118 research outputs found
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
Progress in Nonequilibrium Quantum Field Theory
We review recent developments for the description of far-from-equilibrium
dynamics of quantum fields and subsequent thermalization.Comment: To appear in the proceedings of Strong and Electroweak Matter (SEWM
2002), Heidelberg, Germany, 2-5 Oct 2002, 16 pages, 11 figure
Kinetic equilibration in heavy ion collisions: The role of elastic processes
We study the kinetic equilibration of gluons produced in the very early
stages of a high energy heavy ion collision in a ``self-consistent'' relaxation
time approximation. We compare two scenarios describing the initial state of
the gluon system, namely the saturation and the minijet scenarios, both at RHIC
and LHC energies. We argue that, in order to characterize kinetic
equilibration, it is relevant to test the isotropy of various observables. As a
consequence, we find in particular that in both scenarios elastic processes are
not sufficient for the system to reach kinetic equilibrium at RHIC energies.
More generally, we show that, contrary to what is often assumed in the
literature, elastic collisions alone are not sufficient to rapidly achieve
kinetic equilibration. Because of longitudinal expansion at early times, the
actual equilibration time is at least of the order of a few fermis.Comment: 10 pages, 2 tables, 5 figures, references added, submitted to JHE
Parametric resonance in quantum field theory
We present the first study of parametric resonance in quantum field theory
from a complete next-to-leading order calculation in a 1/N-expansion of the 2PI
effective action, which includes scattering and memory effects. We present a
complete numerical solution for an O(N)-symmetric scalar theory and provide an
approximate analytic description of the nonlinear dynamics in the entire
amplification range. We find that the classical resonant amplification at early
times is followed by a collective amplification regime with explosive particle
production in a broad momentum range, which is not accessible in a
leading-order calculation.Comment: 4 pages, 5 figures, version to appear in Phys. Rev. Lett., results
unchange
Quantum properties of a non-Gaussian state in the large-N approximation
We study the properties of a non-Gaussian density matrix for a O(N) scalar
field in the context of the incomplete description picture. This is of
relevance for studies of decoherence and entropy production in quantum field
theory. In particular, we study how the inclusion of the simplest non-Gaussian
correlator in the set of measured observables modifies the effective (Gaussian)
description one can infer from the knowledge of the two-point functions only.
We compute exactly the matrix elements of the non-Gaussian density matrix at
leading order in a 1/N-expansion. This allows us to study the quantum
properties (purity, entropy, coherence) of the corresponding state for
arbitrarily strong nongaussianity. We find that if the Gaussian and the
non-Gaussian observers essentially agree concerning quantum purity or
correlation entropy, their conclusion can significantly differ for other, more
detailed aspects such as the degree of quantum coherence of the system.Comment: 14 pages, 7 figures. Published version (Phys. Rev. D, minor
corrections
Yang-Mills correlators across the deconfinement phase transition
We compute the finite temperature ghost and gluon propagators of Yang-Mills
theory in the Landau-DeWitt gauge. The background field that enters the
definition of the latter is intimately related with the (gauge-invariant)
Polyakov loop and serves as an equivalent order parameter for the deconfinement
transition. We use an effective gauge-fixed description where the
nonperturbative infrared dynamics of the theory is parametrized by a gluon mass
which, as argued elsewhere, may originate from the Gribov ambiguity. In this
scheme, one can perform consistent perturbative calculations down to infrared
momenta, which have been shown to correctly describe the phase diagram of
Yang-Mills theories in four dimensions as well as the zero-temperature
correlators computed in lattice simulations. In this article, we provide the
one-loop expressions of the finite temperature Landau-DeWitt ghost and gluon
propagators for a large class of gauge groups and present explicit results for
the SU(2) case. These are substantially different from those previously
obtained in the Landau gauge, which corresponds to a vanishing background
field. The nonanalyticity of the order parameter across the transition is
directly imprinted onto the propagators in the various color modes. In the
SU(2) case, this leads, for instance, to a cusp in the electric and magnetic
gluon susceptibilities as well as similar signatures in the ghost sector. We
mention the possibility that such distinctive features of the transition could
be measured in lattice simulations in the background field gauge studied here.Comment: 28 pages, 17 figures; published versio
Deconfinement transition in SU(N) theories from perturbation theory
We consider a simple massive extension of the Landau-DeWitt gauge for SU()
Yang-Mills theory. We compute the corresponding one-loop effective potential
for a temporal background gluon field at finite temperature. At this order the
background field is simply related to the Polyakov loop, the order parameter of
the deconfinement transition. Our perturbative calculation correctly describes
a quark confining phase at low temperature and a phase transition of second
order for and weakly first order for . Our estimates for the
transition temperatures are in qualitative agreement with values from lattice
simulations or from other continuum approaches. Finally, we discuss the
effective gluon mass parameter in relation to the Gribov ambiguities of the
Landau-DeWitt gauge.Comment: 10 pages, 3 figure
Yang-Mills correlators at finite temperature: A perturbative perspective
We consider the two-point correlators of Yang-Mills theories at finite
temperature in the Landau gauge. We employ a model for the corresponding
Yang-Mills correlators based on the inclusion of an effective mass term for
gluons. The latter is expected to have its origin in the existence of Gribov
copies. One-loop calculations at zero temperature have been shown to agree
remarkably well with the corresponding lattice data. We extend on this and
perform a one-loop calculation of the Matsubara gluon and ghost two-point
correlators at finite temperature. We show that, as in the vacuum, an effective
gluon mass accurately captures the dominant infrared physics for the magnetic
gluon and ghost propagators. It also reproduces the gross qualitative features
of the electric gluon propagator. In particular, we find a slight nonmonotonous
behavior of the Debye mass as a function of temperature, however not as
pronounced as in existing lattice results. A more quantitative description of
the electric sector near the deconfinement phase transition certainly requires
another physical ingredient sensitive to the order parameter of the transition.Comment: 16 pages, 12 figures ; Published version (PRD
Two-loop study of the deconfinement transition in Yang-Mills theories: SU(3) and beyond
We study the confinement-deconfinement phase transition of pure Yang-Mills
theories at finite temperature using a simple massive extension of standard
background field methods. We generalize our recent next-to-leading-order
perturbative calculation of the Polyakov loop and of the related background
field effective potential for the SU(2) theory to any compact and connex Lie
group with a simple Lie algebra. We discuss in detail the SU(3) theory, where
the two-loop corrections yield improved values for the first-order transition
temperature as compared to the one-loop result. We also show that certain
one-loop artifacts of thermodynamical observables disappear at two-loop order,
as was already the case for the SU(2) theory. In particular, the entropy and
the pressure are positive for all temperatures. Finally, we discuss the groups
SU(4) and Sp(2) which shed interesting light, respectively, on the relation
between the (de)confinement of static matter sources in the various
representations of the gauge group and on the use of the background field
itself as an order parameter for confinement. In both cases, we obtain
first-order transitions, in agreement with lattice simulations and other
continuum approaches.Comment: 35 pages, 20 figure
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