5,109 research outputs found
Dyson-Schwinger Equations: An Instrument for Hadron Physics
Dyson-Schwinger equations furnish a Poincare' covariant approach to hadron
physics. They reveal that dynamical chiral symmetry breaking is tied to the
long-range behaviour of the strong interaction and make predictions
corroborated by modern lattice-QCD simulations. A hallmark in the contemporary
use of DSEs is the existence of a nonperturbative, symmetry preserving
truncation that enables the proof of exact results. The systematic error
associated with the truncation's leading term has been quantified and this
underpins an efficacious one-parameter model that is being employed to study
meson excited states.Comment: 9 pages; LaTeX2e; Contribution to proceedings of "17th International
Conference on Few-Body Problems in Physics," Duke University/TUNL,
5-10/June/200
Facets of confinement and dynamical chiral symmetry breaking
The gap equation is a cornerstone in understanding dynamical chiral symmetry
breaking and may also provide clues to confinement. A symmetry-preserving
truncation of its kernel enables proofs of important results and the
development of an efficacious phenomenology. We describe a model of the kernel
that yields: a momentum-dependent dressed-quark propagator in fair agreement
with quenched lattice-QCD results; and chiral limit values: f_pi= 68 MeV and
= -(190 MeV)^3. It is compared with models inferred from studies of
the gauge sector.Comment: 5 pages, 3 figures; contribution to the proceedings of Quark Nuclear
Physics (QNP 2002), Juelich, Germany, 9-14 Jun 200
Confinement, DCSB, Bound States, and the Quark-Gluon Vertex
Aspects of the dressed-quark-gluon vertex and their role in the gap and
Bethe-Salpeter equations are briefly surveyed using an intuitive model. The
model allows one to elucidate why a linear extrapolation to the chiral limit of
extant lattice data on the dressed-quark mass-function overestimates this
function and hence the value of the vacuum quark condensate. The diagrammatic
content of the vertex described is explicitly enumerable. This property is
essential to the symmetry preserving study of bound state properties. It
facilitates a realistic analysis of vector and pseudoscalar meson masses, and
also allows the accuracy of standard truncations to be gauged. The splitting
between vector and pseudoscalar meson masses is observed to vanish as the
current-quark mass increases. That argues for the mass of the pseudoscalar
partner of the Upsilon(1S) to be above 9.4GeV. Moreover, in this limit the
rainbow-ladder truncation provides an increasingly accurate estimate of a bound
state's mass.Comment: 6 pages, Contribution to the Proceedings of "QCD Down Under", Special
Centre for the Subatomic Structure of Matter, University of Adelaide,
10-19/March/200
Theory and Phenomenology of Hadrons
This Dyson-Schwinger equation (DSE) aperc,u highlights recent applications to
mesons. It reports features of, and results for, pseudoscalar and scalar
bound-state residues in vacuum polarisations, and exhibits how a restoration of
chiral symmetry in meson trajectories could be manifest in a relationship
between them. It also touches on nucleon studies, emphasising the importance of
both scalar and axial-vector diquark correlations, and reporting the
calculation of mu_n G_E^n(Q^2)/G_M^n(Q^2). The value of respecting symmetries,
including Poincare' covariance, is stressed.Comment: 7 pages, 1 figure, contribution to proceedings of "18th International
IUPAP Conference on Few-Body Problems in Physics," Santos, Brazil, August
21-26, 200
Mind the gap
In this summary of the application of Dyson-Schwinger equations to the theory
and phenomenology of hadrons, some deductions following from a nonperturbative,
symmetry-preserving truncation are highlighted, notable amongst which are
results for pseudoscalar mesons. We also describe inferences from the gap
equation relating to the radius of convergence of a chiral expansion,
applications to heavy-light and heavy-heavy mesons, and quantitative estimates
of the contribution of quark orbital angular momentum in pseudoscalar mesons;
and recapitulate upon studies of nucleon electromagnetic form factors.Comment: 7 pages, 3 figures. Contribution to Proceedings of 4th International
Conference on Quarks and Nuclear Physics (QNP06), Madrid, Spain, 5-10 Jun
200
Strong Decays of Light Vector Mesons
The vector meson strong decays rho-->pi pi, phi-->KK, and K^star-->pi K are
studied within a covariant approach based on the ladder-rainbow truncation of
the QCD Dyson--Schwinger equation for the quark propagator and the
Bethe--Salpeter equation for the mesons. The model preserves the one-loop
behavior of QCD in the ultraviolet, has two infrared parameters, and implements
quark confinement and dynamical chiral symmetry breaking. The 3-point decay
amplitudes are described in impulse approximation. The Bethe--Salpeter study
motivates a method for estimating the masses for heavier mesons within this
model without continuing the propagators into the complex plane. We test the
accuracy via the rho, phi and K^{star} masses and then produce estimates of the
model results for the a_1 and b_1 masses as well as the mass of the proposed
exotic vector pi_1(1400).Comment: Submitted for publication; 10x2-column pages, REVTEX 4, 3 .eps files
making 3fig
Analysis of Yield Spreads on Commercial Mortgage-Backed Securities
Yield spreads on commercial mortgage-backed securities (CMBS) are defined as the difference between the yield on CMBS and the yield on comparable-maturity Treasuries. CMBS yield spreads declined dramatically from 1992 until 1997, then increased in 1998 and 1999. The relationship between CMBS yield spreads and other variables is estimated in an effort to explain recent trends. Results identify several variables that are related to yield spreads on both fixed-rate and variable-rate CMBS. However, even after controlling for other observable factors, the yield spread on CMBS still declined from 1992 until 1997, then increased each of the next two years. Possible explanations for this phenomenon are explored.
K_{l3} transition form factors
The rainbow truncation of the quark Dyson-Schwinger equation is combined with
the ladder Bethe-Salpeter equation for the meson bound state amplitudes and the
dressed quark-W vertex in a manifestly covariant calculation of the K_{l3}
transition form factors and decay width in impulse approximation. With model
gluon parameters previously fixed by the chiral condensate, the pion mass and
decay constant, and the kaon mass, our results for the K_{l3} form factors and
the kaon semileptonic decay width are in good agreement with the experimental
data.Comment: 8 pages, 3 figures, Revte
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