82 research outputs found
Dynamical Chiral Symmetry Breaking in Landau gauge QCD
We summarise results for the propagators of Landau gauge QCD from the Green's
functions approach and lattice calculations. The nonperturbative solutions for
the ghost, gluon and quark propagators from a coupled set of Dyson-Schwinger
equations agree almost quantitatively with corresponding lattice results.
Similar unquenching effects are found in both approaches. The dynamically
generated quark masses are close to `phenomenological' values. The chiral
condensate is found to be large.Comment: 4 pages, 2 figures, talk given by C.F. at 6th Conference on Quark
Confinement and the Hadron Spectrum, Villasimius, Sardinia, Italy, 21-25 Sep
200
Dynamical Mass Generation in Landau gauge QCD
We summarise results on the infrared behaviour of Landau gauge QCD from the
Green's functions approach and lattice calculations. Approximate,
nonperturbative solutions for the ghost, gluon and quark propagators as well as
first results for the quark-gluon vertex from a coupled set of Dyson-Schwinger
equations are compared to quenched and unquenched lattice results. Almost
quantitative agreement is found for all three propagators. Similar effects of
unquenching are found in both approaches. The dynamically generated quark
masses are close to `phenomenological' values. First results for the
quark-gluon vertex indicate a complex tensor structure of the non-perturbative
quark-gluon interaction.Comment: 6 pages, 6 figures, Summary of a talk given at the international
conference QCD DOWN UNDER, March 10 - 19, Adelaide, Australi
Form Factors of Baryons in a Confining and Covariant Diquark-Quark Model
We treat baryons as bound states of scalar or axialvector diquarks and a
constituent quark which interact through quark exchange. This description
results as an approximation to the relativistic Faddeev equation for three
quarks which yields an effective Bethe-Salpeter equation. Octet and decuplet
masses and fully four-dimensional wave functions have been computed for two
cases: assuming an essentially pointlike diquark on the one hand, and a diquark
with internal structure on the other hand. Whereas the differences in the mass
spectrum are fairly small, the nucleon electromagnetic form factors are greatly
improved assuming a diquark with structure. First calculations to the
pion-nucleon form factor also suggest improvements.Comment: 11 pages, uses 'aipproc.sty'. Talk given by M.O. at the Workshop
"Effective Theories of Low Energy QCD", Coimbra, Portugal, Sep 10-15 199
The Infrared Behaviour of the Running Coupling in Landau Gauge QCD
Approximate solutions for the gluon and ghost propagators as well as the
running coupling in Landau gauge Yang-Mills theories are presented. These
propagators obtained from the corresponding Dyson-Schwinger equations are in
remarkable agreement with those of recent lattice calculations. The resulting
running coupling possesses an infrared fixed point,
for all gauge groups SU(). Above one GeV the running coupling rapidly
approaches its perturbative form.Comment: 8 pages, 3 figures, uses ActaStyle.cls, Invited talk given by R.A. at
the conference RENORMALIZATION GROUP 2002, March 10 - 16, 2002, Strba,
Slovaki
Infrared Exponents and the Running Coupling of Landau gauge QCD and their Relation to Confinement
The infrared behaviour of the gluon and ghost propagators in Landau gauge QCD
is reviewed. The Kugo-Ojima confinement criterion and the Gribov-Zwanziger
horizon condition result from quite general properties of the ghost
Dyson-Schwinger equation. The numerical solutions for the gluon and ghost
propagators obtained from a truncated set of Dyson-Schwinger equations provide
an explicit example for the anticipated infrared behaviour. The results are in
good agreement with corresponding lattice data obtained recently. The resulting
running coupling approaches a fix point in the infrared, . Two different fits for the scale dependence of the running coupling
are given and discussed.Comment: 3 pages, 3 figures; talk given by R.A. at the conference Quark
Nuclear Physics 200
What the Infrared Behaviour of QCD Vertex Functions in Landau gauge can tell us about Confinement
The infrared behaviour of Landau gauge QCD vertex functions is investigated
employing a skeleton expansion of the Dyson-Schwinger and Renormalization Group
equations. Results for the ghost-gluon, three-gluon, four-gluon and quark-gluon
vertex functions are presented. Positivity violation of the gluon propagator,
and thus gluon confinement, is demonstrated. Results of the Dyson-Schwinger
equations for a finite volume are compared to corresponding lattice data. It is
analytically demonstrated that a linear rising potential between heavy quarks
can be generated by infrared singularities in the dressed quark-gluon vertex.
The selfconsistent mechanism that generates these singularities necessarily
entails the scalar Dirac amplitudes of the full vertex and the quark
propagator. These can only be present when chiral symmetry is broken, either
explicitly or dynamically.Comment: 13 pages, 13 figures; to appear in the Proceedings of ``X Hadron
Physics 2007'', Florianopolis, Brazil, March 26 - 31, 200
Analytic structure of the Landau gauge gluon propagator
The results of different non-perturbative studies agree on a power law as the
infrared behavior of the Landau gauge gluon propagator. This propagator
violates positivity and thus indicates the absence of the transverse gluons
from the physical spectrum, i.e. gluon confinement. A simple analytic structure
for the gluon propagator is proposed capturing all of its features. We comment
also on related investigations for the Landau gauge quark propagator.Comment: 4 pages, 2 figures, talk given by R.A. at 6th Conference on Quark
Confinement and the Hadron Spectrum, Villasimius, Sardinia, Italy, 21-25 Sep
200
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