8,014 research outputs found
Longevity of supersymmetric flat directions
We examine the fate of supersymmetric flat directions. We argue that the
non-perturbative decay of the flat direction via preheating is an unlikely
event. In order to address this issue, first we identify the physical degrees
of freedom and their masses in presence of a large flat direction VEV (Vacuum
Expectation Value). We explicitly show that the (complex) flat direction and
its fermionic partner are the only light {\it physical} fields in the spectrum.
If the flat direction VEV is much larger than the weak scale, and it has a
rotational motion, there will be no resonant particle production at all. The
case of multiple flat directions is more involved. We illustrate that in many
cases of physical interest, the situation becomes effectively the same as that
of a single flat direction, or collection of independent single directions. In
such cases preheating is not relevant. In an absence of a fast non-perturbative
decay, the flat direction survives long enough to affect thermalization in
supersymmetric models as described in hep-ph/0505050 and hep-ph/0512227. It can
also ``terminate'' an early stage of non-perturbative inflaton decay as
discussed in hep-ph/0603244.Comment: 9 revtex pages, v3: expanded discussion on two flat directions, minor
modifications, conclusions unchange
Constraining Non-thermal and Thermal properties of Dark Matter
We describe the evolution of Dark Matter (DM) abundance from the very onset
of its creation from inflaton decay under the assumption of an instantaneous
reheating. Based on the initial conditions such as the inflaton mass and its
decay branching ratio to DM, reheating temperature, and the DM mass and
interaction rate with the thermal bath, the DM particles can either thermalize
(fully/partially) with the primordial bath or remain non-thermal throughout
their evolution history. In the thermal case, the final abundance is set by the
standard freeze-out mechanism for large annihilation rates, irrespective of the
initial conditions. For smaller annihilation rates, it can be set by the
freeze-in mechanism, also independent of the initial abundance, provided it is
small to begin with. For even smaller interaction rates, the DM decouples while
being non-thermal, and the relic abundance will be essentially set by the
initial conditions. We put model-independent constraints on the DM mass and
annihilation rate from over-abundance by exactly solving the relevant Boltzmann
equations, and identify the thermal freeze-out, freeze-in and non-thermal
regions of the allowed parameter space. We highlight a generic fact that
inflaton decay to DM inevitably leads to an overclosure of the Universe for a
large range of DM parameter space, and thus poses a stringent constraint that
must be taken into account while constructing models of DM. For the thermal DM
region, we also show the complementary constraints from indirect DM search
experiments, Big Bang Nucleosynthesis, Cosmic Microwave Background, Planck
measurements, and theoretical limits due to the unitarity of S-matrix. For the
non-thermal DM scenario, we show the allowed parameter space in terms of the
inflaton and DM masses for a given reheating temperature, and compute the
comoving free-streaming length to identify the hot, warm and cold DM regimes.Comment: 29 pages, 8 figures; some clarifications and references added;
published versio
Cross-Sender Bit-Mixing Coding
Scheduling to avoid packet collisions is a long-standing challenge in
networking, and has become even trickier in wireless networks with multiple
senders and multiple receivers. In fact, researchers have proved that even {\em
perfect} scheduling can only achieve . Here
is the number of nodes in the network, and is the {\em medium
utilization rate}. Ideally, one would hope to achieve ,
while avoiding all the complexities in scheduling. To this end, this paper
proposes {\em cross-sender bit-mixing coding} ({\em BMC}), which does not rely
on scheduling. Instead, users transmit simultaneously on suitably-chosen slots,
and the amount of overlap in different user's slots is controlled via coding.
We prove that in all possible network topologies, using BMC enables us to
achieve . We also prove that the space and time
complexities of BMC encoding/decoding are all low-order polynomials.Comment: Published in the International Conference on Information Processing
in Sensor Networks (IPSN), 201
Angular Inflation from Supergravity
We study supergravity inflationary models where inflation is produced along
the angular direction. For this we express the scalar component of a chiral
superfield in terms of the radial and the angular components. We then express
the supergravity potential in a form particularly simple for calculations
involving polynomial expressions for the superpotential and Kahler potential.
We show for a simple Polonyi model the angular direction may give rise to a
stage of inflation when the radial field is fixed to its minimum. We obtain
analytical expressions for all the relevant inflationary quantities and discuss
the possibility of supersymmetry breaking in the radial direction while
inflating by the angular component.Comment: 7 pages, one figure. Final version. Title changed, two figures
droppe
Towards Scalable Visual Exploration of Very Large RDF Graphs
In this paper, we outline our work on developing a disk-based infrastructure
for efficient visualization and graph exploration operations over very large
graphs. The proposed platform, called graphVizdb, is based on a novel technique
for indexing and storing the graph. Particularly, the graph layout is indexed
with a spatial data structure, i.e., an R-tree, and stored in a database. In
runtime, user operations are translated into efficient spatial operations
(i.e., window queries) in the backend.Comment: 12th Extended Semantic Web Conference (ESWC 2015
Transport and magnetic anomalies due to A-site ionic size mismatch in LaCaBa_{x}MnO
We present results of electrical resistivity, magnetoresistance and ac and dc
magnetic susceptibility on polycrystalline samples of the type
La(0.5)Ca(0.5-x)Ba(x)MnO(3) synthesized under identical heat treatment
conditions. The substitution of larger Ba ions for Ca results in a non-
monotonic variation of the curie temperature as the system evolves from a
charge ordered insulating state for x=0 to a ferromagnetic metallic state for
x=0.5. An intermediate compositino, x=0.1, interestingly exhibits
ferromagnetic. insulating behaviour with thermal hysteresis in ac chi around
the curie tem- perature (120K). The x=0.2 and 0.3 compounds exhibit
semiconducting like behavior as the temperature is lowered below 300K, with a
broad peak in rho around 80-100K: These compositions exhibit a weak increase in
rho as the temperature lowered below 30K, indicative of electron localization
effects. These compositions also undergo ferromagnetic transitions below about
200 and 235K respectively, though these are non-hysteretic; above all, for
these compositions, MR is large and conveniently measurable over the entire
tempera- ture range of measurement below Tc. This experimental finding may be
of interest from the application point of view. We infer that the A-site
ionic-size mismatch plays a crucial role in the deciding these properties.Comment: 5 pages, 6 Figures, Resubmitted with extended abstract on 26 Nov,
199
Effects of 3-d and 4-d-transition metal substitutional impurities on the electronic properties of CrO2
We present first-principles based density functional theory calculations of
the electronic and magnetic structure of CrO2 with 3d (Ti through Cu) and 4d
(Zr through Ag) substitutional impurities. We find that the half-metallicity of
CrO2 remains intact for all of the calculated substitutions. We also observe
two periodic trends as a function of the number of valence electrons: if the
substituted atom has six or fewer valence electrons (Ti-Cr or Zr-Mo), the
number of down spin electrons associated with the impurity ion is zero,
resulting in ferromagnetic (FM) alignment of the impurity magnetic moment with
the magnetization of the CrO2 host. For substituent atoms with eight to ten
(Fe-Ni or Ru-Pd with the exception of Ni), the number of down spin electrons
contributed by the impurity ion remains fixed at three as the number
contributed to the majority increases from one to three resulting in
antiferromagnetic (AFM) alignment between impurity moment and host
magnetization. The origin of this variation is the grouping of the impurity
states into 3 states with approximate "t2g" symmetry and 2 states with
approximate "eg" symmetry. Ni is an exception to the rule because a
Jahn-Teller-like distortion causes a splitting of the Ni eg states. For Mn and
Tc, which have 8 valence electrons, the zero down spin and 3 down spin
configurations are very close in energy. For Cu and Ag atoms, which have 11
valence electrons, the energy is minimized when the substituent ion contributes
5 Abstract down-spin electrons. We find that the interatomic exchange
interactions are reduced for all substitutions except for the case of Fe for
which a modest enhancement is calculated for interactions along certain
crystallographic directions.Comment: 26 pages, 10 figures, 2 table
Ferromagnetically correlated clusters in semi-metallic Ru2NbAl Heusler alloy
In this work, we report the structural, magnetic and electrical and thermal
transport properties of the Heusler-type alloy Ru2NbAl. From the detailed
analysis of magnetization data, we infer the presence of superparamagnetically
interacting clusters with a Pauli paramagnetic background, while short-range
ferromagnetic interaction is developed among the clusters below 5 K. The
presence of this ferromagnetic interaction is confirmed through heat capacity
measurements. The relatively small value of electronic contribution to specific
heat, gamma (~2.7 mJ/mol-K2), as well as the linear nature of temperature
dependence of Seebeck coefficient indicate a semi-metallic ground state with a
pseudo-gap that is also supported by our electronic structure calculations. The
activated nature of resistivity is reflected in the observed negative
temperature coefficient and has its origin in the charge carrier localization
due to antisite defects, inferred from magnetic measurements as well as
structural analysis. Although the absolute value of thermoelectric figure of
merit is rather low (ZT = 5.2*10-3) in Ru2NbAl, it is the largest among all the
reported non-doped full Heusler alloys.Comment: 25 pages, 14 figure
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