27,374 research outputs found
Temperature Dependence of Gluon and Quark Condensates as from Linear Confinement
The gluon and quark condensates and their temperature dependence are
investigated within QCD premises. The input for the former is a gauge invariant
kernel made up of the direct (D), exchange (X) and contact(C) QCD
interactions in the lowest order, but with the perturbative propagator
replaced by a `non-perturbative form obtained via two
differentiations: , ( a scale
parameter), and then setting , to simulate linear confinement. Similarly
for the input kernel the gluon propagator is replaced by the above
form. With these `linear' simulations, the respective condensates are
obtained by `looping' up the gluon and quark lines in the standard manner.
Using Dimensional regularization (DR), the necessary integrals yield the
condensates plus temperature corrections, with a common scale parameter
for both. For gluons the exact result is . Evaluation
of the quark condensate is preceded by an approximate solution of the SDE for
the mass function , giving a recursive formula, with convergence achieved
at the third iteration. Setting the scale parameter equal to the
universal Regge slope , the gluon and quark condensates at T=0 are
found to be and respectively, in fair accord
with QCD sum rule values. Next, the temperature corrections (of order
for both condensates) is determined via finite-temperature field theory a la
Matsubara. Keywords: Gluon Condensate, mass tensor, gauge invariance, linear
confinement, finite-temperature, contour-closing. PACS: 11.15.Tk ; 12.38.Lg ;
13.20.CzComment: 13 pages (LaTeX) including 2 figure
Coherent structures in fully-developed pipe turbulence
A turbulent mean profile for pipe flow is prescribed which closely matches
experimental observations. The nature of perturbations superimposed upon this
profile is then considered. Optimal growth calculations predict two distinct
classes of structures, clearly associated with near-wall and large-scale
structures. Quantitative correspondence of the spanwise wavelength of
wall-structures with experimental observations is very good. The response to
harmonic forcing is also considered, and the linear growth tested with direct
numerical simulation of forced turbulence. Despite the very simple eddy
viscosity assumption, this linear approach predicts well the surprisingly large
growth of outer-scale modes in the bulk flow.
Un profil moyen turbulent est prescrit dans une conduite cylindrique, en
adequation avec les observations experimentales. Nous considerons ensuite la
nature des perturbations a cet ecoulement synthetique. Le calcul des
croissances optimales predit deux types de structures, associees respectivement
aux structures de proche-paroi et de grande echelle. Un excellent accord
quantitatif est trouve avec les resultats experimentaux quant a la longueur
d'onde transversale. La reponse harmonique est egalement etudiee, et la
croissance lineaire observee comparee a des simulations numeriques directes de
turbulence forcee. Malgre de l'hypothese simple de type `Eddy viscosity', cette
approche lineaire predit efficacement la croissance spectaculaire des modes de
grande echelle au coeur de l'ecoulement.Comment: 5 pages; Congres Francais de Mecanique, Marseille (2009
Relativistic Hydrodynamic Cosmological Perturbations
Relativistic cosmological perturbation analyses can be made based on several
different fundamental gauge conditions. In the pressureless limit the variables
in certain gauge conditions show the correct Newtonian behaviors. Considering
the general curvature () and the cosmological constant () in the
background medium, the perturbed density in the comoving gauge, and the
perturbed velocity and the perturbed potential in the zero-shear gauge show the
same behavior as the Newtonian ones in general scales. In the first part, we
elaborate these Newtonian correspondences. In the second part, using the
identified gauge-invariant variables with correct Newtonian correspondences, we
present the relativistic results with general pressures in the background and
perturbation. We present the general super-sound-horizon scale solutions of the
above mentioned variables valid for general , , and generally
evolving equation of state. We show that, for vanishing , the
super-sound-horizon scale evolution is characterised by a conserved variable
which is the perturbed three-space curvature in the comoving gauge. We also
present equations for the multi-component hydrodynamic situation and for the
rotation and gravitational wave.Comment: 16 pages, no figure, To appear in Gen. Rel. Gra
Lines on projective varieties and applications
The first part of this note contains a review of basic properties of the
variety of lines contained in an embedded projective variety and passing
through a general point. In particular we provide a detailed proof that for
varieties defined by quadratic equations the base locus of the projective
second fundamental form at a general point coincides, as a scheme, with the
variety of lines. The second part concerns the problem of extending embedded
projective manifolds, using the geometry of the variety of lines. Some
applications to the case of homogeneous manifolds are included.Comment: 15 pages. One example removed; one remark and some references added;
typos correcte
Field evaluation of the CATT/Trypanosoma brucei gambiense on blood-impregnated filter papers for diagnosis of human African trypanosomiasis in southern Sudan.
Most Human African Trypanosomiasis (HAT) control programmes in areas endemic for Trypanosoma brucei gambiense rely on a strategy of active mass screening with the Card Agglutination Test for Trypanosomiasis (CATT)/T. b. gambiense. We evaluated the performance, stability and reproducibility of the CATT/T. b. gambiense on blood-impregnated filter papers (CATT-FP) in Kajo-Keji County, South-Sudan, where some areas are inaccessible to mobile teams. The CATT-FP was performed with a group of 100 people with a positive CATT on whole blood including 17 confirmed HAT patients and the results were compared with the CATT on plasma (CATT-P). The CATT-FP was repeated on impregnated filter papers stored at ambient and refrigerated temperature for 1, 3, 7 and 14 days. Another 82 patients with HAT, including 78 with a positive parasitology, were tested with the CATT-FP and duplicate filter paper samples were sent to a reference laboratory to assess reproducibility. The CATT-FP was positive in 90 of 99 patients with HAT (sensitivity: 91%). It was less sensitive than the CATT-P (mean dilution difference: -2.5). There was no significant loss of sensitivity after storage for up to 14 days both at ambient and cool temperature. Reproducibility of the CATT-FP was found to be excellent (kappa: 0.84). The CATT-FP can therefore be recommended as a screening test for HAT in areas where the use of CATT-P is not possible. Further studies on larger population samples in different endemic foci are still needed before the CATT-FP can be recommended for universal use
An optical NMR spectrometer for Larmor-beat detection and high-resolution POWER NMR
Optical nuclear magnetic resonance (ONMR) is a powerful probe of electronic properties in III-V semiconductors. Larmor-beat detection (LBD) is a sensitivity optimized, time-domain NMR version of optical detection based on the Hanle effect. Combining LBD ONMR with the line-narrowing method of POWER (perturbations observed with enhanced resolution) NMR further enables atomically detailed views of local electronic features in III-Vs. POWER NMR spectra display the distribution of resonance shifts or line splittings introduced by a perturbation, such as optical excitation or application of an electric field, that is synchronized with a NMR multiple-pulse time-suspension sequence. Meanwhile, ONMR provides the requisite sensitivity and spatial selectivity to isolate local signals within macroscopic samples. Optical NMR, LBD, and the POWER method each introduce unique demands on instrumentation. Here, we detail the design and implementation of our system, including cryogenic, optical, and radio-frequency components. The result is a flexible, low-cost system with important applications in semiconductor electronics and spin physics. We also demonstrate the performance of our systems with high-resolution ONMR spectra of an epitaxial AlGaAs/GaAs heterojunction. NMR linewidths down to 4.1 Hz full width at half maximum were obtained, a 10^3-fold resolution enhancement relative any previous optically detected NMR experiment
Percolation of Immobile Domains in Supercooled Thin Polymeric Films
We present an analysis of heterogeneous dynamics in molecular dynamics
simulations of a thin polymeric film, supported by an absorbing structured
surface. Near the glass transition "immobile" domains occur throughout the
film, yet the probability of their occurrence decreasing with larger distance
from the surface. Still, enough immobile domains are located near the free
surface to cause them to percolate in the direction perpendicular to surface,
at a temperature near the glass transition temperature. This result is in
agreement with a recent theoretical model of glass transition
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