65,391 research outputs found
Effects of medium-induced meson mixing on the equation of state in isospin-asymmetric nuclear matter
We reexamine effects of the meson mixing mediated by nucleon
polarizations on the symmetry energy in isospin-asymmetric nuclear matter.
Taking into account the rearrangement term neglected in previous studies by
others, we evaluate the mixing angle in a novel way within the
Relativistic Mean-Field Models with and without chiral limits. It is found that
the symmetry energy is significantly softened at high densities contrary to the
finding in earlier studies. As the first step of going beyond the lowest-order
calculations, we also solve the RPA equation for the mixing. In
this case, it is found that the symmetry energy is not only significantly
softened by the mixing at supra-saturation densities, similar to
the lowest-order mixing, but interestingly also softened at
subsaturation densities. In addition, the softening of the symmetry energy at
subsaturation densities can be partly suppressed by the nonlinear
self-interaction of the meson.Comment: Significant changes made. Accepted version to appear in PRC (2009
Uniqueness of Bessel models: the archimedean case
In the archimedean case, we prove uniqueness of Bessel models for general
linear groups, unitary groups and orthogonal groups.Comment: 22 page
Self-consistent triaxial de Zeeuw-Carollo Models
We use the usual method of Schwarzschild to construct self-consistent
solutions for the triaxial de Zeeuw & Carollo (1996) models with central
density cusps. ZC96 models are triaxial generalisations of spherical
-models of Dehnen whose densities vary as near the center
and at large radii and hence, possess a central density core for
and cusps for . We consider four triaxial models from
ZC96, two prolate triaxials: with and
1.5, and two oblate triaxials: with and
1.5. We compute 4500 orbits in each model for time periods of .
We find that a large fraction of the orbits in each model are stochastic by
means of their nonzero Liapunov exponents. The stochastic orbits in each model
can sustain regular shapes for or longer, which suggests
that they diffuse slowly through their allowed phase-space. Except for the
oblate triaxial models with , our attempts to construct
self-consistent solutions employing only the regular orbits fail for the
remaining three models. However, the self-consistent solutions are found to
exist for all models when the stochastic and regular orbits are treated in the
same way because the mixing-time, , is shorter than the
integration time, . Moreover, the ``fully-mixed'' solutions can
also be constructed for all models when the stochastic orbits are fully mixed
at 15 lowest energy shells. Thus, we conclude that the self-consistent
solutions exist for our selected prolate and oblate triaxial models with
and 1.5.Comment: 6 Pages, 3 Figures, 2 Tables. Accepted for Publication in A&
Phase-reference VLBI Observations of the Compact Steep-Spectrum Source 3C 138
We investigate a phase-reference VLBI observation that was conducted at 15.4
GHz by fast switching VLBA antennas between the compact steep-spectrum radio
source 3C 138 and the quasar PKS 0528+134 which are about 4 away on the
sky. By comparing the phase-reference mapping with the conventional hybrid
mapping, we demonstrate the feasibility of high precision astrometric
measurements for sources separated by 4. VLBI phase-reference mapping
preserves the relative phase information, and thus provides an accurate
relative position between 3C 138 and PKS 0528+134 of
and
(J2000.0) in right ascension and declination, respectively. This gives an
improved position of the nucleus (component A) of 3C 138 in J2000.0 to be
RA= and Dec= under the
assumption that the position of calibrator PKS 0528+134 is correct. We further
made a hybrid map by performing several iterations of CLEAN and
self-calibration on the phase-referenced data with the phase-reference map as
an input model for the first phase self-calibration. Compared with the hybrid
map from the limited visibility data directly obtained from fringe fitting 3C
138 data, this map has a similar dynamic range, but a higher angular
resolution. Therefore, phase-reference technique is not only a means of phase
connection, but also a means of increasing phase coherence time allowing
self-calibration technique to be applied to much weaker sources.Comment: 9 pages plus 2 figures, accepted by PASJ (Vol.58 No.6
The response of hot wires in high Reynolds-number turbulent pipe flow
Issues concerning the accuracy of hot-wire measurements in turbulent pipe flow are addressed for pipe Reynolds numbers up to 6 × 106 and hot-wire Reynolds numbers up to Rew ap 250. These include the optimization of spatial and temporal resolution and the associated feature of signal-to-noise ratio. Very high wire Reynolds numbers enable the use of wires with reduced length-to-diameter ratios compared to those typical of atmospheric conditions owing to increased wire Nusselt numbers. Simulation of the steady-state heat balance for the wire and the unetched portion of wire are used to assess static end-conduction effects: they are used to calculate wire Biot numbers, \sqrt{c_0}l , and fractional end-conduction losses, σ, which confirm the 'conduction-only' theory described by Corrsin. They show that, at Rew ap 250, the wire length-to-diameter ratio can be reduced to about 50, while keeping \sqrt{c_0}l\gt3 and σ < 7% in common with accepted limits at Rew ap 3. It is shown that these limits depend additionally on the choice of wire material and the length of unetched wire. The dynamic effects of end-cooling are also assessed using the conduction-only theory
Effect of Statistical Fluctuation in Monte Carlo Based Photon Beam Dose Calculation on Gamma Index Evaluation
The gamma-index test has been commonly adopted to quantify the degree of
agreement between a reference dose distribution and an evaluation dose
distribution. Monte Carlo (MC) simulation has been widely used for the
radiotherapy dose calculation for both clinical and research purposes. The goal
of this work is to investigate both theoretically and experimentally the impact
of the MC statistical fluctuation on the gamma-index test when the fluctuation
exists in the reference, the evaluation, or both dose distributions. To the
first order approximation, we theoretically demonstrated in a simplified model
that the statistical fluctuation tends to overestimate gamma-index values when
existing in the reference dose distribution and underestimate gamma-index
values when existing in the evaluation dose distribution given the original
gamma-index is relatively large for the statistical fluctuation. Our numerical
experiments using clinical photon radiation therapy cases have shown that 1)
when performing a gamma-index test between an MC reference dose and a non-MC
evaluation dose, the average gamma-index is overestimated and the passing rate
decreases with the increase of the noise level in the reference dose; 2) when
performing a gamma-index test between a non-MC reference dose and an MC
evaluation dose, the average gamma-index is underestimated when they are within
the clinically relevant range and the passing rate increases with the increase
of the noise level in the evaluation dose; 3) when performing a gamma-index
test between an MC reference dose and an MC evaluation dose, the passing rate
is overestimated due to the noise in the evaluation dose and underestimated due
to the noise in the reference dose. We conclude that the gamma-index test
should be used with caution when comparing dose distributions computed with
Monte Carlo simulation
Finite-Volume Energy Spectrum, Fractionalized Strings, and Low-Energy Effective Field Theory for the Quantum Dimer Model on the Square Lattice
We present detailed analytic calculations of finite-volume energy spectra,
mean field theory, as well as a systematic low-energy effective field theory
for the square lattice quantum dimer model. The analytic considerations explain
why a string connecting two external static charges in the confining columnar
phase fractionalizes into eight distinct strands with electric flux
. An emergent approximate spontaneously broken symmetry
gives rise to a pseudo-Goldstone boson. Remarkably, this soft phonon-like
excitation, which is massless at the Rokhsar-Kivelson (RK) point, exists far
beyond this point. The Goldstone physics is captured by a systematic low-energy
effective field theory. We determine its low-energy parameters by matching the
analytic effective field theory with exact diagonalization results and Monte
Carlo data. This confirms that the model exists in the columnar (and not in a
plaquette or mixed) phase all the way to the RK point.Comment: 35 pages, 16 figure
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