153 research outputs found

### The non-perturbative BRST quartet mechanism in Landau gauge QCD: Ghost-gluon and ghost-quark bound states

The non-perturbative BRST quartet mechanism in infrared Landau gauge QCD is
presented. It is demonstrated that positivity violation for transverse gluons
allows to identify the gluon's non-perturbative BRST quartet. To describe the
respective BRST-daughter state a truncated Bethe-Salpeter equation for the
gluon-ghost bound state is investigated. An analogous construction for quarks
yields a truncated Bethe-Salpeter equation for the quark-ghost bound state. The
gluon-ghost bound state equation in two space-time dimensions has been
numerically solved.Comment: 3 pages, 1 figure; Poster presented at the Xth Quark Confinement and
the Hadron Spectrum, Munich, Germany, 201

### QCD Green Functions and their Application to Hadron Physics

In a functional approach to QCD the infrared behaviour of Landau gauge Green
functions is investigated. It can be proven that the ghost Dyson-Schwinger
equation implies the Gribov-Zwanziger horizon condition. Its relation to the
Kugo-Ojima confinement scenario is elucidated. Positivity violation for gluons
is demonstrated, and the analytic structure of the gluon propagator is studied.
Quark confinement is related to an infrared divergence of the quark-gluon
vertex. It is shown that in the latter various components are non-vanishing due
to the dynamical breaking of chiral symmetry. As a result an infrared finite
running coupling in the Yang-Mills sector is derived whereas the running
coupling related to the quark-gluon vertex is infrared divergent.
In Coulomb gauge QCD already the one-gluon-exchange (over-)confines. This
leads to a vanishing quark propagator, and thus quarks are confined.
Nevertheless colour singlet quantities derived from the quark propagator are
well-defined. Especially the expression for the quark condensate proves that
chiral symmetry is dynamically broken. As expected the properties of mesons can
be directly calculated whereas the mass of coloured diquarks diverges, and thus
diquarks are confined. The latter nevertheless possess a well-defined size.
In the third part the results obtained so far will be used to formulate a
covariant Faddeev approach to nucleons. The resulting amplitudes describe the
quark core of the nucleon. Besides the mass of this state also the
electromagnetic form factors are calculated. The results for charge radii and
magnetic moments as a function of the quark current mass provide some
indication what the missing pion cloud may contribute to the nucleons'
properties.Comment: 21 pages, 32 figures; lectures presented at the conference ``Infrared
QCD in Rio: Propagators, Condensates and Topological Effects (IRQCD 2006)'',
Rio de Janeiro, Brazil, 5-9 Jun 2006; a few typos corrected (no other
changes

### On Confinement, Chiral Symmetry Breaking, and the UA(1) anomaly in Functional Approaches

The so-called decoupling and scaling solutions of functional equations of
Landau gauge Yang-Mills theory are briefly reviewed. In both types of solutions
the positivity violation seen in the gluon propagator is taken as an indication
of gluon confinement. In the scaling solution the resulting infrared
singularities of the quark-gluon vertex are responsible for the linear
potential between static quarks and are therefore signaling quark confinement.
A corresponding description of the UA(1) anomaly in functional approaches is
only known for the scaling solution. Nevertheless, it seems puzzling at first
sight that quark confinement is related to the dynamical and anomalous breaking
of chiral symmetry in a self-consistent manner: One obtains either all these
phenomena or none. For the scaling solution also fundamental scalar fields are
confined. This provides evidence that within functional approaches static
confinement is an universal property of the gauge sector even though it is
formally represented in the functional equations of the matter sector.Comment: 10 pages, 2 figures; talk given at the workshop "The many faces of
QCD", 1-5 Nov 2010, Ghent, Belgiu

### The effect of an infrared divergent quark-antiquark interaction kernel on other Green functions

The n-point Green functions of Landau gauge QCD are systematically
investigated in a Dyson-Schwinger approach assuming a static linearly rising
potential between fundamental color charges. Besides quarks also scalar matter
in the fundamental representation is considered. Starting from the hypothesis
of an 1/k^4 infrared divergent matter-antimatter vertex restrictions on the
general color tensor structure of this divergence are derived. Consequences for
the other four-point functions of QCD, resp., scalar QCD, are presented. Hereby
Casimir scaling is found. It is shown that possible singular contributions to
the three-point functions vanish due to cancellations within the color algebra.
On the other hand, higher n-point functions inherit the infrared singularity of
the matter-antimatter vertex in certain color channels. The presented results
show that linear confinement is consistently possible in a Greens function
approach, however, at the expense that the decoupling theorem is circumvented
by infrared singularities.Comment: 7 pages, 2figures, talk presented at the Xth Quark Confinement and
the Hadron Spectrum, Munich, Germany, 201

### On the effect of time-dependent inhomogeneous magnetic fields in electron-positron pair production

Electron-positron pair production in space- and time-dependent
electromagnetic fields is investigated. Especially, the influence of a
time-dependent, inhomogeneous magnetic field on the particle momenta and the
total particle yield is analyzed for the first time. The role of the Lorentz
invariant $\mathbf{E}^2 - \mathbf{B}^2$, including its sign and local values,
in the pair creation process is emphasized.Comment: 6 pages, 5 figures; Revised version: some improvements, references
updated; version will be published in Phys. Lett.

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