22,579 research outputs found
Are Topological Charge Fluctuations in QCD Instanton Dominated?
We consider a recent proposal by Horv\'ath {\em et al.} to address the
question whether topological charge fluctuations in QCD are instanton dominated
via the response of fermions using lattice fermions with exact chiral symmetry,
the overlap fermions. Considering several volumes and lattice spacings we find
strong evidence for chirality of a finite density of low-lying eigenvectors of
the overlap-Dirac operator in the regions where these modes are peaked. This
result suggests instanton dominance of topological charge fluctuations in
quenched QCD.Comment: LaTeX, 15 pages, 8 postscript figures, minor improvements, version to
appear in PR
Topology and chiral symmetry breaking in SU(N) gauge theories
We study the low-lying eigenmodes of the lattice overlap Dirac operator for
SU(N) gauge theories with N=2,3,4 and 5 colours. We define a fermionic
topological charge from the zero-modes of this operator and show that, as N
grows, any disagreement with the topological charge obtained by cooling the
fields, becomes rapidly less likely. By examining the fields where there is a
disagreement, we are able to show that the Dirac operator does not resolve
instantons below a critical size of about rho = 2.5 a, but resolves the larger,
more physical instantons. We investigate the local chirality of the near-zero
modes and how it changes as we go to larger N. We observe that the local
chirality of these modes, which is prominent for SU(2) and SU(3), becomes
rapidly weaker for larger N and is consistent with disappearing entirely in the
limit of N -> infinity. We find that this is not due to the observed
disappearance of small instantons at larger N.Comment: 41 pages, 12 figures, RevTe
Tracing Slow Winds from T Tauri Stars via Low Velocity Forbidden Line Emission
Using Keck/HIRES spectra {\Delta}v ~ 7 km/s, we analyze forbidden lines of [O
I] 6300 {\AA}, [O I] 5577 {\AA} and [S II] 6731 {\AA} from 33 T Tauri stars
covering a range of disk evolutionary stages. After removing a high velocity
component (HVC) associated with microjets, we study the properties of the low
velocity component (LVC). The LVC can be attributed to slow disk winds that
could be magnetically (MHD) or thermally (photoevaporative) driven. Both of
these winds play an important role in the evolution and dispersal of
protoplanetary material.
LVC emission is seen in all 30 stars with detected [O I] but only in 2 out of
eight with detected [S II] , so our analysis is largely based on the properties
of the [O I] LVC. The LVC itself is resolved into broad (BC) and narrow (NC)
kinematic components. Both components are found over a wide range of accretion
rates and their luminosity is correlated with the accretion luminosity, but the
NC is proportionately stronger than the BC in transition disks.
The FWHM of both the BC and NC correlates with disk inclination, consistent
with Keplerian broadening from radii of 0.05 to 0.5 AU and 0.5 to 5 AU,
respectively. The velocity centroids of the BC suggest formation in an MHD disk
wind, with the largest blueshifts found in sources with closer to face-on
orientations. The velocity centroids of the NC however, show no dependence on
disk inclination. The origin of this component is less clear and the evidence
for photoevaporation is not conclusive
Instantons and Chiral Symmetry on the Lattice
I address the question of how much of QCD in the chiral limit is reproduced
by instantons. After reconstructing the instanton content of smoothed Monte
Carlo lattice configurations, I compare hadron spectroscopy on this instanton
ensemble to the spectroscopy on the original ``physical'' smoothed
configurations using a chirally optimised clover fermion action. By studying
the zero mode zone in simple instances I find that the optimised action gives a
satisfactory description of it. Through the Banks-Casher formula, instantons by
themselves are shown to break chiral symmetry but hadron correlators on the
instanton backgrounds are strongly influenced by free quark propagation. This
results in unnaturally light hadrons and a small splitting between the vector
and the pseudoscalar meson channels. Superimposing a perturbative ensemble of
zero momentum gauge field fluctuations (torons) on the instantons is found to
be enough to eliminate the free quarks and restore the physical hadron
correlators. I argue that the torons that are present only in finite volumes,
are probably needed to compensate the unnaturally large finite size effects due
to the lack of confinement in the instanton ensemble.Comment: 32 pages, LaTeX with 14 eps figure
On the chirality of quark modes
A model for the QCD vacuum based on a domainlike structured background gluon
field with definite duality attributed to the domains has been shown elsewhere
to give confinement of static quarks, a reasonable value for the topological
susceptibility and indications that chiral symmetry is spontaneously broken. In
this paper we study in detail the eigenvalue problem for the Dirac operator in
such a gluon mean field. A study of the local chirality parameter shows that
the lowest nonzero eigenmodes possess a definite mean chirality correlated with
the duality of a given domain. A probability distribution of the local
chirality qualitatively reproduces histograms seen in lattice simulations.Comment: RevTeX4, 5 figures, 14 page
Assessment of Effectiveness of Buffer Zones in Removing Impurites in Runoff from Areas Treated with Poultry Litter
Land application of animal manures (e.g. poultry litter, poultry manure, and swine manure) to pasture and range can lead to runoff quality degradation during storms that occur soon after application. Vegetative filter strips (VFS) have been shown to reduce pollution in runoff from row-cropped areas but have not been extensively studied in pasture and range settings. This research involved characterizing performance of fescue VFS in improving quality of runoff from pasture land areas treated with poultry litter and swine manure. The VFS were found to be quite effective in reducing off-site transport of ammonia nitrogen (NH3-N), total Kjeldahl nitrogen (TKN), ortho-phosphorus (P04-P), total phospnorus (TP) , and fecal coliform (FC) for simulated storms occurring 2-5 days following poultry litter and swine manure applicati on. The VFS were from 81 to 99% effective (at a VFS length of 21.4 m) in reducing incoming mass transport of NH3-N, TKN, P04-P, TP, and FC in runoff from poultry litter-treated plots. Similar performance was observed for the VFS installed below plots treated with swine manure. Transport of suspended solids and chemical oxygen demand was also reduced by the VFS, but generally not to the extent of other litter and manure constituents. Transport of poultry litter and swine manure constituents were well-described by first-order kinetics
Comment on ``Evidence Against Instanton Dominance of Topological Charge Fluctuations in QCD''
We comment on the recent paper (hep-lat/0102003) by Horvath, Isgur, McCune,
and Thacker, which concludes that the local chiral structure of fermionic
eigenmodes is not consistent with instanton dominance. Our calculations, done
with an overlap action, suggest the opposite conclusion.Comment: 5 pages, Revtex, 4 postscript figures. COLO-HEP-45
Chirality Correlation within Dirac Eigenvectors from Domain Wall Fermions
In the dilute instanton gas model of the QCD vacuum, one expects a strong
spatial correlation between chirality and the maxima of the Dirac eigenvectors
with small eigenvalues. Following Horvath, {\it et al.} we examine this
question using lattice gauge theory within the quenched approximation. We
extend the work of those authors by using weaker coupling, , larger
lattices, , and an improved fermion formulation, domain wall fermions. In
contrast with this earlier work, we find a striking correlation between the
magnitude of the chirality density, , and the
normal density, , for the low-lying Dirac eigenvectors.Comment: latex, 25 pages including 12 eps figure
Risk factors for active trachoma in The Gambia.
Trachoma has been endemic in The Gambia for decades but national surveys indicate that the prevalence is falling. Risk factor data can help guide trachoma control efforts. This study investigated risk factors for active trachoma and ocular Chlamydia trachomatis infection in children aged below 10 years in two Gambian regions. The overall prevalence of C. trachomatis infection was only 0.3% (3/950) compared with 10.4% (311/2990) for active trachoma, therefore analyses were only performed for active trachoma. After adjustment, increased risk of trachoma was associated with being aged 1-2 years (odds ratio (OR) 2.20, 95% CI 1.07-4.52) and 3-5 years (OR 3.62, 95% CI 1.80-7.25) compared with <1 year, nasal discharge (OR 2.07, 95% CI 1.53-2.81), ocular discharge (OR 2.68, 95% CI 1.76-4.09) and there being at least one other child in the household with active trachoma (OR 11.28, 95% CI 8.31-15.31). Compared with other occupations, children of traders had reduced risk (OR 0.53, 95% CI 0.30-0.94). At the household level, only the presence of another child in the household with active trachoma was associated with increased risk of active trachoma, suggesting that current trachoma control interventions are effective at this level. In contrast, child-level factors were associated with increased risk after adjustment, indicating a need to increase control efforts at the child level
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