62,765 research outputs found
Analytic ranks of elliptic curves over number fields
Let be an elliptic curves over the rational numbers. Let be a cyclic
extension of prime degree . Then, we show that the average of analytic ranks
of over all cyclic extension of prime degree is at most
, where is the analytic rank of . This bound is independent of the degree of the cyclic extension. Also, we
also obtain some average rank result over -fields
Velocity statistics from spectral line data: effects of density-velocity correlations, magnetic field, and shear
In a previous work Lazarian and Pogosyan suggested a technique to extract
velocity and density statistics, of interstellar turbulence, by means of
analysing statistics of spectral line data cubes. In this paper we test that
technique, by studying the effect of correlation between velocity and density
fields, providing a systematic analysis of the uncertainties arising from the
numerics, and exploring the effect of a linear shear. We make use of both
compressible MHD simulations and synthetic data to emulate spectroscopic
observations and test the technique. With the same synthetic spectroscopic
data, we also studied anisotropies of the two point statistics and related
those anisotropies with the magnetic field direction. This presents a new
technique for magnetic field studies. The results show that the velocity and
density spectral indices measured are consistent with the analytical
predictions. We identified the dominant source of error with the limited number
of data points along a given line of sight. We decrease this type of noise by
increasing the number of points and by introducing Gaussian smoothing. We argue
that in real observations the number of emitting elements is essentially
infinite and that source of noise vanishes.Comment: 12 pages, 10 figures. Accepted for publication in MNRA
Magnetic Reconnection and Turbulent Mixing: From ISM to Clusters of Galaxies
Magnetic reconnection, or the ability of the magnetic field lines that are
frozen in plasma to change their topology, is a fundamental problem of
magnetohydrodynamics (MHD). We briefly examine the problem starting with the
well-known Sweet-Parker scheme, discuss effects of tearing modes, anomalous
resistivity and the concept of hyperresistivity. We show that the field
stochasticity by itself provides a way to enable fast reconnection even if, at
the scale of individual turbulent wiggles, the reconnection happens at the slow
Sweet-Parker rate. We show that fast reconnection allows efficient mixing of
magnetic field in the direction perpendicular to the local direction of
magnetic field. While the idea of stochastic reconnection still requires
numerical confirmation, our numerical simulations testify that mixing motions
perpendicular to the local magnetic field are up to high degree hydrodynamical.
This suggests that the turbulent heat transport should be similar to that in
non-magnetized turbulent fluid, namely, should have a diffusion coefficient
\sim LV_L, where V_L is the amplitude of the turbulent velocity and L is the
scale of the turbulent motions. We present numerical simulations which support
this conclusion. The application of this idea to thermal conductivity in
clusters of galaxies shows that this mechanism may dominate the diffusion of
heat and may be efficient enough to prevent cooling flow formation.Comment: 12 pages, 2 figures, invited talk at JENAM2002 - The Unsolved
Universe:Challenges for the Future (v2: minor changes
Kinetically-controlled thin-film growth of layered - and NaCoO cobaltate
We report growth characteristics of epitaxial -NaCoO and
-NaCoO thin films on (001) sapphire substrates grown by
pulsed-laser deposition. Reduction of deposition rate could change structure of
NaCoO thin film from -phase with island growth mode to
-phase with layer-by-layer growth mode. The
-NaCoO thin film exhibits spiral surface growth with
multiterraced islands and highly crystallized texture compared to that of the
-NaCoO thin film. This heterogeneous epitaxial film growth
can give opportunity of strain effect of physical properties and growth
dynamics of NaCoO as well as subtle nature of structural change.Comment: accepted for publication in Applied Physics Letter
Dilaton as a Dark Matter Candidate and its Detection
Assuming that the dilaton is the dark matter of the universe, we propose an
experiment to detect the relic dilaton using the electromagnetic resonant
cavity, based on the dilaton-photon conversion in strong electromagnetic
background. We calculate the density of the relic dilaton, and estimate the
dilaton mass for which the dilaton becomes the dark matter of the universe.
With this we calculate the dilaton detection power in the resonant cavity, and
compare it with the axion detection power in similar resonant cavity
experiment.Comment: 23 pages, 2 figure
Comment on Decay
We calculate the rate for decay using Chiral
Perturbation Theory. This isospin violating process results from -
mixing, and its amplitude is proportional to . Experimental information on the branching
ratio for can provide insight into the pattern of
violation in radiative decays.Comment: 7 pages with 2 figures not included but available upon request,
CALT-68-191
Lineal Trails of D2-D2bar Superstrings
We study the superstrings suspended between a D2- and an anti-D2-brane. We
quantize the string in the presence of some general configuration of gauge
fields over the (anti-)D-brane world volumes. The interstring can move only in
a specific direction that is normal to the difference of the electric fields of
each (anti-)D-branes. Especially when the electric fields are the same, the
interstring cannot move. We obtain the condition for the tachyons to disappear
from the spectrum.Comment: 15 pages with 4 figures, referenced added, Sec. 5 on the spectrum
made cleare
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