2,263 research outputs found
Nature
The sluices of the Bergenmeersen FCA-CRT were opened for the first time on 25 April 2013. At the time of writing, the natural development of the area was still in a primordial stage. Therefore the actual observed nature development in Bergenmeersen could not be described in this book (although some actual results are given in Chapter 8). Therefore, this chapter explains the expected nature development with relation to similar environments along the river Scheldt.The nature target scenario was defined in the objective of the updated Sigma Plan. In this plan, just one coordinating objective was set out for Bergenmeersen: the development of 40 ha of estuarine nature. What this precisely implies is described below
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
Symmetry breaking via fermion 4-point functions
We construct the effective action and gap equations for nonperturbative
fermion 4-point functions. Our results apply to situations in which fermion
masses can be ignored, which is the case for theories of strong flavor
interactions involving standard quarks and leptons above the electroweak scale.
The structure of the gap equations is different from what a naive
generalization of the 2-point case would suggest, and we find for example that
gauge exchanges are insufficient to generate nonperturbative 4-point functions
when the number of colors is large.Comment: 36 pages, uses Revtex and eps files for figure
Confinement Phenomenology in the Bethe-Salpeter Equation
We consider the solution of the Bethe-Salpeter equation in Euclidean metric
for a qbar-q vector meson in the circumstance where the dressed quark
propagators have time-like complex conjugate mass poles. This approximates
features encountered in recent QCD modeling via the Dyson-Schwinger equations;
the absence of real mass poles simulates quark confinement. The analytic
continuation in the total momentum necessary to reach the mass shell for a
meson sufficiently heavier than 1 GeV leads to the quark poles being within the
integration domain for two variables in the standard approach. Through Feynman
integral techniques, we show how the analytic continuation can be implemented
in a way suitable for a practical numerical solution. We show that the would-be
qbar-q width to the meson generated from one quark pole is exactly cancelled by
the effect of the conjugate partner pole; the meson mass remains real and there
is no spurious qbar-q production threshold. The ladder kernel we employ is
consistent with one-loop perturbative QCD and has a two-parameter infrared
structure found to be successful in recent studies of the light SU(3) meson
sector.Comment: Submitted for publication; 10.5x2-column pages, REVTEX 4, 3
postscript files making 3 fig
N=1 Supersymmetric Spin-Charge Separation in effective gauge theories of planar magnetic superconductors
We present a N=1 Supersymmetric extension of a spin-charge separated
effective `particle-hole' gauge theory of excitations
about the nodes of the gap of a d-wave planar magnetic superconductor. The
supersymmetry is achieved without introducing extra degrees of freedom, as
compared to the non-supersymmetric models. The only exception, the introduction
of gaugino fieds, finds a natural physical interpretation as describing
interlayer coupling in the statistical model. The low-energy continuum theory
is described by a relativistic (2+1)-dimensional supersymmetric
-model with Gross-Neveu-Thirring-type four-fermion interactions. We
emphasize the crucial r\^ole of the constraint in inducing a non-trivial
dynamical mass generation for fermions (and thus superconductivity), in a way
compatible with manifest N=1 supersymmetry. We also give a preliminary
discussion of non-perturbative effects. We argue that supersymmetry suppresses
the dangerous for superconductivity instanton contributions to the mass of the
perturbatively massless gauge boson of the unbroken U(1) subgroup of SU(2).
Finally, we point out the possibility of applying these ideas to effective
gauge models of spin-charge separation in one-space dimensional superconducting
chains of holons, which, for example, have recently been claimed to be
important in the stripe phase of underdoped cuprates.Comment: 19 pages LATEX (minor misprints in formula (43) corrected
Mean field exponents and small quark masses
We demonstrate that the restoration of chiral symmetry at finite-T in a class
of confining Dyson-Schwinger equation (DSE) models of QCD is a mean field
transition, and that an accurate determination of the critical exponents using
the chiral and thermal susceptibilities requires very small values of the
current-quark mass: log_{10}(m/m_u) < -5. Other classes of DSE models
characterised by qualitatively different interactions also exhibit a mean field
transition. Incipient in this observation is the suggestion that mean field
exponents are a result of the gap equation's fermion substructure and not of
the interaction.Comment: 13 pages, 3 figures, REVTEX, epsfi
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
Bethe-Salpeter equation and a nonperturbative quark-gluon vertex
A Ward-Takahashi identity preserving Bethe-Salpeter kernel can always be
calculated explicitly from a dressed-quark-gluon vertex whose diagrammatic
content is enumerable. We illustrate that fact using a vertex obtained via the
complete resummation of dressed-gluon ladders. While this vertex is planar, the
vertex-consistent kernel is nonplanar and that is true for any dressed vertex.
In an exemplifying model the rainbow-ladder truncation of the gap and
Bethe-Salpeter equations yields many results; e.g., pi- and rho-meson masses,
that are changed little by including higher-order corrections. Repulsion
generated by nonplanar diagrams in the vertex-consistent Bethe-Salpeter kernel
for quark-quark scattering is sufficient to guarantee that diquark bound states
do not exist.Comment: 16 pages, 12 figures, REVTEX
Manifestation of three-body forces in three-body Bethe-Salpeter and light-front equations
Bethe-Salpeter and light-front bound state equations for three scalar
particles interacting by scalar exchange-bosons are solved in ladder
truncation. In contrast to two-body systems, the three-body binding energies
obtained in these two approaches differ significantly from each other: the
ladder kernel in light-front dynamics underbinds by approximately a factor of
two compared to the ladder Bethe-Salpeter equation. By taking into account
three-body forces in the light-front approach, generated by two exchange-bosons
in flight, we find that most of this difference disappears; for small exchange
masses, the obtained binding energies coincide with each other.Comment: 24 pages, 8 figures, submitted in Few-Body System
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