1,208 research outputs found
Covariant QCD Modeling of Light Meson Physics
We summarize recent progress in soft QCD modeling based on the set of
Dyson--Schwinger equations truncated to ladder-rainbow level. This covariant
approach to hadron physics accommodates quark confinement and implements the
QCD one-loop renormalization group behavior. We compare the dressed quark
propagator, pseudoscalar and vector meson masses as a function of quark mass,
and the rho -> pi pi coupling to recent lattice-QCD data. The error in the
Gell-Mann--Oakes--Renner relation with increasing quark mass is quantified by
comparison to the exact pseudoscalar mass relation as evaluated within the
ladder-rainbow Dyson-Schwinger model.Comment: Presented at the International School on Nuclear Physics, 24th
course: Quarks in Nuclei, Erice, Sicily, September 2002; to be published in
Prog. Part. Nucl. Phys.; 6 pages, 6 fig
Selected nucleon form factors and a composite scalar diquark
A covariant, composite scalar diquark, Fadde'ev amplitude model for the
nucleon is used to calculate pseudoscalar, isoscalar- and isovector-vector,
axial-vector and scalar nucleon form factors. The last yields the nucleon
sigma-term and on-shell sigma-nucleon coupling. The calculated form factors are
soft, and the couplings are generally in good agreement with experiment and
other determinations. Elements in the dressed-quark-axial-vector vertex that
are not constrained by the Ward-Takahashi identity contribute ~20% to the
magnitude of g_A. The calculation of the nucleon sigma-term elucidates the only
unambiguous means of extrapolating meson-nucleon couplings off the meson
mass-shell.Comment: 12 pages, REVTEX, 5 figures, epsfi
Meson Transition Form Factors From A QCD Model Field Theory
We discuss form factors and coupling constants for the , and interactions generated by a model
field theory that produces finite size meson modes. The approach
implements dressing of the vertices and propagators consistent with dynamical
chiral symmetry breaking, gauge invariance, quark confinement and perturbative
QCD.Comment: 10 pages incl 4 figs in a single gzip-ed, uuencoded Postscript file;
Paper presented at {\it The International School of Nuclear Physics: Quarks
in Hadrons and Nuclei}, Erice, September 1995; Proceedings to appear in Prog.
Part. Nucl. Phys. {\bf 36
Nucleon form factors and a nonpointlike diquark
Nucleon form factors are calculated on q^2 in [0,3] GeV^2 using an Ansatz for
the nucleon's Fadde'ev amplitude motivated by quark-diquark solutions of the
relativistic Fadde'ev equation. Only the scalar diquark is retained, and it and
the quark are confined. A good description of the data requires a nonpointlike
diquark correlation with an electromagnetic radius of 0.8 r_pi. The composite,
nonpointlike nature of the diquark is crucial. It provides for diquark-breakup
terms that are of greater importance than the diquark photon absorption
contribution.Comment: 5 pages, REVTEX, epsfig, 3 figure
Charge symmetry breaking via rho-omega mixing from model quark-gluon dynamics
The quark-loop contribution to the mixing self-energy
function is calculated using a phenomenologically successful QCD-based model
field theory in which the and mesons are composite
bound states. In this calculation the dressed quark propagator, obtained from a
model Dyson-Schwinger equation, is confining. In contrast to previous studies,
the meson- vertex functions are characterised by a strength and range
determined by the dynamics of the model; and the calculated off-mass-shell
behaviour of the mixing amplitude includes the contribution from the calculated
diagonal meson self-energies. The mixing amplitude is shown to be very
sensitive to the small isovector component of dynamical chiral symmetry
breaking. The spacelike quark-loop mixing-amplitude generates an insignificant
charge symmetry breaking nuclear force.Comment: 11 Pages, 3 figures uuencoded and appended to this file, REVTEX 3.0.
ANL-PHY-7718-TH-94, KSUCNR-004-94. [!! PostScript file format corrected.
Retrieve by anonymous ftp from theory.phy.anl.gov (130.202.20.190), directory
pub: mget wpfig*.ps Three files.
K -> pi pi and a light scalar meson
We explore the Delta-I= 1/2 rule and epsilon'/epsilon in K -> pi pi
transitions using a Dyson-Schwinger equation model. Exploiting the feature that
QCD penguin operators direct K^0_S transitions through 0^{++} intermediate
states, we find an explanation of the enhancement of I=0 K -> pi pi transitions
in the contribution of a light sigma-meson. This mechanism also affects
epsilon'/epsilon.Comment: 7 pages, REVTE
Semiclassical Instability of the Cauchy Horizon in Self-Similar Collapse
Generic spherically symmetric self-similar collapse results in strong
naked-singularity formation. In this paper we are concerned with particle
creation during a naked-singularity formation in spherically symmetric
self-similar collapse without specifying the collapsing matter. In the generic
case, the power of particle emission is found to be proportional to the inverse
square of the remaining time to the Cauchy horizon (CH). The constant of
proportion can be arbitrarily large in the limit to marginally naked
singularity. Therefore, the unbounded power is especially striking in the case
that an event horizon is very close to the CH because the emitted energy can be
arbitrarily large in spite of a cutoff expected from quantum gravity. Above
results suggest the instability of the CH in spherically symmetric self-similar
spacetime from quantum field theory and seem to support the existence of a
semiclassical cosmic censor. The divergence of redshifts and blueshifts of
emitted particles is found to cause the divergence of power to positive or
negative infinity, depending on the coupling manner of scalar fields to
gravity. On the other hand, it is found that there is a special class of
self-similar spacetimes in which the semiclassical instability of the CH is not
efficient. The analyses in this paper are based on the geometric optics
approximation, which is justified in two dimensions but needs justification in
four dimensions.Comment: 14 pages, 4 figures, minor errors corrected and some sentences added
in the introduction, accepted for publication in Physical Review
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
Diquarks: condensation without bound states
We employ a bispinor gap equation to study superfluidity at nonzero chemical
potential: mu .neq. 0, in two- and three-colour QCD. The two-colour theory,
QC2D, is an excellent exemplar: the order of truncation of the quark-quark
scattering kernel: K, has no qualitative impact, which allows a straightforward
elucidation of the effects of mu when the coupling is strong. In rainbow-ladder
truncation, diquark bound states appear in the spectrum of the three-colour
theory, a defect that is eliminated by an improvement of K. The corrected gap
equation describes a superfluid phase that is semi-quantitatively similar to
that obtained using the rainbow truncation. A model study suggests that the
width of the superfluid gap and the transition point in QC2D provide reliable
quantitative estimates of those quantities in QCD.Comment: 7 pages, 3 figures, REVTEX, epsfi
The Quark-Photon Vertex and the Pion Charge Radius
The rainbow truncation of the quark Dyson-Schwinger equation is combined with
the ladder Bethe-Salpeter equation for the dressed quark-photon vertex to study
the low-momentum behavior of the pion electromagnetic form factor. With model
gluon parameters previously fixed by the pion mass and decay constant, the pion
charge radius is found to be in excellent agreement with the data. When
the often-used Ball-Chiu Ansatz is used to construct the quark-photon vertex
directly from the quark propagator, less than half of is generated.
The remainder of is seen to be attributable to the presence of the
-pole in the solution of the ladder Bethe-Salpeter equation.Comment: 21 pages, 9 figure
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