210,190 research outputs found
Color-suppression of non-planar diagrams in bosonic bound states
We study the suppression of non-planar diagrams in a scalar QCD model of a
meson system in space-time dimensions due to the inclusion of the color
degrees of freedom. As a prototype of the color-singlet meson, we consider a
flavor-nonsinglet system consisting of a scalar-quark and a scalar-antiquark
with equal masses exchanging a scalar-gluon of a different mass, which is
investigated within the framework of the homogeneous Bethe-Salpeter equation.
The equation is solved by using the Nakanishi representation for the manifestly
covariant bound-state amplitude and its light-front projection. The resulting
non-singular integral equation is solved numerically. The damping of the impact
of the cross-ladder kernel on the binding energies are studied in detail. The
color-suppression of the cross-ladder effects on the light-front wave function
and the elastic electromagnetic form factor are also discussed. As our results
show, the suppression appears significantly large for , which supports
the use of rainbow-ladder truncations in practical nonperturbative calculations
within QCD.Comment: 12 pages, 7 figures. To appear in Physics Letters
Bound state structure and electromagnetic form factor beyond the ladder approximation
We investigate the response of the bound state structure of a two-boson
system, within a Yukawa model with a scalar boson exchange, to the inclusion of
the cross-ladder contribution to the ladder kernel of the Bethe-Salpeter
equation. The equation is solved by means of the Nakanishi integral
representation and light-front projection. The valence light-front wave
function and the elastic electromagnetic form factor beyond the impulse
approximation, with the inclusion of the two-body current, generated by the
cross-ladder kernel, are computed. The valence wave function and
electromagnetic form factor, considering both ladder and ladder plus
cross-ladder kernels, are studied in detail. Their asymptotic forms are found
to be quite independent of the inclusion of the cross-ladder kernel, for a
given binding energy. The asymptotic decrease of form factor agrees with the
counting rules. This analysis can be generalized to fermionic systems, with a
wide application in the study of the meson structure.Comment: 19 pages, 6 figures, submitted to Phys. Lett.
Chiral symmetry breaking with lattice propagators
We study chiral symmetry breaking using the standard gap equation,
supplemented with the infrared-finite gluon propagator and ghost dressing
function obtained from large-volume lattice simulations. One of the most
important ingredients of this analysis is the non-abelian quark-gluon vertex,
which controls the way the ghost sector enters into the gap equation.
Specifically, this vertex introduces a numerically crucial dependence on the
ghost dressing function and the quark-ghost scattering amplitude. This latter
quantity satisfies its own, previously unexplored, dynamical equation, which
may be decomposed into individual integral equations for its various form
factors. In particular, the scalar form factor is obtained from an approximate
version of the "one-loop dressed" integral equation, and its numerical impact
turns out to be rather considerable. The detailed numerical analysis of the
resulting gap equation reveals that the constituent quark mass obtained is
about 300 MeV, while fermions in the adjoint representation acquire a mass in
the range of (750-962) MeV.Comment: 32 pages, 13 figure
Two-dimensional effective action for matter fields coupled to the dilaton
We revise the calculation of the one-loop effective action for scalar and
spinor fields coupled to the dilaton in two dimensions. Applying the method of
covariant perturbation theory for the heat kernel we derive the effective
action in an explicitly covariant form that produces both the conformally
invariant and the conformally anomalous terms.For scalar fields the conformally
invariant part of the action is nonlocal. The obtained effective action is
proved to be infrared finite. We also compute the one-loop effective action for
scalar fields at finite temperature.Comment: LaTeX, 25 page
QCD modeling of hadron physics
We review recent developments in the understanding of meson properties as
solutions of the Bethe-Salpeter equation in rainbow-ladder truncation. Included
are recent results for the pseudoscalar and vector meson masses and leptonic
decay constants, ranging from pions up to c\bar{c} bound states; extrapolation
to b\bar{b} states is explored. We also present a new and improved calculation
of F_\pi(Q^2) and an analysis of the \pi\gamma\gamma transition form factor for
both \pi(140) and \pi(1330). Lattice-QCD results for propagators and the
quark-gluon vertex are analyzed, and the effects of quark-gluon vertex dressing
and the three-gluon coupling upon meson masses are considered.Comment: 17 pages, 19 postscript figures, contribution to the proceedings of
LC05, Cairns, Australia, July 200
Pion cloud effects on baryon masses
In this work we explore the effect of pion cloud contributions to the mass of
the nucleon and the delta baryon. To this end we solve a coupled system of
Dyson-Schwinger equations for the quark propagator, a Bethe-Salpeter equation
for the pion and a three-body Faddeev equation for the baryons. In the
quark-gluon interaction we explicitly resolve the term responsible for the
back-coupling of the pion onto the quark, representing rainbow-ladder like pion
cloud effects in bound states. We study the dependence of the resulting baryon
masses on the current quark mass and discuss the internal structure of the
baryons in terms of a partial wave decomposition. We furthermore determine
values for the nucleon and delta sigma-terms.Comment: 9 pages, 4 figures, 2 tables. v2: Numerics corrected; results
updated; discussion extended. Version accepted for publication in Phys.Lett.
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