16,887 research outputs found
Impact of Dynamical Fermions on QCD Vacuum Structure
We examine how dynamical fermions affect both the UV and infrared structure
of the QCD vacuum. We consider large lattices from the MILC
collaboration, using a gluonic definition of the topological charge density,
founded on a new over-improved stout-link smearing algorithm. The algorithm
reproduces established results from the overlap formalism and is designed to
preserve nontrivial topological objects including instantons. At short
distances we focus on the topological charge correlator, , where
negative values at small $x$ reveal a sign-alternating layered structure to the
topological-charge density of the QCD vacuum. We find that the magnitudes of
the negative dip in the correlator and the positive
contact term are both increased with the introduction of dynamical fermion
degrees of freedom. This is in accord with expectations based on charge
renormalization and the vanishing of the topological susceptibility in the
chiral limit. At large distances we examine the extent to which instanton-like
objects are found on the lattice, and how their distributions vary between
quenched and dynamical gauge fields. We show that dynamical gauge fields
contain more instanton-like objects with an average size greater than in the
quenched vacuum. Finally, we directly visualize the topological charge density
in order to investigate the effects of dynamical sea-quark degrees of freedom
on topology.Comment: 9 pages, 8 figure
Over-Improved Stout-Link Smearing
A new over-improved stout-link smearing algorithm, designed to stabilise
instanton-like objects, is presented. A method for quantifying the selection of
the over-improvement parameter, , is demonstrated. The new smearing
algorithm is compared with the original stout-link smearing, and Symanzik
improved smearing through calculations of the topological charge and
visualisations of the topological charge density.Comment: 9 pages, 18 figures, submitted to Physical Review
The estimation of the propagation delay through the troposphere from microwave radiometer data
The uncertainity in propagation delay estimates is due primarily to tropospheric water, the total amount and vertical distribution of which is variable. Because water vapor both delays and attenuates microwave signals, the propagation delay, or wet path length, can be estimated from the microwave brightness temperature near the 22.235 GHz transition of water vapor. The data from a total of 240 radiosonde launches taken simultaneously were analyzed. Estimates of brightness temperature at 19 and 22 GHz and wet path length were made from these data. The wet path length in the zenith direction could be estimated from the surface water vapor density to an accuracy of 5 cm for the summer data and 2 cm for winter data. Using the brightness temperatures, the wet path could be estimated to an accuracy of 0.3 cm. Two dual frequency radiometers were refurbished in order to test these techniques. These radiometers were capable of measuring the difference in the brightness temperature at 30 deg elevation angle and at the zenith to an accuracy of about 1 K. In August 1975, 45 radiosondes were launched over an 11 day period. Brightness temperature measurements were made simultaneously at 19 and 22 GHz with the radiometers. The rms error for the estimation of wet path length from surface meteorological parameters was 3.2 cm, and from the radiometer brightness temperatures, 1.5 cm
Impact of stout-link smearing in lattice fermion actions
The impact of stout-link smearing in lattice fermion actions is examined
through the consideration of the mass and renormalization functions of the
overlap quark propagator over a variety of smeared configurations. Up to six
sweeps of stout-link smearing are investigated. For heavy quark masses, the
quark propagator is strongly affected by the smearing procedure. For moderate
masses, the effect appears to be negligible. A small effect is seen for light
quark masses, where dynamical mass generation is suppressed through the
smearing procedure.Comment: 7 pages, 3 figures, presented at the XXVII International Symposium on
Lattice Field Theory - LAT2009, July 26-31 2009, Peking University, Beijing,
Chin
Stout-link smearing in lattice fermion actions
The properties of the momentum space quark propagator in Landau gauge are
studied for the overlap quark action in quenched lattice QCD. Numerical
calculations are performed over four ensembles of gauge configurations, where
three are smeared using either 1, 3, or 6 sweeps of stout-link smearing. We
calculate the non-perturbative wave function renormalization function
and the non-perturbative mass function for a variety of bare quark
masses. We find that the wave-function renormalization function is slightly
sensitive to the number of stout-link smearing sweeps. For the mass function we
find the effect of the stout-link smearing algorithm to be small for moderate
to light bare quark masses. For a heavy bare quark mass we find a strong
dependence on the number of smearing sweeps.Comment: 8 pages, 4 figure
Precision electromagnetic structure of decuplet baryons in the chiral regime
The electromagnetic properties of the baryon decuplet are calculated in
quenched QCD on a 20^3 x 40 lattice with a lattice spacing of 0.128 fm using
the fat-link irrelevant clover (FLIC) fermion action with quark masses
providing a pion mass as low as 300 MeV. Magnetic moments and charge radii are
extracted from the electric and magnetic form factors for each individual quark
sector. From these, the corresponding baryon properties are constructed. We
present results for the higher order moments of the spin-3/2 baryons, including
the electric quadrupole moment E2 and the magnetic octupole moment M3. The
world's first determination of a non-zero M3 form factor for the Delta baryon
is presented. With these results we provide a conclusive analysis which shows
that decuplet baryons are deformed. We compare the decuplet baryon results from
a similar lattice calculation of the octet baryons. We establish that the
environment sensitivity is far less pronounced in the case of the decuplet
baryons compared to that in the octet baryons. A surprising result is that the
charge radii of the decuplet baryons are generally smaller than that of the
octet baryons. The magnetic moment of the Delta^+ reveals a turn over in the
low quark mass region, making it smaller than the proton magnetic moment. These
results are consistent with the expectations of quenched chiral perturbation
theory. A similar turn over is also noticed in the magnetic moment of the
Sigma^*0, but not for Xi^* where only kaon loops can appear in quenched QCD.
The electric quadrupole moment of the Omega^- baryon is positive when the
negative charge factor is included, and is equal to 0.86 +- 0.12 x 10^-2 fm^2,
indicating an oblate shape.Comment: 30 pages, 32 figure
A technique for passive attitude control of solar oriented interplanetary spacecraft
Passive damping technique for attitude control of solar orientated interplanetary spacecraf
X-ray Emission from the Weak-lined T Tauri Binary System KH 15D
The unique eclipsing, weak-lined T Tauri star KH 15D has been detected as an
X-ray source in a 95.7 ks exposure from the Chandra X-ray Observatory archives.
A maximum X-ray luminosity of 1.5 x 10^{29} erg s is derived in the
0.5--8 keV band, corresponding to L_{X}/L_bol = 7.5 x 10^{-5}. Comparison with
samples of stars of similar effective temperature in NGC 2264 and in the Orion
Nebula Cluster shows that this is about an order of magnitude low for a typical
star of its mass and age. We argue that the relatively low luminosity cannot be
attributed to absorption along the line of sight but implies a real deficiency
in X-ray production. Possible causes for this are considered in the context of
a recently proposed eccentric binary model for KH 15D. In particular, we note
that the visible component rotates rather slowly for a weak-lined T Tauri star
and has possibly been pseudosynchronized by tidal interaction with the primary
near periastron
Low-lying Odd-parity States of the Nucleon in Lattice QCD
The world's first examination of the odd-parity nucleon spectrum at light
quark masses in 2+1 flavor lattice QCD is presented. Configurations generated
by the PACS-CS collaboration and made available through the ILDG are used, with
the lightest pion mass at 156 MeV. A novel method for tracking the individual
energy eigenstates as the quark mass changes is introduced. The success of this
approach reveals the flow of the states towards the physical masses. Using the
correlation matrix method, the two lowest-energy states revealed are found to
be in accord with the physical spectrum of Nature.Comment: 5 page
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