18,500 research outputs found
QCD effective charges from lattice data
We use recent lattice data on the gluon and ghost propagators, as well as the
Kugo-Ojima function, in order to extract the non-perturbative behavior of two
particular definitions of the QCD effective charge, one based on the pinch
technique construction, and one obtained from the standard ghost-gluon vertex.
The construction relies crucially on the definition of two dimensionful
quantities, which are invariant under the renormalization group, and are built
out of very particular combinations of the aforementioned Green's functions.
The main non-perturbative feature of both effective charges, encoded in the
infrared finiteness of the gluon propagator and ghost dressing function used in
their definition, is the freezing at a common finite (non-vanishing) value, in
agreement with a plethora of theoretical and phenomenological expectations. We
discuss the sizable discrepancy between the freezing values obtained from the
present lattice analysis and the corresponding estimates derived from several
phenomenological studies, and attribute its origin to the difference in the
gauges employed. A particular toy calculation suggests that the modifications
induced to the non-perturbative gluon propagator by the gauge choice may indeed
account for the observed deviation of the freezing values.Comment: 23 pages, 7 figure
Infrared finite effective charge of QCD
We show that the gauge invariant treatment of the Schwinger-Dyson equations
of QCD leads to an infrared finite gluon propagator, signaling the dynamical
generation of an effective gluon mass, and a non-enhanced ghost propagator, in
qualitative agreement with recent lattice data. The truncation scheme employed
is based on the synergy between the pinch technique and the background field
method. One of its most powerful features is that the transversality of the
gluon self-energy is manifestly preserved, exactly as dictated by the BRST
symmetry of the theory. We then explain, for the first time in the literature,
how to construct non-perturbatively a renormalization group invariant quantity
out of the conventional gluon propagator. This newly constructed quantity
serves as the natural starting point for defining a non-perturbative effective
charge for QCD, which constitutes, in all respects, the generalization in a
non-Abelian context of the universal QED effective charge. This strong
effective charge displays asymptotic freedom in the ultraviolet, while in the
low-energy regime it freezes at a finite value, giving rise to an infrared
fixed point for QCD. Some possible pitfalls related to the extraction of such
an effective charge from infrared finite gluon propagators, such as those found
on the lattice, are briefly discussed.Comment: Invited talk given at LIGHT CONE 2008 Relativistic Nuclear and
Particle Physics, July 7-11 2008 Mulhouse, Franc
Nonperturbative gluon and ghost propagators for d=3 Yang-Mills
We study a manifestly gauge invariant set of Schwinger-Dyson equations to
determine the nonperturbative dynamics of the gluon and ghost propagators in
Yang-Mills. The use of the well-known Schwinger mechanism, in the Landau
gauge, leads to the dynamical generation of a mass for the gauge boson (gluon
in ), which, in turn, gives rise to an infrared finite gluon propagator
and ghost dressing function. The propagators obtained from the numerical
solution of these nonperturbative equations are in very good agreement with the
results of lattice simulations.Comment: 25 pages, 8 figure
The gluon mass generation mechanism: a concise primer
We present a pedagogical overview of the nonperturbative mechanism that
endows gluons with a dynamical mass. This analysis is performed based on pure
Yang-Mills theories in the Landau gauge, within the theoretical framework that
emerges from the combination of the pinch technique with the background field
method. In particular, we concentrate on the Schwinger-Dyson equation satisfied
by the gluon propagator and examine the necessary conditions for obtaining
finite solutions within the infrared region. The role of seagull diagrams
receives particular attention, as do the identities that enforce the
cancellation of all potential quadratic divergences. We stress the necessity of
introducing nonperturbative massless poles in the fully dressed vertices of the
theory in order to trigger the Schwinger mechanism, and explain in detail the
instrumental role of these poles in maintaining the Becchi-Rouet-Stora-Tyutin
symmetry at every step of the mass-generating procedure. The dynamical equation
governing the evolution of the gluon mass is derived, and its solutions are
determined numerically following implementation of a set of simplifying
assumptions. The obtained mass function is positive definite, and exhibits a
power law running that is consistent with general arguments based on the
operator product expansion in the ultraviolet region. A possible connection
between confinement and the presence of an inflection point in the gluon
propagator is briefly discussed.Comment: 37 pages, 11 figures. Based on the talk given at the Workshop
Dyson-Schwinger equations in modern mathematics and physics, ECT* (Trento)
22-26 September 2014. Review article contribution to the special issue of
Frontiers of Physics (Eds. M. Pitschmann and C. D. Roberts
Schwinger mechanism in linear covariant gauges
In this work we explore the applicability of a special gluon mass generating
mechanism in the context of the linear covariant gauges. In particular, the
implementation of the Schwinger mechanism in pure Yang-Mills theories hinges
crucially on the inclusion of massless bound-state excitations in the
fundamental nonperturbative vertices of the theory. The dynamical formation of
such excitations is controlled by a homogeneous linear Bethe-Salpeter equation,
whose nontrivial solutions have been studied only in the Landau gauge. Here,
the form of this integral equation is derived for general values of the
gauge-fixing parameter, under a number of simplifying assumptions that reduce
the degree of technical complexity. The kernel of this equation consists of
fully-dressed gluon propagators, for which recent lattice data are used as
input, and of three-gluon vertices dressed by a single form factor, which is
modelled by means of certain physically motivated Ans\"atze. The
gauge-dependent terms contributing to this kernel impose considerable
restrictions on the infrared behavior of the vertex form factor; specifically,
only infrared finite Ans\"atze are compatible with the existence of nontrivial
solutions. When such Ans\"atze are employed, the numerical study of the
integral equation reveals a continuity in the type of solutions as one varies
the gauge-fixing parameter, indicating a smooth departure from the Landau
gauge. Instead, the logarithmically divergent form factor displaying the
characteristic "zero crossing", while perfectly consistent in the Landau gauge,
has to undergo a dramatic qualitative transformation away from it, in order to
yield acceptable solutions. The possible implications of these results are
briefly discussed.Comment: 27 pages, 9 figures; v2: typos corrected, version matching the
published on
Unified description of seagull cancellations and infrared finiteness of gluon propagators
We present a generalized theoretical framework for dealing with the important
issue of dynamical mass generation in Yang-Mills theories, and, in particular,
with the infrared finiteness of the gluon propagators, observed in a multitude
of recent lattice simulations. Our analysis is manifestly gauge-invariant, in
the sense that it preserves the transversality of the gluon self-energy, and
gauge-independent, given that the conclusions do not depend on the choice of
the gauge-fixing parameter within the linear covariant gauges. The central
construction relies crucially on the subtle interplay between the Abelian Ward
identities satisfied by the nonperturbative vertices and a special integral
identity that enforces a vast number of 'seagull cancellations' among the one-
and two-loop dressed diagrams of the gluon Schwinger-Dyson equation. The key
result of these considerations is that the gluon propagator remains rigorously
massless, provided that the vertices do not contain (dynamical) massless poles.
When such poles are incorporated into the vertices, under the pivotal
requirement of respecting the gauge symmetry of the theory, the terms
comprising the Ward identities conspire in such a way as to still enforce the
total annihilation of all quadratic divergences, inducing, at the same time,
residual contributions that account for the saturation of gluon propagators in
the deep infrared.Comment: 40 pages, 7 figures; v2: typos corrected, version matching the
published on
Nonperturbative results on the quark-gluon vertex
We present analytical and numerical results for the Dirac form factor of the
quark-gluon vertex in the quark symmetric limit, where the incoming and
outgoing quark momenta have the same magnitude but opposite sign. To accomplish
this, we compute the relevant components of the quark-ghost scattering kernel
at the one-loop dressed approximation, using as basic ingredients the full
quark propagator, obtained as a solution of the quark gap equation, and the
gluon propagator and ghost dressing function, obtained from large-volume
lattice simulations.Comment: 8 pages, 6 figures. Talk presented by A.C.A at Xth Quark Confinement
and the Hadron Spectrum, 8-12 October 2012, TUM Campus Garching, Munich,
German
Evidence of ghost suppression in gluon mass dynamics
In this work we study the impact that the ghost sector of pure Yang-Mills
theories may have on the generation of a dynamical gauge boson mass, which
hinges on the appearance of massless poles in the fundamental vertices of the
theory, and the subsequent realization of the well-known Schwinger mechanism.
The process responsible for the formation of such structures is itself
dynamical in nature, and is governed by a set of Bethe-Salpeter type of
integral equations. While in previous studies the presence of massless poles
was assumed to be exclusively associated with the background-gauge three-gluon
vertex, in the present analysis we allow them to appear also in the
corresponding ghost-gluon vertex. The full analysis of the resulting
Bethe-Salpeter system reveals that the contribution of the poles associated
with the ghost-gluon vertex are particularly suppressed, their sole discernible
effect being a slight modification in the running of the gluon mass, for
momenta larger than a few GeV. In addition, we examine the behavior of the
(background-gauge) ghost-gluon vertex in the limit of vanishing ghost momentum,
and derive the corresponding version of Taylor's theorem. These considerations,
together with a suitable Ansatz, permit us the full reconstruction of the pole
sector of the two vertices involved.Comment: 30 pages, 10 figure
Recommended from our members
Effects of drop and film viscosity on drop impacts onto thin films
While drop-film impacts have been studied extensively in the past, little thought has been given towards separating the effects of the drop fluid properties from those of the film. Distinguishing between the behaviors resulting from characteristics of each independently could provide insight into the underlying physical phenomena with a clarity that is unavailable when the drop and the film consist of identical liquids. In this study, the viscosity is the central parameter varied in both drop and film liquid. Using water, aqueous glycerol mixtures, and Fluoroinert FC-72, a range of kinematic viscosity covering 3 orders of magnitude (4 × 10-7 - 6.5 × 10 -4 m2/s) is examined; a smaller range of surface tension (0.024-0.072 N/m) is covered, as well. Drop impacts occur over a range of Weber numbers from 20 to 3000 and Reynolds numbers from 20 to 14000. Impact outcomes categorized are both formation of a crown and splashing from the crown. Criteria for each impact outcome are presented in light of both film and drop properties; certain outcomes are found to depend more strongly on either the properties of the drop or the film individually. Crown formation appears to relate more strongly to the film's properties, whereas crown splashing has some dependence on the drop properties. Existing splashing correlations are examined in light of the separation of properties. © 2013 by Begell House, Inc
Charging Interacting Rotating Black Holes in Heterotic String Theory
We present a formulation of the stationary bosonic string sector of the whole
toroidally compactified effective field theory of the heterotic string as a
double Ernst system which, in the framework of General Relativity describes, in
particular, a pair of interacting spinning black holes; however, in the
framework of low--energy string theory the double Ernst system can be
particularly interpreted as the rotating field configuration of two interacting
sources of black hole type coupled to dilaton and Kalb--Ramond fields. We
clarify the rotating character of the --component of the
antisymmetric tensor field of Kalb--Ramond and discuss on its possible torsion
nature. We also recall the fact that the double Ernst system possesses a
discrete symmetry which is used to relate physically different string vacua.
Therefore we apply the normalized Harrison transformation (a charging symmetry
which acts on the target space of the low--energy heterotic string theory
preserving the asymptotics of the transformed fields and endowing them with
multiple electromagnetic charges) on a generic solution of the double Ernst
system and compute the generated field configurations for the 4D effective
field theory of the heterotic string. This transformation generates the
vector field content of the whole low--energy heterotic string
spectrum and gives rise to a pair of interacting rotating black holes endowed
with dilaton, Kalb--Ramond and multiple electromagnetic fields where the charge
vectors are orthogonal to each other.Comment: 15 pages in latex, revised versio
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