3,779 research outputs found
Status of center dominance in various center gauges
We review arguments for center dominance in center gauges where vortex
locations are correctly identified. We introduce an appealing interpretation of
the maximal center gauge, discuss problems with Gribov copies, and a cure to
the problems through the direct Laplacian center gauge. We study correlations
between direct and indirect Laplacian center gauges.Comment: Presented by S. Olejnik at the NATO Advanced Research Workshop
"Confinement, Topology, and other Non-Perturbative Aspects of QCD", Jan.
21-27, 2002, Stara Lesna, Slovakia. 10 pages, 3 figures (8 EPS files), uses
crckapb.st
The Changing AGN Population
We investigate how the fraction of broad-line sources in the AGN population
changes with X-ray luminosity and redshift. We first construct the rest-frame
hard-energy (2-8 keV) X-ray luminosity function (HXLF) at z=0.1-1 using Chandra
Lockman Hole-Northwest wide-area data, Chandra Deep Field-North 2 Ms data,
other Chandra deep field data, and the ASCA Large Sky Survey data. We find that
broad-line AGNs dominate above 3e43 ergs/s and have a mean luminosity of 1.3e44
ergs/s. Type II AGNs can only become an important component of the X-ray
population at Seyfert-like X-ray luminosities. We then construct z=0.1-0.5 and
z=0.5-1 HXLFs and compare them with both the local HXLF measured from HEAO-1 A2
survey data and the z=1.5-3 HXLF measured from soft-energy (0.5-2 keV) Chandra
and ROSAT data. We find that the number density of >1e44 ergs/s sources
(quasars) steadily declines with decreasing redshift, while the number density
of 1e43-1e44 ergs/s sources peaks at z=0.5-1. Strikingly, however, the number
density of broad-line AGNs remains roughly constant with redshift while their
average luminosities decline at the lower redshifts, showing another example of
cosmic downsizing.Comment: Accepted by The Astrophysical Journal Letters, 5 page
Remarks on the Gribov Problem in Direct Maximal Center Gauge
We review the equivalence of maximal center gauge fixing to the problem of
finding the best fit, to a given lattice gauge field, by a thin vortex
configuration. This fit is necessarily worst at the location of P-plaquettes.
We then compare the fits achieved in Gribov copies generated by (i)
over-relaxation; (ii) over-relaxation after Landau gauge preconditioning; and
(iii) simulated annealing. Simulated annealing yields the best fit if all links
on the lattice are included, but the situation changes if we consider only the
lattice volume exterior to P-plaquettes. In this exterior region, the fit is
best for Gribov copies generated by over-relaxation, and worst for Gribov
copies generated after Landau gauge preconditioning. The two fitting criteria
(including or not including the P-plaquettes) yield string tensions differing
by -34% to +20% respectively, relative to the full string tension. Our usual
procedure (``quenched minimization'') seems to be a compromise between these
criteria, and yields string tensions at an intermediate value close to the full
string tension.Comment: 14 pages, 6 figure
On the relevance of center vortices to QCD
In a numerical experiment, we remove center vortices from an ensemble of
lattice SU(2) gauge configurations. This removal adds short-range disorder.
Nevertheless, we observe long-range order in the modified ensemble: confinement
is lost and chiral symmetry is restored (together with trivial topology),
proving that center vortices are responsible for both phenomena. As for the
Abelian monopoles, they survive but their percolation properties are lost.Comment: 4 pages, 5 figures; discussion expanded, text compressed... to appear
in Phys. Rev. Let
Importance of cooling in triggering the collapse of hypermassive neutron stars
The inspiral and merger of a binary neutron star (NSNS) can lead to the
formation of a hypermassive neutron star (HMNS). As the HMNS loses thermal
pressure due to neutrino cooling and/or centrifugal support due to
gravitational wave (GW) emission, and/or magnetic breaking of differential
rotation it will collapse to a black hole. To assess the importance of
shock-induced thermal pressure and cooling, we adopt an idealized equation of
state and perform NSNS simulations in full GR through late inspiral, merger,
and HMNS formation, accounting for cooling. We show that thermal pressure
contributes significantly to the support of the HMNS against collapse and that
thermal cooling accelerates its "delayed" collapse. Our simulations demonstrate
explicitly that cooling can induce the catastrophic collapse of a hot
hypermassive neutron star formed following the merger of binary neutron stars.
Thus, cooling physics is important to include in NSNS merger calculations to
accurately determine the lifetime of the HMNS remnant and to extract
information about the NS equation of state, cooling mechanisms, bar
instabilities and B-fields from the GWs emitted during the transient phase
prior to BH formation.Comment: 13 pages, 7 figures, matches published versio
Some Cautionary Remarks on Abelian Projection and Abelian Dominance
Some critical remarks are presented, concerning the abelian projection theory
of quark confinement.Comment: Talk presented at LATTICE96(topology) plenary session, uses psfig and
espcrc2 package
Manipulating the torsion of molecules by strong laser pulses
A proof-of-principle experiment is reported, where torsional motion of a
molecule, consisting of a pair of phenyl rings, is induced by strong laser
pulses. A nanosecond laser pulse spatially aligns the carbon-carbon bond axis,
connecting the two phenyl rings, allowing a perpendicularly polarized, intense
femtosecond pulse to initiate torsional motion accompanied by an overall
rotation about the fixed axis. The induced motion is monitored by femtosecond
time-resolved Coulomb explosion imaging. Our theoretical analysis accounts for
and generalizes the experimental findings.Comment: 4 pages, 4 figures, submitted to PRL; Major revision of the
presentation of the material; Correction of ion labels in Fig. 2(a
Abelian Dominance in Wilson Loops
It has been conjectured that the Abelian projection of QCD is responsible for
the confinement of color. Using a gauge independent definition of the Abelian
projection which does {\it not} employ any gauge fixing, we provide a strong
evidence for the Abelian dominance in Wilson loop integral. In specific we
prove that the gauge potential which contributes to the Wilson loop integral is
precisely the one restricted by the Abelian projection.Comment: 4 pages, no figure, revtex. Phys. Rev. D in pres
Time-frequency detection algorithm for gravitational wave bursts
An efficient algorithm is presented for the identification of short bursts of
gravitational radiation in the data from broad-band interferometric detectors.
The algorithm consists of three steps: pixels of the time-frequency
representation of the data that have power above a fixed threshold are first
identified. Clusters of such pixels that conform to a set of rules on their
size and their proximity to other clusters are formed, and a final threshold is
applied on the power integrated over all pixels in such clusters. Formal
arguments are given to support the conjecture that this algorithm is very
efficient for a wide class of signals. A precise model for the false alarm rate
of this algorithm is presented, and it is shown using a number of
representative numerical simulations to be accurate at the 1% level for most
values of the parameters, with maximal error around 10%.Comment: 26 pages, 15 figures, to appear in PR
The static and dynamic conductivity of warm dense Aluminum and Gold calculated within a density functional approach
The static resistivity of dense Al and Au plsmas are calculated where all the
needed inputs are obtained from density functional theory (DFT). This is used
as input for a study of the dynamic conductivity. These calculations involve a
self-consistent determination of (i) the equation of state (EOS) and the
ionization balance, (ii) evaluation of the ion-ion, and ion-electron
pair-distribution functions, (iii) Determination of the scattering amplitudes,
and finally the conductivity. We present data for the static resistivity of Al
for compressions 0.1-2.0, and in the temperature range T= 0.1 - 10 eV. Results
for Au in the same temperature range and for compressions 0.1-1.0 is also
given. In determining the dynamic conductivity for a range of frequencies
consistent with standard laser probes, a knowledge of the electronic
eigenstates and occupancies of Al- or Au plasma becomes necessary. They are
calculated using a neutral-pseudoatom model. We examine a number of
first-principles approaches to the optical conductivity, including many-body
perturbation theory, molecular-dynamics evaluations, and simplified
time-dependent DFT. The modification to the Drude conductivity that arises from
the presence of shallow bound states in typical Al-plasmas is examined and
numerical results are given at the level of the Fermi Golden rule and an
approximate form of time-dependent DFT.Comment: 5 figures, Latex original. Cross-referencced to PLASMA PHYSIC
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