112,668 research outputs found
Non-Thermal Production of WIMPs and the Sub-Galactic Structure of the Universe
There is increasing evidence that conventional cold dark matter (CDM) models
lead to conflicts between observations and numerical simulations of dark matter
halos on sub-galactic scales. Spergel and Steinhardt showed that if the CDM is
strongly self-interacting, then the conflicts disappear. However, the
assumption of strong self-interaction would rule out the favored candidates for
CDM, namely weakly interacting massive particles (WIMPs), such as the
neutralino. In this paper we propose a mechanism of non-thermal production of
WIMPs and study its implications on the power spectrum. We find that the
non-vanishing velocity of the WIMPs suppresses the power spectrum on small
scales compared to what it obtained in the conventional CDM model. Our results
show that, in this context, WIMPs as candidates for dark matter can work well
both on large scales and on sub-galactic scales.Comment: 6 pages, 2 figures; typo corrected; to appear in PR
Increase of cold tolerance in cotton plant (Gossypium hirsutum L.) by mepiquat chloride
Three mepiquat chloride (MC) concentrations - 40, 70, and 100 g a.i./ha - were used to spray cotton (Gossypium hirsutum L., cultival McNair 220) plants to determine whether or not MC would increase their cold tolerance. Seven to ten days after the spray, the plants were exposed to three different cold treatments. No important difference in cold damage was noticed between the control and the MC-treated plants when they were exposed repeatedly to 4.5 C. No plants died when exposed to 0.5 C for 12 h; however, 90% of the 1st and 2nd leaves of the control plants were damaged. This was three times more damage than those leaves of plants treated with 70 and 100 g a.i./ha MC concentrations; 60% f the control and 10-20% of the MC-treated plants died when the plants were subjected to a cold hardening process with 15.5 C day (12 h) and 1.7 C night (12 h) for 10 days, and then, held at -2.2 C for 24 hours. The electrolyte leakage and reflectance measurement data showed that the cell membranes of the MC-treated plants sustained much less damage than those of the control. Freezing injury was easily assessed by reflectance measurements at the 1.65 micrometer wavelength
Study of HST counterparts to Chandra X-ray sources in the Globular Cluster M71
We report on archival Hubble Space Telescope (HST) observations of the
globular cluster M71 (NGC 6838). These observations, covering the core of the
globular cluster, were performed by the Advanced Camera for Surveys (ACS) and
the Wide Field Planetary Camera 2 (WFPC2). Inside the half-mass radius (r_h =
1.65') of M71, we find 33 candidate optical counterparts to 25 out of 29
Chandra X-ray sources while outside the half-mass radius, 6 possible optical
counterparts to 4 X-ray sources are found. Based on the X-ray and optical
properties of the identifications, we find 1 certain and 7 candidate
cataclysmic variables (CVs). We also classify 2 and 12 X-ray sources as certain
and potential chromospherically active binaries (ABs), respectively. The only
star in the error circle of the known millisecond pulsar (MSP) is inconsistent
with being the optical counterpart. The number of X-ray faint sources with
L_x>4x10^{30} ergs/s (0.5-6.0 keV) found in M71 is higher than extrapolations
from other clusters on the basis of either collision frequency or mass. Since
the core density of M71 is relatively low, we suggest that those CVs and ABs
are primordial in origin.Comment: 12 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
Making Sense of the Legendre Transform
The Legendre transform is an important tool in theoretical physics, playing a
critical role in classical mechanics, statistical mechanics, and
thermodynamics. Yet, in typical undergraduate or graduate courses, the power of
motivation and elegance of the method are often missing, unlike the treatments
frequently enjoyed by Fourier transforms. We review and modify the presentation
of Legendre transforms in a way that explicates the formal mathematics,
resulting in manifestly symmetric equations, thereby clarifying the structure
of the transform algebraically and geometrically. Then we bring in the physics
to motivate the transform as a way of choosing independent variables that are
more easily controlled. We demonstrate how the Legendre transform arises
naturally from statistical mechanics and show how the use of dimensionless
thermodynamic potentials leads to more natural and symmetric relations.Comment: 11 pages, 3 figure
Effects of Zeeman spin splitting on the modular symmetry in the quantum Hall effect
Magnetic-field-induced phase transitions in the integer quantum Hall effect
are studied under the formation of paired Landau bands arising from Zeeman spin
splitting. By investigating features of modular symmetry, we showed that
modifications to the particle-hole transformation should be considered under
the coupling between the paired Landau bands. Our study indicates that such a
transformation should be modified either when the Zeeman gap is much smaller
than the cyclotron gap, or when these two gaps are comparable.Comment: 8 pages, 4 figure
Dual Monte Carlo and Cluster Algorithms
We discuss the development of cluster algorithms from the viewpoint of
probability theory and not from the usual viewpoint of a particular model. By
using the perspective of probability theory, we detail the nature of a cluster
algorithm, make explicit the assumptions embodied in all clusters of which we
are aware, and define the construction of free cluster algorithms. We also
illustrate these procedures by rederiving the Swendsen-Wang algorithm,
presenting the details of the loop algorithm for a worldline simulation of a
quantum 1/2 model, and proposing a free cluster version of the
Swendsen-Wang replica method for the random Ising model. How the principle of
maximum entropy might be used to aid the construction of cluster algorithms is
also discussed.Comment: 25 pages, 4 figures, to appear in Phys.Rev.
Duality Relation among Periodic Potential Problems in the Lowest Landau Level
Using a momentum representation of a magnetic von Neumann lattice, we study a
two-dimensional electron in a uniform magnetic field and obtain one-particle
spectra of various periodic short-range potential problems in the lowest Landau
level.We find that the energy spectra satisfy a duality relation between a
period of the potential and a magnetic length. The energy spectra consist of
the Hofstadter-type bands and flat bands. We also study the connection between
a periodic short-range potential problem and a tight-binding model.Comment: 6 pages, 3 figures, final version to appear in PR
Exact Asymptotic Results for a Model of Sequence Alignment
Finding analytically the statistics of the longest common subsequence (LCS)
of a pair of random sequences drawn from c alphabets is a challenging problem
in computational evolutionary biology. We present exact asymptotic results for
the distribution of the LCS in a simpler, yet nontrivial, variant of the
original model called the Bernoulli matching (BM) model which reduces to the
original model in the large c limit. We show that in the BM model, for all c,
the distribution of the asymptotic length of the LCS, suitably scaled, is
identical to the Tracy-Widom distribution of the largest eigenvalue of a random
matrix whose entries are drawn from a Gaussian unitary ensemble. In particular,
in the large c limit, this provides an exact expression for the asymptotic
length distribution in the original LCS problem.Comment: 4 pages Revtex, 2 .eps figures include
- …