36,714 research outputs found
How CMB and large-scale structure constrain chameleon interacting dark energy
We explore a chameleon type of interacting dark matter-dark energy scenario
in which a scalar field adiabatically traces the minimum of an effective
potential sourced by the dark matter density. We discuss extensively the effect
of this coupling on cosmological observables, especially the parameter
degeneracies expected to arise between the model parameters and other
cosmological parameters, and then test the model against observations of the
cosmic microwave background (CMB) anisotropies and other cosmological probes.
We find that the chameleon parameters and , which determine
respectively the slope of the scalar field potential and the dark matter-dark
energy coupling strength, can be constrained to and using CMB data alone. The latter parameter in particular is constrained
only by the late Integrated Sachs-Wolfe effect. Adding measurements of the
local Hubble expansion rate tightens the bound on by a factor of
two, although this apparent improvement is arguably an artefact of the tension
between the local measurement and the value inferred from Planck data in
the minimal CDM model. The same argument also precludes chameleon
models from mimicking a dark radiation component, despite a passing similarity
between the two scenarios in that they both delay the epoch of matter-radiation
equality. Based on the derived parameter constraints, we discuss possible
signatures of the model for ongoing and future large-scale structure surveys.Comment: 25 pages, 6 figure
On the Kinetic Roughening in Polymer Film Growth by Vapor Deposition
This is a Comment on a recent publication: Y.-P. Zhao et al., Phys. Rev.
Lett. 85, 3229 (2000). In the Letter, the authors report on an experimental
investigation of polymeric (p-xylene) thin film growth and propose a new
universality class not previously known. Here, we point out that the critical
exponents reported in the Letter are consistent with the critical exponents of
Das Sarma-Tamborenea growth model.Comment: 2 pages, 1 figure include
Magnetocaloric properties of nanocrystalline LaCaMnO
Some recent experimental studies show the invisibility of antiferromagnetic
transition in the cases of manganites when their particle size is reduced to
nanometer scale. In complete contrast to these cases, we have observed the
signature of antiferromagnetic transition in the magnetocaloric properties of
nanocrystalline LaCaMnO of average particle size 70
and 60 nm similar to its polycrystalline bulk form. The system exhibit inverse
magnetocaloric effect in its polycrystalline and nanocrystalline form. An extra
ferromagnetic phase is stabilized at low temperature for the sample with
particle size nm.Comment: 3 Figure
Bulk Higgs and Gauge fields in a multiply warped braneworld model
We readdress the problems associated with bulk Higgs and the gauge fields in
a 5-dimensional Randall-Sundrum model by extending the model to six dimensions
with double warping along the two extra spatial dimensions. In this
6-dimensional model we have a freedom of two moduli scales as against one
modulus in the 5-dimensional model. With a little hierarchy between these
moduli we can obtain the right magnitude for and boson masses from the
Kaluza-Klein modes of massive bulk gauge fields where the spontaneous symmetry
breaking is triggered by bulk Higgs . We also have determined the gauge
couplings of the standard model fermions with Kaluza-Klein modes of the gauge
fields. Unlike the case of 5-dimensional model with a massless bulk gauge
field, here we have shown that the gauge couplings and the masses of the
Kaluza-Klein gauge fields satisfy the precision electroweak constraints and
also obey the Tevatron bounds.Comment: 15 Pages, Late
Exploration of Finite 2D Square Grid by a Metamorphic Robotic System
We consider exploration of finite 2D square grid by a metamorphic robotic
system consisting of anonymous oblivious modules. The number of possible shapes
of a metamorphic robotic system grows as the number of modules increases. The
shape of the system serves as its memory and shows its functionality. We
consider the effect of global compass on the minimum number of modules
necessary to explore a finite 2D square grid. We show that if the modules agree
on the directions (north, south, east, and west), three modules are necessary
and sufficient for exploration from an arbitrary initial configuration,
otherwise five modules are necessary and sufficient for restricted initial
configurations
Magnetic-field-induced chiral hidden order in URu2Si2
Two of the most striking and yet unresolved manifestations of the hidden
order (HO) in URu2Si2, are associated on one hand with the double-step
metamagnetic transitions and on the other with the giant anomalous Nernst
signal. Both are observed when a magnetic field is applied along the c-axis.
Here we provide for the first time a unified understanding of these puzzling
phenomena and the related field-temperature (B-T) phase diagram. We demonstrate
that the HO phase at finite fields can be explained with a chiral dxy+idx2-y2
spin density wave, assuming that the zero field HO contains only the
time-reversal symmetry preserving idx2-y2 component. We argue that the presence
of the field-induced chiral HO can be reflected in a distinctive non-linear
B-dependence of the Kerr angle, when a Kerr experiment is conducted for finite
fields. This fingerprint can be conclusive for the possible emergence of
chirality in the HO.Comment: 8 pages and 9 figures main text + 6 pages supplementary material.
Philosophical Magazine: Special Issue: Focused Issue on Hidden Order in
URu2Si2 (May 2014
Neutrino Masses and Mixings in a Minimal SO(10) Model
We consider a minimal formulation of SO(10) Grand Unified Theory wherein all
the fermion masses arise from Yukawa couplings involving one 126 and one 10 of
Higgs multiplets. It has recently been recognized that such theories can
explain, via the type-II seesaw mechanism, the large \nu_\mu - \nu_\tau mixing
as a consequence of b-tau unification at the GUT scale. In this picture,
however, the CKM phase \delta lies preferentially in the second quadrant, in
contradiction with experimental measurements. We revisit this minimal model and
show that the conventional type-I seesaw mechanism generates phenomenologically
viable neutrino masses and mixings, while being consistent with CKM CP
violation. We also present improved fits in the type-II seesaw scenario and
suggest fully consistent fits in a mixed scenario.Comment: 27 pages, 13 eps figures, revtex4; references added, some minor
correction
Motion of a random walker in a quenched power law correlated velocity field
We study the motion of a random walker in one longitudinal and d transverse
dimensions with a quenched power law correlated velocity field in the
longitudinal x-direction. The model is a modification of the Matheron-de
Marsily (MdM) model, with long-range velocity correlation. For a velocity
correlation function, dependent on transverse co-ordinates y as 1/(a+|{y_1 -
y_2}|)^alpha, we analytically calculate the two-time correlation function of
the x-coordinate. We find that the motion of the x-coordinate is a fractional
Brownian motion (fBm), with a Hurst exponent H = max [1/2, (1- alpha/4),
(1-d/4)]. From this and known properties of fBM, we calculate the disorder
averaged persistence probability of x(t) up to time t. We also find the lines
in the parameter space of d and alpha along which there is marginal behaviour.
We present results of simulations which support our analytical calculation.Comment: 8 pages, 4 figures. To appear in Physical Review
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