56 research outputs found
Lifting the Gribov ambiguity in Yang-Mills theories
We report on the work presented in Phys. Lett. B712 (2012) 97, where a new
one-parameter family of Landau gauges has been proposed for Yang-Mills
theories, inspired by an analogy with disordered systems in condensed matter
physics. This is based on a particular average over Gribov copies which avoids
the Neuberger zero problem of the standard Fadeev-Popov construction. The
proposed gauge fixing can be formulated as a local renormalizable field theory
in four dimensions and is well-suited for analytical calculations. A remarkable
feature is that, for what concerns the calculation of ghost and gauge field
correlators, the gauged-fixed action is perturbatively equivalent to a simple
massive extension of the Faddeev-Popov action. The renormalization group flow
of the theory admits infrared safe trajectories, with no Landau pole. The
one-loop calculations of Yang-Mills two-point correlators show remarkable
agreement with lattice simulations all the way to the deep infrared.Comment: Contribution to the Xth conference on Quark Confinement and the
Hadron Spectrum, October 8-12, 2012, TUM Campus Garching, Munich, Germany; 8
pages, 2 figure
Renormalization group flow and symmetry restoration in de Sitter space
We compute the renormalization group flow of O(N) scalar field theories in de
Sitter space using nonperturbative renormalization group techniques in the
local potential approximation. We obtain the flow of the effective potential on
superhorizon scales for arbitrary space-time dimension D=d+1. We show that, due
to strong infrared fluctuations, the latter is qualitatively similar to the
corresponding one in Euclidean space with D=0. It follows that spontaneously
broken symmetries are radiatively restored in any space-time dimension and for
any value of N.Comment: 5 pages; 1 figure, minor corrections (published version
Nonperturbative infrared enhancement of nonGaussian correlators in de Sitter space
We compute the four-point correlation function of a light O(N) scalar field
in de Sitter space in the large-N limit. For superhorizon momentum modes,
infrared effects strongly enhance the size of loop contributions. We find that
in the deep infrared limit, the latter are of the same order as the tree-level
one. The tree-level momentum structure, characteristic of a contact term, gets
renormalized by a factor of order unity. In addition loop contributions give
rise to a new momentum structure, characteristic of an exchange diagram,
corresponding to the exchange of an effective composite scalar degree of
freedom.Comment: 7 pages, 5 figures. Material added. To appear in Phys. Lett.
Infrared dynamics in de Sitter space from Schwinger-Dyson equations
We study the two-point correlator of an O(N) scalar field with quartic
self-coupling in de Sitter space. For light fields in units of the expansion
rate, perturbation theory is plagued by large logarithmic terms for
superhorizon momenta. We show that a proper treatment of the infinite series of
self-energy insertions through the Schwinger-Dyson equations resums these
infrared logarithms into well defined power laws. We provide an exact
analytical solution of the Schwinger-Dyson equations for infrared momenta when
the self-energy is computed at two-loop order. The obtained correlator exhibits
a rich structure with a superposition of free-field-like power laws. We extract
mass and field-strength renormalization factors from the asymptotic infrared
behavior. The latter are nonperturbative in the coupling in the case of a
vanishing tree-level mass.Comment: 9 pages, 3 figures, published version, some material has been added,
title change
Influence of Gribov ambiguities in a class of nonlinear covariant gauges
We consider Yang-Mills theories in a recently proposed family of nonlinear
covariant gauges that consistently deals with the issue of Gribov ambiguities.
Such gauges provide a generalization of the Curci-Ferrari-Delbourgo-Jarvis
gauges which can be formulated as an extremization procedure and might be
implemented in numerical calculations. This would allow for nonperturbative
studies of Yang-Mills correlators in a broad class of covariant gauges
continuously connected to the well-studied Landau gauge. We compute the ghost
and gluon propagators in the continuum formulation at one-loop order in
perturbation theory and we study their momentum dependence down to the deep
infrared regime, with and without renormalization group improvement. In
particular, we show that the theory admits infrared-safe renormalization-group
trajectories with no Landau pole. Both the gluon and the ghost behave as
massive fields at low energy, and the gluon propagator is transverse even away
from the Landau gauge limit. We compare our results to those obtained in the
usual Curci-Ferrari model, which allows us to pinpoint the specific effects
arising from our treatment of Gribov copies.Comment: 23 pages. 24 figures. One suplemental material: Mathematica file with
the full expressions of the one-loop Feynman diagram
The Likelihood of DCC Formation
We estimate the probability that a disoriented chiral condensate forms during
the spherical expansion of a hot medium described by the linear sigma model.Comment: 12 pages, latex2e, 2 figures in postscript, uses (included)
boldgreek.sty and standard epsf packages, reference adde
Small parameters in infrared QCD: The pion decay constant
We continue our investigation of the QCD dynamics in terms of the
Curci-Ferrari effective Lagrangian, a deformation of the Faddeev-Popov one in
the Landau gauge with a tree-level gluon mass term. In a previous work we have
studied the dynamics of chiral symmetry breaking at the level of the quark
propagator and, in particular, the dynamical generation of a constituent quark
mass. In the present article, we study the associated Goldstone mode, the pion,
and we compute the pion decay constant in the chiral limit. Our approach
exploits the fact that the coupling (defined in the Taylor scheme) in the pure
gauge sector is perturbative, as observed in lattice simulations which,
together with a -expansion, allows for a systematic, controllable
approximation scheme in the low energy regime of QCD. At leading order, this
leads to the well-known rainbow-ladder resummation. We study the region of
parameter space of the model that gives physical values of the pion decay
constant. This allows one to constrain the gluon mass parameter as a function
of the coupling using a physically measured quantity.Comment: 21 pages, 11 figure
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