30,364 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
Test and evaluation of water vapor radiometers
The accuracy of very long baseline interferometry in geodetic and astrometric applications is primarily limited by the propagation delays through the troposphere. The part of this delay that is most difficult to predict is due to atmospheric water vapor which can contribute up to about 40 cm of excess propagation path at microwave frequencies. The water vapor content in the atmosphere is variable and is not well correlated with surface meteorological variables. The brightness temperature measured near the transition of water vapor at 22.2 GHz and the propagation delay due to water vapor, or wet path length, are well correlated. This correlation is not perfect because the absorption coefficient and index of refraction do not have the same dependence on temperature and pressure
A computational method to model radar return range in a polygonally based, computer-generated-imagery simulation
Described is a method for modeling a ground-mapping radar system for use in simulations where the terrain is in a polygonal form commonly used with computer generated imagery (CGI). The method employs a unique approach for rapidly rejecting polygons not visible to the radar to facilitate the real-time simulation of the radar return. This rapid rejection of the nonvisible polygons requires the precalculation and storage of a set of parameters that do not vary during the simulation. The calculation of a radar range as a function of the radar forward-looking angle to the CGI terrain is carried out only for the visible polygons. This method was used as part of a simulation for terrain-following helicopter operations on the vertical motion simulator at the NASA Ames Research Center. It proved to be an efficient means for returning real-time simulated radar range data
Barriers to application for judicial appointment research: lesbian, gay, bisexual and transgender experiences
A 43GHz VLBI mapping of SiO maser emission associated with Orion-KL IRC-2
A milliarcsecond resolution spot map of the SiO maser emission associated with IRC-2 in Orion-KL is presented. The two dominant groups of spectral features, near V(LRS) = -6 and 16 km/s, were observed in the 43 GHz, v = 1 to 0 transition of SiO, using a Mark III VLBI system. The 74 km baseline ran from Haystack Observatory in Westford, Massachusetts to Five College Radio Astronomy Observatory (FCRAO) in New Salem, Massachusetts. Five distinct maser features were observed: -8.5 to -6.5 km/s; -5 to -1.5 km/s; 12 to 13.5 km/s; 16.5 to 19 km/s; and 20 to 21 km/s (stellar velocity = 5 km/s). The relative positions were established, from an analysis of fringe phases, to an accuracy of about 5 milliarcseconds. All the features lay within an area of radius 0.08 arcseconds or 6x10(14) cm, at a distance of 500 pc. Previous interferometric studies were only able to measure the gross separation between the red and the blue shifted groups. Our measurement of the separation between these two gropus is consistent with those of the previous studies, indicating the persistence of these two centers of activity. The absolute positions of the masers with respect to IRC-2 are only known to an accuracy of about 1 arcsecond. It is assumed that IRC-2 is centered between the red shifted and the blue shifted maser features. The relative placement of these two groups of maser features agrees with observations of thermal emission from SO, which traces the outflow on a much larger scale. The SiO masers trace the neutral outflow from IRC-2 on the smallest scale yet observed
New receivers for DS-SS in time variant multipath channels based on the PN alignment concept
We present new combined blind equalization and detection schemes for a DS-SS system. The new proposed algorithms improve the bit error rate compared to traditional RAKE receivers in time-variant channels with multipath. This improvement is obtained in both simulated and a real ionospheric HF link. Its very low computational complexity makes them suitable to be implemented in real receivers.Peer ReviewedPostprint (published version
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
DSP-based ionospheric radiolink using DS-CDMA and on-line channel estimation
In this paper, a new blind multiuser detection algorithm is presented. It can both cancel multiuser interference and estimate the multipath channel response in a blind way. The method has been specially conceived for low coherence bandwidth channels such as the ionospheric channel and exhibits very low computational requirements. Real-time measurements from a fully digital HF radio-link are presented that confirm the reliability of the method for the ionospheric channel.Peer ReviewedPostprint (published version
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