2,206 research outputs found
Gauge covariance and the fermion-photon vertex in three- and four- dimensional, massless quantum electrodynamics
In the quenched approximation, the gauge covariance properties of three
vertex Ans\"{a}tze in the Schwinger-Dyson equation for the fermion self energy
are analysed in three- and four- dimensional quantum electrodynamics. Based on
the Cornwall-Jackiw-Tomboulis effective action, it is inferred that the
spectral representation used for the vertex in the gauge technique cannot
support dynamical chiral symmetry breaking. A criterion for establishing
whether a given Ansatz can confer gauge covariance upon the Schwinger-Dyson
equation is presented and the Curtis and Pennington Ansatz is shown to satisfy
this constraint. We obtain an analytic solution of the Schwinger-Dyson equation
for quenched, massless three-dimensional quantum electrodynamics for arbitrary
values of the gauge parameter in the absence of dynamical chiral symmetry
breaking.Comment: 17 pages, PHY-7143-TH-93, REVTE
European food-safety concerns: From farm to market
In May of this year, 14 students from Iowa State University (ISU) had the opportunity to visit England and Scotland to examine food-safety issues, with emphasis on increasing the exposure to and understanding of European food-safety concerns, farming practices,the regulatory environment,research activities, and consumer attitudes
The analytic structure of heavy quark propagators
The renormalised quark Dyson-Schwinger equation is studied in the limit of
the renormalised current heavy quark mass m_R --> infinity. We are particularly
interested in the analytic pole structure of the heavy quark propagator in the
complex momentum plane. Approximations in which the quark-gluon vertex is
modelled by either the bare vertex or the Ball-Chiu Ansatz, and the Landau
gauge gluon propagator takes either a gaussian form or a gaussian form with an
ultraviolet asymptotic tail are used.Comment: 21 pages Latex and 5 postscript figures. The original version of this
paper has been considerably extended to include a formalism dealing with the
renormalised heavy quark Dyson-Schwinger equation and uses a more realistic
Ansatz for the gluon propagator
Vector Positronium States in QED3
The homogeneous Bethe-Salpeter equation is solved in the quenched ladder
approximation for the vector positronium states of 4-component quantum
electrodynamics in 2 space and 1 time dimensions. Fermion propagator input is
from a Rainbow approximation Dyson-Schwinger solution, with a broad range of
fermion masses considered. This work is an extension of earlier work on the
scalar spectrum of the same model. The non-relativistic limit is also
considered via the large fermion mass limit. Classification of states via their
transformation properties under discrete parity transformations allows
analogies to be drawn with the meson spectrum of QCD.Comment: 24 pages, 2 encapsulated postscript figure
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
Zero Lattice Sound
We study the N_f-flavor Gross-Neveu model in 2+1 dimensions with a baryon
chemical potential mu, using both analytical and numerical methods. In
particular, we study the self-consistent Boltzmann equation in the Fermi liquid
framework using the quasiparticle interaction calculated to O(1/N_f), and find
solutions for zero sound propagation for almost all mu > mu_c, the critical
chemical potential for chiral symmetry restoration. Next we present results of
a numerical lattice simulation, examining temporal correlation functions of
mesons defined using a point-split interpolating operator, and finding evidence
for phonon-like behaviour characterised by a linear dispersion relation in the
long wavelength limit. We argue that our results provide the first evidence for
a collective excitation in a lattice simulation.Comment: 18 pages, 6 figure
Pseudoscalar Meson Nonet at Zero and Finite Temperature
Theoretical understanding of experimental results from relativistic heavy-ion
collisions requires a microscopic approach to the behavior of QCD n-point
functions at finite temperatures, as given by the hierarchy of Dyson-Schwinger
equations, properly generalized within the Matsubara formalism. The convergence
of sums over Matsubara modes is studied. The technical complexity of
finite-temperature calculations mandates modeling. We present a model where the
QCD interaction in the infrared, nonperturbative domain, is modeled by a
separable form. Results for the mass spectrum of light quark flavors (u, d, s)
and for the pseudoscalar bound-state amplitudes at finite temperature are
presented.Comment: 14 pages, 11 figures, accepted for publication in Physics of
Particles and Nuclei Letters, based on invited lectures at "Dense Matter In
Heavy Ion Collisions and Astrophysics", 21.08-01.09 2006, Dubna, Russi
Nonperturbative Vertices in Supersymmetric Quantum Electrodynamics
We derive the complete set of supersymmetric Ward identities involving only
two- and three- point proper vertices in supersymmetric QED. We also present
the most general form of the proper vertices consistent with both the
supersymmetric and U(1) gauge Ward identities. These vertices are the
supersymmetric equivalent of the non supersymmetric Ball-Chiu vertices.Comment: seventeen pages late
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