2,007 research outputs found
Analytic structure of the Landau gauge gluon propagator
The results of different non-perturbative studies agree on a power law as the
infrared behavior of the Landau gauge gluon propagator. This propagator
violates positivity and thus indicates the absence of the transverse gluons
from the physical spectrum, i.e. gluon confinement. A simple analytic structure
for the gluon propagator is proposed capturing all of its features. We comment
also on related investigations for the Landau gauge quark propagator.Comment: 4 pages, 2 figures, talk given by R.A. at 6th Conference on Quark
Confinement and the Hadron Spectrum, Villasimius, Sardinia, Italy, 21-25 Sep
200
Exposure to the Dental Environment and Prevalence of Respiratory Illness in Dental Student Populations
Objective: To determine if the prevalence of respiratory disease among dental students and dental residents varies with their exposure to the clinical dental environment.
Methods: A detailed questionnaire was administered to 817 students at 3 dental schools. The questionnaire sought information concerning demographic characteristics, school year, exposure to the dental environment and dental procedures, and history of respiratory disease. The data obtained were subjected to bivariate and multiple logistic regression analysis.
Results: Respondents reported experiencing the following respiratory conditions during the previous year: asthma (26 cases), bronchitis (11 cases), chronic lung disease (6 cases), pneumonia (5 cases) and streptococcal pharyngitis (50 cases). Bivariate statistical analyses indicated no significant associations between the prevalence of any of the respiratory conditions and year in dental school, except for asthma, for which there was a significantly higher prevalence at 1 school compared to the other 2 schools. When all cases of respiratory disease were combined as a composite variable and subjected to multivariate logistic regression analysis controlling for age, sex, race, dental school, smoking history and alcohol consumption, no statistically significant association was observed between respiratory condition and year in dental school or exposure to the dental environment as a dental patient.
Conclusion: No association was found between the prevalence of respiratory disease and a student\u27s year in dental school or previous exposure to the dental environment as a patient. These results suggest that exposure to the dental environment does not increase the risk for respiratory infection in healthy dental health care workers
Chiral Extrapolation of Lattice Data for Heavy Meson Hyperfine Splittings
We investigate the chiral extrapolation of the lattice data for the
light-heavy meson hyperfine splittings D^*-D and B^*-B to the physical region
for the light quark mass. The chiral loop corrections providing non-analytic
behavior in m_\pi are consistent with chiral perturbation theory for heavy
mesons. Since chiral loop corrections tend to decrease the already too low
splittings obtained from linear extrapolation, we investigate two models to
guide the form of the analytic background behavior: the constituent quark
potential model, and the covariant model of QCD based on the ladder-rainbow
truncation of the Dyson-Schwinger equations. The extrapolated hyperfine
splittings remain clearly below the experimental values even allowing for the
model dependence in the description of the analytic background.Comment: 14 pages, 4 figures, typos corrected, presentation clarifie
Infrared divergence in QED at finite temperature
We consider various ways of treating the infrared divergence which appears in
the dynamically generated fermion mass, when the transverse part of the photon
propagator in N flavour at finite temperature is included in the
Matsubara formalism. This divergence is likely to be an artefact of taking into
account only the leading order term in the expansion when we
calculate the photon propagator and is handled here phenomenologically by means
of an infrared cutoff. Inserting both the longitudinal and the transverse part
of the photon propagator in the Schwinger-Dyson equation we find the dependence
of the dynamically generated fermion mass on the temperature and the cutoff
parameters. It turns out that consistency with certain statistical physics
arguments imposes conditions on the cutoff parameters. For parameters in the
allowed range of values we find that the ratio is approximately 6, consistently with previous calculations which
neglected the transverse photon contribution.Comment: 37 pages, 12 figures, typos corrected, references added, Introduction
rewritte
Analytic structure of the gluon and quark propagators in Landau gauge QCD
In Landau gauge QCD the infrared behavior of the propagator of transverse
gluons can be analytically determined to be a power law from Dyson-Schwinger
equations. This propagator clearly shows positivity violation, indicating the
absence of the transverse gluons from the physical spectrum, i.e. gluon
confinement. A simple analytic structure for the gluon propagator is proposed
capturing all important features. We provide arguments that the Landau gauge
quark propagator possesses a singularity on the real timelike axis. For this
propagator we find a positive definite Schwinger function.Comment: 6 pages, 3 figures; summary of a talk given at several occasions; to
be published in the proceedings of the international conference QCD DOWN
UNDER, March 10 - 19, Adelaide, Australi
Quantum rejection sampling
Rejection sampling is a well-known method to sample from a target
distribution, given the ability to sample from a given distribution. The method
has been first formalized by von Neumann (1951) and has many applications in
classical computing. We define a quantum analogue of rejection sampling: given
a black box producing a coherent superposition of (possibly unknown) quantum
states with some amplitudes, the problem is to prepare a coherent superposition
of the same states, albeit with different target amplitudes. The main result of
this paper is a tight characterization of the query complexity of this quantum
state generation problem. We exhibit an algorithm, which we call quantum
rejection sampling, and analyze its cost using semidefinite programming. Our
proof of a matching lower bound is based on the automorphism principle which
allows to symmetrize any algorithm over the automorphism group of the problem.
Our main technical innovation is an extension of the automorphism principle to
continuous groups that arise for quantum state generation problems where the
oracle encodes unknown quantum states, instead of just classical data.
Furthermore, we illustrate how quantum rejection sampling may be used as a
primitive in designing quantum algorithms, by providing three different
applications. We first show that it was implicitly used in the quantum
algorithm for linear systems of equations by Harrow, Hassidim and Lloyd.
Secondly, we show that it can be used to speed up the main step in the quantum
Metropolis sampling algorithm by Temme et al.. Finally, we derive a new quantum
algorithm for the hidden shift problem of an arbitrary Boolean function and
relate its query complexity to "water-filling" of the Fourier spectrum.Comment: 19 pages, 5 figures, minor changes and a more compact style (to
appear in proceedings of ITCS 2012
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
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
Effect of retardation on dynamical mass generation in two-dimensional QED at finite temperature
The effect of retardation on dynamical mass generation in is studied, in the
imaginary time formalism. The photon porarization tensor is evaluated to
leading order in 1/N (N is the number of flavours), and simple closed form
expressions are found for the fully retarded longitudinal and transverse
propagators, which have the correct limit when T goes to zero. The resulting
S-D equation for the fermion mass (at order 1/N) has an infrared divergence
associated with the contribution of the transverse photon propagator; only the
longitudinal contribution is retained, as in earlier treatments. For solutions
of constant mass, it is found that the retardation reduces the value of the
parameter r (the ratio of twice the mass to the critical temperature) from
about 10 to about 6. The gap equation is then solved allowing for the mass to
depend on frequency. It was found that the r value remained close to 6.
Possibilities for including the transverse propagator are discussed.Comment: 26 pages 8 figure
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