108 research outputs found
Bound States of Pseudo-Dirac Dark Matter
We study the bound-state spectrum in a simple model of pseudo-Dirac dark
matter, and examine how the rate of bound-state formation through radiative
capture compares to Sommerfeld-enhanced annihilation. We use this model as an
example to delineate the new features induced by the presence of a mass
splitting between the dark matter and a nearly-degenerate partner, compared to
the case where only a single dark-matter-like state is present. We provide a
simple analytic prescription for estimating the spectrum of bound states in
systems containing a mass splitting, which in turn allows characterization of
the resonances due to near-zero-energy bound states, and validate this estimate
both for pseudo-Dirac dark matter and for the more complex case of wino dark
matter. We demonstrate that for pseudo-Dirac dark matter the capture rate into
deeply bound states is, to a good approximation, simply related to the
Sommerfeld enhancement factor.Comment: 26 pages, 8 figures, Comments welcom
Social Cataloguing Sites: An Analysis through Webometric Approach
The basic purpose of this study is to compare and evaluate the social cataloguing sites via content analysis from various parameters and also through SEO analyzer. The nine social cataloguing sites have been considered for this webometric analysis. Collected data were analyzed according to forty criteria under nine main aspects. Finally it was found that LibraryThing, Goodreads, aNobii etc. are good example of social cataloguing sites
Thinning-free Polygonal Approximation of Thick Digital Curves Using Cellular Envelope
Since the inception of successful rasterization of curves and objects in the digital space, several algorithms have been proposed for approximating a given digital curve. All these algorithms, however, resort to thinning as preprocessing before approximating a digital curve with changing thickness. Described in this paper is a novel thinning-free algorithm for polygonal approximation of an arbitrarily thick digital curve, using the concept of "cellular envelope", which is newly introduced in this paper. The cellular envelope, defined as the smallest set of cells containing the given curve, and hence bounded by two tightest (inner and outer) isothetic polygons, is constructed using a combinatorial technique. This envelope, in turn, is analyzed to determine a polygonal approximation of the curve as a sequence of cells using certain attributes of digital straightness. Since a real-world curve=curve-shaped object with varying thickness, unexpected disconnectedness, noisy information, etc., is unsuitable for the existing algorithms on polygonal approximation, the curve is encapsulated by the cellular envelope to enable the polygonal approximation. Owing to the implicit Euclidean-free metrics and combinatorial properties prevailing in the cellular plane, implementation of the proposed algorithm involves primitive integer operations only, leading to fast execution of the algorithm. Experimental results that include output polygons for different values of the approximation parameter corresponding to several real-world digital curves, a couple of measures on the quality of approximation, comparative results related with two other well-referred algorithms, and CPU times, have been presented to demonstrate the elegance and efficacy of the proposed algorithm
Sudakov Shoulder Resummation for Thrust and Heavy Jet Mass
When the allowed range of an observable grows order-by-order in perturbation
theory, its perturbative expansion can have discontinuities (as in the
parameter) or discontinuities in its derivatives (as in thrust or heavy jet
mass) called Sudakov shoulders. We explore the origin of these logarithms using
both perturbation theory and effective field theory. We show that for thrust
and heavy jet mass, the logarithms arise from kinematic configurations with
narrow jets and deduce the next-to-leading logarithmic series. The
left-shoulder logarithms in heavy jet mass () of the form with are particularly dangerous, because they
invalidate fixed order perturbation theory in regions traditionally used to
extract . Although the factorization formula shows there are no
non-global logarithms, we find Landau-pole like singularities in the resummed
distribution associated with the cusp anomalous dimension, and that power
corrections are exceptionally important.Comment: 49 pages, 8 figures. Published in Physical Review
Large magnetocapacitance in electronic ferroelectric manganite systems
We have observed a sizable positive magnetocapacitance () in
perovskite PrCaMnO and bilayer
Pr(SrCa)MnO system under 5T magnetic field across
20-100 K below the magnetic transition point T. The magnetodielectric
effect, on the other hand, exhibits a crossover: (a) from positive to negative
for the perovskite system and (b) from negative to positive for the bilayer
system over the same temperature range. The bilayer
Pr(SrCa)MnO system exhibits a sizable anisotropy as
well. We have also noticed the influence of magnetic field on the dielectric
relaxation characteristics of these systems. These systems belong to a class of
improper ferroelectrics and are expected to exhibit charge/orbital order driven
ferroelectric polarization below the transition point T. Large
magnetocapacitance in these systems shows typical multiferroic behavior even
though the ferroelectric polarization is small in comparison to that of other
ferroelectrics.Comment: 6 pages with 5 embedded figures; accepted for publication in J. Appl.
Phy
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