13,736 research outputs found
Top-Quark FCNC Decay t->cgg in Topcolor-assisted Technicolor Model
The topcolor-assisted technicolor (TC2) model predicts several pseudo-scalars
called top-pions and at loop level they can induce the FCNC top quark decay
t->cgg which is extremely suppressed in the Standard Model (SM). We find that
in the allowed parameter space the TC2 model can greatly enhance such a FCNC
decay and push the branching ratio up to 10^{-3}, which is much larger than the
predictions in the SM (10^{-9}) and in the minimal supersymmetric model
(10^{-4}). We also compare the result with the two-body FCNC decay t-> cg and
find that the branching ratio of t-> cgg is slightly larger than t-> cg. Such
enhanced FCNC top quark decays may serve as a good probe of TC2 model at the
future top quark factory.Comment: 11 pages, 4 figure
Onset of unsteady horizontal convection in rectangle tank at
The horizontal convection within a rectangle tank is numerically simulated.
The flow is found to be unsteady at high Rayleigh numbers. There is a Hopf
bifurcation of from steady solutions to periodic solutions, and the
critical Rayleigh number is obtained as for the
middle plume forcing at , which is much larger than the formerly obtained
value. Besides, the unstable perturbations are always generated from the
central jet, which implies that the onset of instability is due to velocity
shear (shear instability) other than thermally dynamics (thermal instability).
Finally, Paparella and Young's [J. Fluid Mech. 466 (2002) 205] first hypotheses
about the destabilization of the flow is numerically proved, i.e. the middle
plume forcing can lead to a destabilization of the flow.Comment: 4pages, 6 figures, extension of Chin. Phys. Lett. 2008, 25(6), in
pres
Possible under the peak in photoproduction
The LEPS collaboration has recently reported a measurement of the reaction
with linearly polarized photon beam at
resonance region. The observed beam asymmetry is sizably negative at
, in contrast to the presented theoretical
prediction. In this paper, we calculate this process in the framework of the
effective Lagrangian approach. By including a newly proposed
state with mass around 1380~MeV, the experimental
data for both and experiments can be well reproduced. It
is found that the and/or the contact term may play
important role and deserve further investigation.Comment: modified version to be published at Phys. Rev.
Multimodal estimation of distribution algorithms
Taking the advantage of estimation of distribution algorithms (EDAs) in preserving high diversity, this paper proposes a multimodal EDA. Integrated with clustering strategies for crowding and speciation, two versions of this algorithm are developed, which operate at the niche level. Then these two algorithms are equipped with three distinctive techniques: 1) a dynamic cluster sizing strategy; 2) an alternative utilization of Gaussian and Cauchy distributions to generate offspring; and 3) an adaptive local search. The dynamic cluster sizing affords a potential balance between exploration and exploitation and reduces the sensitivity to the cluster size in the niching methods. Taking advantages of Gaussian and Cauchy distributions, we generate the offspring at the niche level through alternatively using these two distributions. Such utilization can also potentially offer a balance between exploration and exploitation. Further, solution accuracy is enhanced through a new local search scheme probabilistically conducted around seeds of niches with probabilities determined self-adaptively according to fitness values of these seeds. Extensive experiments conducted on 20 benchmark multimodal problems confirm that both algorithms can achieve competitive performance compared with several state-of-the-art multimodal algorithms, which is supported by nonparametric tests. Especially, the proposed algorithms are very promising for complex problems with many local optima
Spatial Interference: From Coherent To Incoherent
It is well known that direct observation of interference and diffraction
pattern in the intensity distribution requires a spatially coherent source.
Optical waves emitted from portions beyond the coherence area possess
statistically independent phases, and will degrade the interference pattern. In
this paper we show an optical interference experiment, which seems contrary to
our common knowledge, that the formation of the interference pattern is related
to a spatially incoherent light source. Our experimental scheme is very similar
to Gabor's original proposal of holography[1], just with an incoherent source
replacing the coherent one. In the statistical ensemble of the incoherent
source, each sample field produces a sample interference pattern between object
wave and reference wave. These patterns completely differ from each other due
to the fluctuation of the source field distribution. Surprisingly, the sum of a
great number of sample patterns exhibits explicitly an interference pattern,
which contains all the information of the object and is equivalent to a
hologram in the coherent light case. In this sense our approach would be
valuable in holography and other interference techniques for the case where
coherent source is unavailable, such as x-ray and electron sources.Comment: 8 pages, 5 figure
Dynamics of a deformable body in a fast flowing soap film
We study the behavior of an elastic loop embedded in a flowing soap film.
This deformable loop is wetted into the film and is held fixed at a single
point against the oncoming flow. We interpret this system as a two-dimensional
flexible body interacting in a two-dimensional flow. This coupled
fluid-structure system shows bistability, with both stationary and oscillatory
states. In its stationary state, the loop remains essentially motionless and
its wake is a von K\'arm\'an vortex street. In its oscillatory state, the loop
sheds two vortex dipoles, or more complicated vortical structures, within each
oscillation period. We find that the oscillation frequency of the loop is
linearly proportional to the flow velocity, and that the measured Strouhal
numbers can be separated based on wake structure
Fluctuation-Driven Vortex Fractionalization in Topologically Ordered Superfluids of Cold Atoms
We have studied spin structures of fluctuation-driven fractionalized vortices
and topological spin order in 2D nematic superfluids of cold sodium atoms. Our
Monte Carlo simulations suggest a softened pi-spin disclination structure in a
half-quantum vortex when spin correlations are short ranged; in addition,
calculations indicate that a unique non-local topological spin order emerges
simultaneously as cold atoms become a superfluid below a critical temperature.
We have also estimated fluctuation-dependent critical frequencies for
half-quantum vortex nucleation in rotating optical traps and discussed probing
these excitations in experiments.Comment: 5 pages, 2 figures; revised version accepted by Europhysics Letter
Isolating Geometry in Weak Lensing Measurements
Given a foreground galaxy-density field or shear field, its cross-correlation
with the shear field from a background population of source galaxies scales
with the source redshift in a way that is specific to lensing. Such a
source-scaling can be exploited to effectively measure geometrical distances as
a function of redshift and thereby constrain dark energy properties, free of
any assumptions about the galaxy-mass/mass power spectrum (its shape, amplitude
or growth). Such a geometrical method can yield a ~ 0.03 - 0.07 f_{sky}^{-1/2}
measurement on the dark energy abundance and equation of state, for a
photometric redshift accuracy of dz ~ 0.01 - 0.05 and a survey with median
redshift of ~ 1. While these constraints are weaker than conventional weak
lensing methods, they provide an important consistency check because the
geometrical method carries less theoretical baggage: there is no need to assume
any structure formation model (e.g. CDM). The geometrical method is at the most
conservative end of a whole spectrum of methods which obtain smaller errorbars
by making more restrictive assumptions -- we discuss some examples. Our
geometrical approach differs from previous investigations along similar lines
in three respects. First, the source-scaling we propose to use is less
demanding on the photometric redshift accuracy. Second, the scaling works for
both galaxy-shear and shear-shear correlations. Third, we find that previous
studies underestimate the statistical errors associated with similar
geometrical methods, the origin of which is discussed.Comment: 13 pages, 4 figures, submitted to Ap
First Principles Studies on 3-Dimentional Strong Topological Insulators: Bi2Te3, Bi2Se3 and Sb2Te3
Bi2Se3, Bi2Te3 and Sb2Te3 compounds are recently predicted to be
3-dimentional (3D) strong topological insulators. In this paper, based on
ab-initio calculations, we study in detail the topological nature and the
surface states of this family compounds. The penetration depth and the
spin-resolved Fermi surfaces of the surface states will be analyzed. We will
also present an procedure, from which highly accurate effective Hamiltonian can
be constructed, based on projected atomic Wannier functions (which keep the
symmetries of the systems). Such Hamiltonian can be used to study the
semi-infinite systems or slab type supercells efficiently. Finally, we discuss
the 3D topological phase transition in Sb2(Te1-xSex)3 alloy system.Comment: 8 pages,17 figure
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