14 research outputs found
Gluon propagator in diffractive scattering
In this work, we perform a comparison of the employ of distinct gluon
propagators with the experimental data in diffractive processes, elastic
scattering and light meson photo-production. The gluon propagators are
calculated through non-perturbative methods, being justified their use in this
class of events, due to the smallness of the momentum transfer. Our results are
not able to select the best choice for the modified gluon propagator among the
analyzed ones, showing that the application of this procedure in this class of
high energy processes, although giving a reasonable fit to the experimental
data, should be taken with same caution.Comment: 14 pages, 4 figures, accepted for publication in Int. J. Mod. Phys. A
(uses ws-ijmpa.cls). Authors correcte
Self-consistent solution of the Schwinger-Dyson equations for the nucleon and meson propagators
The Schwinger-Dyson equations for the nucleon and meson propagators are
solved self-consistently in an approximation that goes beyond the Hartree-Fock
approximation. The traditional approach consists in solving the nucleon
Schwinger-Dyson equation with bare meson propagators and bare meson-nucleon
vertices; the corrections to the meson propagators are calculated using the
bare nucleon propagator and bare nucleon-meson vertices. It is known that such
an approximation scheme produces the appearance of ghost poles in the
propagators. In this paper the coupled system of Schwinger-Dyson equations for
the nucleon and the meson propagators are solved self-consistently including
vertex corrections. The interplay of self-consistency and vertex corrections on
the ghosts problem is investigated. It is found that the self-consistency does
not affect significantly the spectral properties of the propagators. In
particular, it does not affect the appearance of the ghost poles in the
propagators.Comment: REVTEX, 7 figures (available upon request), IFT-P.037/93,
DOE/ER/40427-12-N9
Analytic Structure of the Quark Propagator in a Model with an infrared vanishing Gluon Propagator
The Dyson-Schwinger equation for the quark self energy is solved in rainbow
approximation using an infrared (IR) vanishing gluon propagator that introduces
an IR mass scale . There exists a dependent critical coupling indicating
the spontaneous breakdown of chiral symmetry. If one chooses realistic QCD
coupling constants the strength and the scale of spontaneous chiral symmetry
breaking decouple from the IR scale for small while for large no
dynamical chiral symmetry breaking occurs. At timelike momenta the quark
propagator possesses a pole, at least for a large range of the parameter .
Therefore it is suggestive that quarks are not confined in this model for all
values of . Furthermore, we argue that the quark propagator is analytic
within the whole complex momentum plane except on the timelike axis. Hence the
na\"{\i}ve Wick rotation is allowed.Comment: 19 pages, revtex, 7 figures, improved analysis of asymptotic
behaviour and slight changes in conclusion; to appear in Phys.Rev.
Meson Form Factors and Non-Perturbative Gluon Propagators
The meson (pion and kaon) form factor is calculated in the perturbative
framework with alternative forms for the running coupling constant and the
gluon propagator in the infrared kinematic region. These modified forms are
employed to test the sensibility of the meson form factor to the
nonperturbative contributions. Its is a powerful discriminating quantity and
the results obtained with a particular choice of modified running coupling
constant and gluon propagator have a good agreement with the available data,
for both mesons, indicating the robustness of the method of calculation.
Nevertheless, nonperturbative aspects may be included in the perturbative
framework of calculation of exclusive processes.Comment: 18 pages, 7 figures. Discutions added, clarifing figures. Accepted to
be published in Phys. Rev.
A Systematic Extended Iterative Solution for QCD
An outline is given of an extended perturbative solution of Euclidean QCD
which systematically accounts for a class of nonperturbative effects, while
allowing renormalization by the perturbative counterterms. Proper vertices
Gamma are approximated by a double sequence Gamma[r,p], with r the degree of
rational approximation w.r.t. the QCD mass scale Lambda, nonanalytic in the
coupling g, and p the order of perturbative corrections in g-squared,
calculated from Gamma[r,0] - rather than from the perturbative Feynman rules
Gamma(0)(pert) - as a starting point. The mechanism allowing the
nonperturbative terms to reproduce themselves in the Dyson-Schwinger equations
preserves perturbative renormalizability and is tied to the divergence
structure of the theory. As a result, it restricts the self-consistency problem
for the Gamma[r,0] rigorously - i.e. without decoupling approximations - to the
superficially divergent vertices. An interesting aspect of the scheme is that
rational-function sequences for the propagators allow subsequences describing
short-lived excitations. The method is calculational, in that it allows known
techniques of loop computation to be used while dealing with integrands of
truly nonperturbative content.Comment: 48 pages (figures included). Scope of replacement: correction of a
technical defect; no changes in conten
On the Infrared Exponent for Gluon and Ghost Propagation in Landau Gauge QCD
In the covariant description of confinement, one expects the ghost
correlations to be infrared enhanced. Assuming ghost dominance, the long-range
behavior of gluon and ghost correlations in Landau gauge QCD is determined by
one exponent kappa. The gluon propagator is infrared finite (vanishing) for
kappa =1/2 (kappa > 1/2) which is still under debate. Here, we study critical
exponent and coupling for the infrared conformal behavior from the asymptotic
form of the solutions to the Dyson-Schwinger equations in an ultraviolet finite
expansion scheme. The value for kappa is directly related to the ghost-gluon
vertex. Assuming that it is regular in the infrared, one obtains kappa = 0.595.
This value maximizes the critical coupling alpha_c(kappa), yielding alpha_c^max
= (4 Pi/Nc) 0.709 approx. 2.97 for Nc=3. For larger kappa the vertex acquires
an infrared singularity in the gluon momentum, smaller ones imply infrared
singular ghost legs. Variations in alpha_c remain within 5% from kappa = 0.5 to
0.7. Above this range, alpha_c decreases more rapidly with alpha_c -> 0 as
kappa -> 1 which sets the upper bound on kappa.Comment: 22 Pages, 10 Figures, LaTeX2e, revtex4, some notes and references
added in response to communication
Dynamical chiral symmetry breaking and confinement with an infrared-vanishing gluon propagator?
We study a model Dyson-Schwinger equation for the quark propagator closed
using an {\it Ansatz} for the gluon propagator of the form \mbox{} and two {\it Ans\"{a}tze} for the quark-gluon vertex: the
minimal Ball-Chiu and the modified form suggested by Curtis and Pennington.
Using the quark condensate as an order parameter, we find that there is a
critical value of such that the model does not support dynamical chiral
symmetry breaking for . We discuss and apply a confinement test which
suggests that, for all values of , the quark propagator in the model {\bf is
not} confining. Together these results suggest that this Ansatz for the gluon
propagator is inadequate as a model since it does not yield the expected
behaviour of QCD.Comment: 21 Pages including 4 PostScript figures uuencoded at the end of the
file. Replacement: slight changes of wording and emphasis. ADP-93-215/T133,
ANL-PHY-7599-TH-93, FSU-SCRI-93-108, REVTEX 3.
Analysis of a quenched lattice-QCD dressed-quark propagator
Quenched lattice-QCD data on the dressed-quark Schwinger function can be
correlated with dressed-gluon data via a rainbow gap equation so long as that
equation's kernel possesses enhancement at infrared momenta above that
exhibited by the gluon alone. The required enhancement can be ascribed to a
dressing of the quark-gluon vertex. The solutions of the rainbow gap equation
exhibit dynamical chiral symmetry breaking and are consistent with confinement.
The gap equation and related, symmetry-preserving ladder Bethe-Salpeter
equation yield estimates for chiral and physical pion observables that suggest
these quantities are materially underestimated in the quenched theory: |<bar-q
q>| by a factor of two and f_pi by 30%.Comment: 9 pages, LaTeX2e, REVTEX4, 6 figure
Asymptotic Scaling and Infrared Behavior of the Gluon Propagator
The Landau gauge gluon propagator for the pure gauge theory is evaluated on a
32^3x64 lattice with a physical volume of (3.35^3x6.7)fm^4. Comparison with two
smaller lattices at different lattice spacings allows an assessment of finite
volume and finite lattice spacing errors. Cuts on the data are imposed to
minimize these errors. Scaling of the gluon propagator is verified between
beta=6.0 and beta=6.2. The tensor structure is evaluated and found to be in
good agreement with the Landau gauge form, except at very small momentum
values, where some small finite volume errors persist. A number of functional
forms for the momentum dependence of the propagator are investigated. The form
D(q^2)=D_ir+D_uv, where D_ir(q^2) ~ (q^2+M^2)^-\eta and D_uv is an infrared
regulated one-loop asymptotic form, is found to provide an adequate description
of the data over the entire momentum region studied - thereby bridging the gap
between the infrared confinement region and the ultraviolet asymptotic region.
The best estimate for the exponent \eta is 3.2(+0.1/-0.2)(+0.2/-0.3), where the
first set of errors represents the uncertainty associated with varying the
fitting range, while the second set of errors reflects the variation arising
from different choices of infrared regulator in D_uv. Fixing the form of D_uv,
we find that the mass parameter M is (1020+/-100)MeV.Comment: 37 pages, RevTeX, 16 postscript figures, 7 gif figures. Revised
version accepted for publication in Phys. Rev. D. Model functions and
discussion of asymptotic behaviour modified; all model fits have been redone.
This paper, including postscript version of all figures, can be found at
http://www.physics.adelaide.edu.au/~jskuller/papers
Infrared Behaviour of the Gluon Propagator: Confining or Confined?
The possible infrared behaviour of the gluon propagator is studied
analytically, using the Schwinger-Dyson equations, in both the axial and the
Landau gauge. The possibility of a gluon propagator less singular than
when is investigated and found to be
inconsistent, despite claims to the contrary, whereas an infrared enhanced one
is consistent. The implications for confinement are discussed.Comment: 20 pages, latex, 2 figure