7,264 research outputs found
The Sigma Commutator from Lattice QCD
As 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 Corrections to Baryon Masses Calculated within Lattice QCD
Consideration 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
1/Nc Expansion in QCD: Double-Line Counting Rules and the Undeservingly Discarded U(1) Ghost
The 1/Nc expansion is one of the very few methods we have for generating a
systematic expansion of QCD at the energy scale relevant to hadron structure.
The present formulation of this theory relies on the double-line notation for
calculating the leading order of a diagram in the 1/Nc expansion, where the
local SU(Nc) gauge symmetry is substituted by a U(Nc) symmetry and the
associated U(1) ghost field is ignored. In the current work we demonstrate the
insufficiency of this formulation for describing certain non-planar diagrams.
We derive a more complete set of Feynman rules that include the U(1) ghost
field and provide a useful tool for calculating both color factors and 1/Nc
orders of all color-singlet diagrams
Chiral Nonanalytic Behaviour: The Edinburgh Plot
The 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
Recommended from our members
Changes in the intestinal microbiota after a short period of dietary over-indulgence, representative of a holiday or festival season
The effects on the intestinal microbiota of a short period of marginal over-eating, characteristic of holiday or
festival periods, were investigated in a pilot study. Fourteen healthy male subjects consumed a diet rich in
animal protein and fat for seven days. During this period, the subjects significantly increased their dietary
energy, protein, carbohydrate and fat intakes by 56, 59, 53 and 58%, respectively (all P < 0.05). The mean
weight gain of 0.27 kg was less than the expected 1 kg, but this was consistent with a degree of under-reporting
on the baseline diet. Fluorescence in situ hybridisation analysis confirmed the relative stability of each
individual’s faecal microbiota but showed considerable variations between them. The diet was associated with
a significant increase in numbers of total faecal bacteria and the bacteroides group, as detected by the universal
bacterial probe (DAPI) and Bacteroides probe (Bac 303), respectively. Overall, there was a decrease in
numbers of the Lactobacillus/Enterococcus group (Lab 158 probe; 2.8 ± 3.0% to 1.8 ± 1.8%) and the Bifidobacterium
group (Bif 164 probe; 3.0 ± 3.7% to 1.7 ± 1.2%), although there was considerable inter-individual
variation. Analysis of the relative proportions of each bacterial group as a percentage of the subject’s total
bacteria showed a trend for a change in the intestinal microbiota that might be considered potentially
unhealthy
Baryon Mass Extrapolation
Consideration 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
Corneoscleral laceration and ocular burns caused by electronic cigarette explosions
PURPOSE: To report cases of acute globe rupture and bilateral corneal burns from electronic cigarette (EC) explosions. METHODS: Case series. RESULTS: We describe a series of patients with corneal injury caused by EC explosions. Both patients suffered bilateral corneal burns and decreased visual acuity, and one patient sustained a unilateral corneoscleral laceration with prolapsed iris tissue and hyphema. A review of the scientific literature revealed no prior reported cases of ocular injury secondary to EC explosions; however, multiple media and government agency articles describe fires and explosions involving ECs, including at least 4 with ocular injuries. CONCLUSIONS: Given these cases and the number of recent media reports, ECs pose a significant public health risk. Users should be warned regarding the possibility of severe injury, including sight-threatening ocular injuries ranging from corneal burns to full-thickness corneoscleral laceration
Chiral Extrapolation: An Analogy with Effective Field Theory
We 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
- …