit the renormalization of Phi-derivable approximations from a
slightly different point of view than the one which is usually followed in
previous works. We pay particular attention to the question of the existence of
a solution to the self-consistent equation that defines the two-point function
in the Cornwall-Jackiw-Tomboulis formalism and to the fact that some of the
ultraviolet divergences which appear if one formally expands the solution in
powers of the bare coupling do not always appear as divergences at the level of
the solution itself. We discuss these issues using a particular truncation of
the Phi functional, namely the simplest truncation which brings non-trivial
momentum and field dependence to the two-point function.Comment: 30 pages, 12 figure
2PI renormalized effective action for gauge theories
We show how to perform renormalization in the framework of the 2PI effective
action for abelian gauge theories. In addition to the usual renormalization
conditions one needs to incorporate new ones in order to remove non-transverse
UV divergences in the truncated two- and four-photon functions. The
corresponding counterterms are allowed by gauge symmetry, in-medium independent
and suppressed with respect to the accuracy of the truncation.Comment: 4 pages, to appear in the proceedings of Strong and Electroweak
Matter 2006, BNL, New-York, May 200
2PI effective action for gauge theories: Renormalization
We discuss the application of two-particle-irreducible (2PI) functional
techniques to gauge theories, focusing on the issue of non-perturbative
renormalization. In particular, we show how to renormalize the photon and
fermion propagators of QED obtained from a systematic loop expansion of the 2PI
effective action. At any finite order, this implies introducing new
counterterms as compared to the usual ones in perturbation theory. We show that
these new counterterms are consistent with the 2PI Ward identities and are
systematically of higher order than the approximation order, which guarantees
the convergence of the approximation scheme. Our analysis can be applied to any
theory with linearly realized gauge symmetry. This is for instance the case of
QCD quantized in the background field gauge.Comment: 21 pages, 8 figures. Uses JHEP3.cl
Thermodynamics and phase transition of the O(N) model from the two-loop Phi-derivable approximation
We discuss the thermodynamics of the O(N) model across the corresponding
phase transition using the two-loop Phi-derivable approximation of the
effective potential and compare our results to those obtained in the literature
within the Hartree-Fock approximation. In particular, we find that in the
chiral limit the transition is of the second order, whereas it was found to be
of the first order in the Hartree-Fock case. These features are manifest at the
level of the thermodynamical observables. We also compute the thermal sigma and
pion masses from the curvature of the effective potential. In the chiral limit,
this guarantees that the Goldstone's theorem is obeyed in the broken phase. A
realistic parametrization of the model in the N=4 case, based on the vacuum
values of the curvature masses, shows that a sigma mass of around 450 MeV can
be obtained. The equations are renormalized after extending our previous
results for the N=1 case by means of the general procedure described in [J.
Berges et al., Annals Phys. 320, 344-398 (2005)]. When restricted to the
Hartree-Fock approximation, our approach reveals that certain problems raised
in the literature concerning the renormalization are completely lifted.
Finally, we introduce a new type of Phi-derivable approximation in which the
gap equation is not solved at the same level of accuracy as the accuracy at
which the potential is computed. We discuss the consistency and applicability
of these types of "hybrid" approximations and illustrate them in the two-loop
case by showing that the corresponding effective potential is renormalizable
and that the transition remains of the second order.Comment: 26 pages, 9 figures, uses RevTeX4-1, published versio
Loss of solution in the symmetry improved Phi-derivable expansion scheme
We consider the two-loop Phi-derivable approximation for the O(2)-symmetric
scalar model, augmented by the symmetry improvement introduced in [A. Pilaftsis
and D. Teresi, Nucl. Phys. B874, 594 (2013)], which enforces Goldstone's
theorem in the broken phase. Although the corresponding equations admit a
solution in the presence of a large enough infrared (IR) regulating scale, we
argue that, for smooth ultraviolet (UV) regulators, the solution is lost when
the IR scale becomes small enough. Infrared regular solutions exist for certain
non-analytic UV regulators, but we argue that these solutions are artifacts
which should disappear when the sensitivity to the UV regulator is removed by a
renormalization procedure. The loss of solution is observed both at zero and at
finite temperature, although it is simpler to identify in the latter case. We
also comment on possible ways to cure this problem.Comment: 20 pages, 7 figures, uses elsarticle, published versio
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