8,681 research outputs found
Infrared-suppressed gluon propagator in 4d Yang-Mills theory in a Landau-like gauge
The infrared behavior of the gluon propagator is directly related to
confinement in QCD. Indeed, the Gribov-Zwanziger scenario of confinement
predicts an infrared vanishing (transverse) gluon propagator in Landau-like
gauges, implying violation of reflection positivity and gluon confinement.
Finite-volume effects make it very difficult to observe (in the minimal Landau
gauge) an infrared suppressed gluon propagator in lattice simulations of the
four-dimensional case. Here we report results for the SU(2) gluon propagator in
a gauge that interpolates between the minimal Landau gauge (for gauge parameter
lambda equal to 1) and the minimal Coulomb gauge (corresponding to lambda = 0).
For small values of lambda we find that the spatially-transverse gluon
propagator D^tr(0,|\vec p|), considered as a function of the spatial momenta
|\vec p|, is clearly infrared suppressed. This result is in agreement with the
Gribov-Zwanziger scenario and with previous numerical results in the minimal
Coulomb gauge. We also discuss the nature of the limit lambda -> 0 (complete
Coulomb gauge) and its relation to the standard Coulomb gauge (lambda = 0). Our
findings are corroborated by similar results in the three-dimensional case,
where the infrared suppression is observed for all considered values of lambda.Comment: 5 pages, 2 figures, one figure with additional results and extended
discussion of some aspects of the results added and some minor
clarifications. In v3: Various small changes and addition
Muscle force is determined also by muscle relative position: isolated effects
Effects on force of changes of the position of extensor digitorum longus muscle (EDL) relative to surrounding tissues were investigated in rat. Connective tissue at the muscle bellies of tibialis anterior (TA), extensor hallucis longus (EHL) and EDL was left intact, to allow myofascial force transmission. The position of EDL muscle was altered, without changing EDL muscle–tendon complex length, and force exerted at proximal and distal tendons of EDL as well as summed force exerted at the distal tendons of TA and EHL muscles (TA+EHL) were measured. Proximal and distal EDL forces as well as distal TA+EHL force changed significantly on repositioning EDL muscle.\ud
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These muscle position–force characteristics were assessed at two EDL lengths and two TA+EHL lengths. It was shown that changes of muscle force with length changes of a muscle is the result of the length changes per se, as well as of changes of relative position of parts of the muscle. It is concluded that in addition to length, muscle position relative to its surroundings co-determines isometric muscle force.\ud
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Keywords: Intermuscular and extramuscular connective tissue; Myofascial force transmission; Rat m. extensor digitorum longus (EDL); Sarcomere length; Muscle relative positio
Phosphorus Abundances in FGK Stars
We measured phosphorus abundances in 22 FGK dwarfs and giants that span
--0.55 [Fe/H] 0.2 using spectra obtained with the Phoenix high
resolution infrared spectrometer on the Kitt Peak National Observatory Mayall
4m telescope, the Gemini South Telescope, and the Arcturus spectral atlas. We
fit synthetic spectra to the P I feature at 10581 to determine abundances
for our sample. Our results are consistent with previously measured phosphorus
abundances; the average [P/Fe] ratio measured in [Fe/H] bins of 0.2 dex for our
stars are within 1 compared to averages from other IR
phosphorus studies. Our study provides more evidence that models of chemical
evolution using the results of theoretical yields are under producing
phosphorus compared to the observed abundances. Our data better fit a chemical
evolution model with phosphorus yields increased by a factor of 2.75 compared
to models with unadjusted yields. We also found average [P/Si] = 0.02
0.07 and [P/S] = 0.15 0.15 for our sample, showing no significant
deviations from the solar ratios for [P/Si] and [P/S] ratios.Comment: 11 pages, 5 figures, Accepted to Ap
A luminosity monitor for the A4 parity violation experiment at MAMI
A water Cherenkov luminosity monitor system with associated electronics has
been developed for the A4 parity violation experiment at MAMI. The detector
system measures the luminosity of the hydrogen target hit by the MAMI electron
beam and monitors the stability of the liquid hydrogen target. Both is required
for the precise study of the count rate asymmetries in the scattering of
longitudinally polarized electrons on unpolarized protons. Any helicity
correlated fluctuation of the target density leads to false asymmetries. The
performance of the luminosity monitor, investigated in about 2000 hours with
electron beam, and the results of its application in the A4 experiment are
presented.Comment: 22 pages, 12 figures, submitted to NIM
Damping of quasi-2D internal wave attractors by rigid-wall friction
The reflection of internal gravity waves at sloping boundaries leads to
focusing or defocusing. In closed domains, focusing typically dominates and
projects the wave energy onto 'wave attractors'. For small-amplitude internal
waves, the projection of energy onto higher wave numbers by geometric focusing
can be balanced by viscous dissipation at high wave numbers. Contrary to what
was previously suggested, viscous dissipation in interior shear layers may not
be sufficient to explain the experiments on wave attractors in the classical
quasi-2D trapezoidal laboratory set-ups. Applying standard boundary layer
theory, we provide an elaborate description of the viscous dissipation in the
interior shear layer, as well as at the rigid boundaries. Our analysis shows
that even if the thin lateral Stokes boundary layers consist of no more than 1%
of the wall-to-wall distance, dissipation by lateral walls dominates at
intermediate wave numbers. Our extended model for the spectrum of 3D wave
attractors in equilibrium closes the gap between observations and theory by
Hazewinkel et al. (2008)
Doctor of Philosophy
dissertationThe wireless radio channel is typically thought of as a means to move information from transmitter to receiver, but the radio channel can also be used to detect changes in the environment of the radio link. This dissertation is focused on the measurements we can make at the physical layer of wireless networks, and how we can use those measurements to obtain information about the locations of transceivers and people. The first contribution of this work is the development and testing of an open source, 802.11b sounder and receiver, which is capable of decoding packets and using them to estimate the channel impulse response (CIR) of a radio link at a fraction of the cost of traditional channel sounders. This receiver improves on previous implementations by performing optimized matched filtering on the field-programmable gate array (FPGA) of the Universal Software Radio Peripheral (USRP), allowing it to operate at full bandwidth. The second contribution of this work is an extensive experimental evaluation of a technology called location distinction, i.e., the ability to identify changes in radio transceiver position, via CIR measurements. Previous location distinction work has focused on single-input single-output (SISO) radio links. We extend this work to the context of multiple-input multiple-output (MIMO) radio links, and study system design trade-offs which affect the performance of MIMO location distinction. The third contribution of this work introduces the "exploiting radio windows" (ERW) attack, in which an attacker outside of a building surreptitiously uses the transmissions of an otherwise secure wireless network inside of the building to infer location information about people inside the building. This is possible because of the relative transparency of external walls to radio transmissions. The final contribution of this dissertation is a feasibility study for building a rapidly deployable radio tomographic (RTI) imaging system for special operations forces (SOF). We show that it is possible to obtain valuable tracking information using as few as 10 radios over a single floor of a typical suburban home, even without precise radio location measurements
Inertial waves and modes excited by the libration of a rotating cube
We report experimental measurements of the flow in a cubic container
submitted to a longitudinal libration, i.e. a rotation modulated in time.
Velocity fields in a vertical and a horizontal plane are measured in the
librating frame using a corotating particle image velocimetry system. When the
libration frequency is smaller than twice the mean rotation rate
, inertial waves can propagate in the interior of the fluid. At
arbitrary excitation frequencies , the oscillating flow
shows two contributions: (i) a basic flow induced by the libration motion, and
(ii) inertial wave beams propagating obliquely upward and downward from the
horizontal edges of the cube. In addition to these two contributions, inertial
modes may also be excited at some specific resonant frequencies. We
characterize in particular the resonance of the mode of lowest order compatible
with the symmetries of the forcing, noted [2,1,+]. By comparing the measured
flow fields to the expected inviscid inertial modes computed numerically
[L.R.M. Maas, Fluid Dyn. Res. \textbf{33}, 373 (2003)], we show that only a
subset of inertial modes, matching the symmetries of the forcing, can be
excited by the libration.Comment: Phys. Fluids (in press
Implementation of the Scalar Dissipation Rate in the REDIM Concept and its Validation for the Piloted Non-Premixed Turbulent Jet Flames
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