2,057 research outputs found
A Perspective on Hadron Physics
Full text linkThe phenomena of confinement and dynamical chiral symmetry breaking are basic
to understanding hadron observables. They can be explored using Dyson-Schwinger
equations. The existence of a systematic, nonperturbative and symmetry
preserving truncation of these equations enables the proof of exact results in
QCD, and their illustration using simple but accurate models. We provide a
sketch of the material qualitative and quantitative success that has been
achieved in the study of pseudoscalar and vector mesons. Efforts are now
turning to the study of baryons, which we exemplify via a calculation of
nucleon weak and pionic form factors.Comment: 16 pages, 5 figures. Contribution to the proceedings of the "Xth
Mexican Workshop on Particles and Fields," Morelia, Mexico, 6-12 Nov. 200
The Sigma Commutator from Lattice QCD
Get PDFAs a direct source of information on chiral symmetry breaking within QCD, the
sigma commutator is of considerable importance. Since hadron structure is a
non-perturbative problem, numerical calculations on a space-time lattice are
currently the only rigorous approach. With recent advances in the calculation
of hadron masses within full QCD, it is of interest to see whether the sigma
commutator can be calculated directly from the dependence of the nucleon mass
on the input quark mass. We show that, provided the correct chiral behaviour of
QCD is respected in the extrapolation to realistic quark masses, one can indeed
obtain a fairly reliable determination of the sigma commutator using present
lattice data. For two-flavour dynamical fermion QCD the sigma commutator lies
between 45 and 55 MeV based on recent data from CP-PACS and UKQCD.Comment: 4 pages, 3 figures, uses espcrc1.sty and epsfig.sty. Contribution to
the proceedings of the International Conference on Quark Nuclear Physics held
in Adelaide Feb. 200
Chiral Nonanalytic Behaviour: The Edinburgh Plot
Get PDFThe Edinburgh Plot is a scale independent way of presenting lattice QCD
calculations over a wide range of quark masses. In this sense it is appealing
as an indicator of how the approach to physical quark masses is progressing.
The difficulty remains that even the most state of the art calculations are
still at quark masses that are too heavy to apply dimensionally-regulated
chiral perturbation theory. We present a method allowing predictions of the
behaviour of the Edinburgh plot, in both the continuum, and on the lattice.Comment: 3 pages, 4 figures, Lattice2002(Spectrum
Baryon Mass Extrapolation
Get PDFConsideration of the analytical properties of pion-induced baryon
self-energies leads to new functional forms for the extrapolation of light
baryon masses. These functional forms reproduce the leading non-analytic
behavior of chiral perturbation theory, the correct heavy-quark limit and have
the advantage of containing information on the extended structure of hadrons.
The forms involve only three unknown parameters which may be optimized by
fitting to present lattice data. Recent dynamical fermion results from CP-PACS
and UK-QCD are extrapolated using these new functional forms. We also use these
functions to probe the limit of the chiral perturbative regime and shed light
on the applicability of chiral perturbation theory to the extrapolation of
present lattice QCD results.Comment: LATTICE99 (QCD Spectrum and Quark Masses
Chiral Corrections to Baryon Masses Calculated within Lattice QCD
Get PDFConsideration of the analytic properties of pion-induced baryon self energies
leads to new functional forms for the extrapolation of light baryon masses.
These functional forms reproduce the leading non-analytic behavior of chiral
perturbation theory, the correct non-analytic behavior at the threshold
and the appropriate heavy-quark limit. They involve only three unknown
parameters, which may be obtained by fitting lattice QCD data. Recent dynamical
fermion results from CP-PACS and UKQCD are extrapolated using these new
functional forms. We also use these functions to probe the limit of
applicability of chiral perturbation theory.Comment: 4 pages, 2 figures, Contribution to the Proceedings of the 15th
Particles and Nuclei International Conference (PANIC 99), Uppsala, Sweden,
June 10-16, 199
Dynamical chiral symmetry breaking and a critical mass
Get PDFOn a bounded, measurable domain of non-negative current-quark mass, realistic
models of QCD's gap equation can simultaneously admit two inequivalent
dynamical chiral symmetry breaking (DCSB) solutions and a solution that is
unambiguously connected with the realisation of chiral symmetry in the Wigner
mode. The Wigner solution and one of the DCSB solutions are destabilised by a
current-quark mass and both disappear when that mass exceeds a critical value.
This critical value also bounds the domain on which the surviving DCSB solution
possesses a chiral expansion. This value can therefore be viewed as an upper
bound on the domain within which a perturbative expansion in the current-quark
mass around the chiral limit is uniformly valid for physical quantities. For a
pseudoscalar meson constituted of equal mass current-quarks, it corresponds to
a mass m_{0^-}~0.45GeV. In our discussion we employ properties of the two DCSB
solutions of the gap equation that enable a valid definition of in
the presence of a nonzero current-mass. The behaviour of this condensate
indicates that the essentially dynamical component of chiral symmetry breaking
decreases with increasing current-quark mass.Comment: 9 pages, 7 figures. Minor wording change
Chiral Extrapolation: An Analogy with Effective Field Theory
Get PDFWe draw an analogy between the chiral extrapolation of lattice QCD
calculations from large to small quark masses and the interpolation between the
large mass (weak field) and small mass (strong field) limits of the
Euler--Heisenberg QED effective action. In the latter case, where the exact
answer is known, a simple extrapolation of a form analogous to those proposed
for the QCD applications is shown to be surprisingly accurate over the entire
parameter range.Comment: 6 pp, revtex, 3 figs; minor changes -- version to appear in PL
Debris disc candidates in systems with transiting planets
Full text linkDebris discs are known to exist around many planet-host stars, but no debris
dust has been found so far in systems with transiting planets. Using publicly
available catalogues, we searched for infrared excesses in such systems. In the
recently published Wide-Field Infrared Survey Explorer (WISE) catalogue, we
found 52 stars with transiting planets. Two systems with one transiting "hot
Jupiter" each, TrES-2 and XO-5, exhibit small excesses both at 12 and 22
microns at a > 3 sigma level. Provided that one or both of these detections are
real, the frequency of warm excesses in systems with transiting planets of 2-4
% is comparable to that around solar-type stars probed at similar wavelengths
with Spitzer's MIPS and IRS instruments. Modelling suggests that the observed
excesses would stem from dust rings with radii of several AU. The inferred
amount of dust is close to the maximum expected theoretically from a
collisional cascade in asteroid belt analogues. If confirmed, the presence of
debris discs in systems with transiting planets may put important constraints
onto formation and migration scenarios of hot Jupiters.Comment: Accepted for publication in MNRAS Letter
A Large Animal Survival Model to Evaluate Bariatric Surgery Mechanisms
Get PDFBACKGROUND:
The impact of Roux-en-Y gastric bypass (RYGB) on type 2 diabetes mellitus is thought to result from upper and/or lower gut hormone alterations. Evidence supporting these mechanisms is incomplete, in part because of limitations in relevant bariatric-surgery animal models, specifically the lack of naturally insulin-resistant large animals. With overfeeding, Ossabaw swine develop a robust metabolic syndrome, and may be suitable for studying post-surgical physiology. Whether bariatric surgery is feasible in these animals with acceptable survival is unknown.
METHODS:
Thirty-two Ossabaws were fed a high-fat, high-cholesterol diet to induce obesity and insulin resistance. These animals were assigned to RYGB (n = 8), RYGB with vagotomy (RYGB-V, n = 5), gastrojejunostomy (GJ, n = 10), GJ with duodenal exclusion (GJD, n = 7), or sham operation (n = 2) and were euthanized 60 days post-operatively. Post-operative changes in weight and food intake are reported.
RESULTS:
Survival to scheduled necropsy among surgical groups was 77%, living an average of 57 days post-operatively. Cardiac arrest under anesthesia occurred in 4 pigs. Greatest weight loss (18.0% ± 6%) and food intake decrease (57.0% ± 20%) occurred following RYGB while animals undergoing RYGB-V showed only 6.6% ± 3% weight loss despite 50.8% ± 25% food intake decrease. GJ (12.7% ± 4%) and GJD (1.2% ± 1%) pigs gained weight, but less than sham controls (13.4% ± 10%).
CONCLUSIONS:
A survival model of metabolic surgical procedures is feasible, leads to significant weight loss, and provides the opportunity to evaluate new interventions and subtle variations in surgical technique (e.g. vagus nerve sparing) that may provide new mechanistic insights
Participatory Budgeting in the United States: A Preliminary Analysis of Chicago's 49 th
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