43,638 research outputs found
Primordial B-mode Diagnostics and Self Calibrating the CMB Polarization
Distortions in the primordial cosmic microwave background (CMB) along the
line-of-sight can be modeled and described using 11 fields. These distortion
fields correspond to various cosmological signals such as weak gravitational
lensing of the CMB by large-scale structure, screening from patchy
reionization, rotation of the plane of polarization due to magnetic fields or
parity violating physics. Various instrumental systematics such as gain
fluctuations, pixel rotation, differential gain, pointing, differential
ellipticity are also described by the same distortion model. All these
distortions produce B-mode that contaminate the primordial tensor B-modes
signal. In this paper we show that apart from generating B-modes, each
distortion uniquely couples different modes (\bfl_1\ne \bfl_2) of the CMB
anisotropies, generating correlations which for the primordial CMB are
zero. We describe and implement unbiased minimum variance quadratic estimators
which using the off diagonal correlations in the CMB can extract the map of
distortions. We perform Monte-Carlo simulations to characterize the estimators
and illustrate the level of distortions that can be detected with current and
future experiments. The estimators can be used to look for cosmological
signals, or to check for any residual systematics in the data. As a specific
example of primordial tensor B-mode diagnostics we compare the level of minimum
detectable distortions using our method with maximum allowed distortion level
for the B-modes detection. We show that for any experiment, the distortions
will be detected at high significance using correlations before they would show
up as spurious B-modes in the power spectrum.Comment: 14 pages, 4 figure
Towards Bose-Einstein Condensation of Electron Pairs: Role of Schwinger Bosons
It can be shown that the bosonic degree of freedom of the tightly bound
on-site electron pairs could be separated as Schwinger bosons. This is
implemented by projecting the whole Hilbert space into the Hilbert subspace
spanned by states of two kinds of Schwinger bosons (to be called binon and
vacanon) subject to a constraint that these two kinds of bosonic quasiparticles
cannot occupy the same site. We argue that a binon is actually a kind of
quantum fluctuations of electron pairs, and a vacanon corresponds to a vacant
state. These two bosonic quasiparticles may be responsible for the
Bose-Einstein condensation (BEC) of the system associated with electron pairs.
These concepts are also applied to the attractive Hubbard model with strong
coupling, showing that it is quite useful. The relevance of the present
arguments to the existing theories associated with the BEC of electron pairs is
briefly commented.Comment: Revtex, one figur
Constraining a spatially dependent rotation of the Cosmic Microwave Background Polarization
Following Kamionkowski (2008), a quadratic estimator of the rotation of the
plane of polarization of the CMB is constructed. This statistic can estimate a
spatially varying rotation angle. We use this estimator to quantify the
prospects of detecting such a rotation field with forthcoming experiments. For
PLANCK and CMBPol we find that the estimator containing the product of the E
and B components of the polarization field is the most sensitive. The variance
of this EB estimator, N(L) is roughly independent of the multipole L, and is
only weakly dependent on the instrumental beam. For FWHM of the beam size ~
5'-50', and instrument noise $\Delta_p ~ 5-50 uK-arcmin, the scaling of
variance N(L) can be fitted by a power law N(L)=3.3 x 10^{-7} \Delta^2_p
(FWHM)^{1.3} sq-deg. For small instrumental noise \Delta_p \leq 5 uK-arcmin,
the lensing B-modes become important, saturating the variance to ~10^{-6}
sq-deg even for an ideal experiment. Upcoming experiments like PLANCK will be
able to detect a power spectrum of the rotation angle, C^{\alpha \alpha}(L), as
small as 0.01 sq-deg, while futuristic experiment like CMBPol will be able to
detect rotation angle power spectrum as small as 2.5 x 10^{-5} sq-deg. We
discuss the implications of such constraints, both for the various physical
effects that can rotate the polarization as photons travel from the last
scattering surface as well as for constraints on instrumental systematics that
can also lead to a spurious rotation signal. Rotation of the CMB polarization
generates B-modes which will act as contamination for the primordial B-modes
detection. We discuss an application of our estimator to de-rotate the CMB to
increase the sensitivity for the primordial B-modes.Comment: 11 pages, 5 figure
Numerical Study of the Correspondence Between the Dissipative and Fixed Energy Abelian Sandpile Models
We consider the Abelian sandpile model (ASM) on the large square lattice with
a single dissipative site (sink). Particles are added by one per unit time at
random sites and the resulting density of particles is calculated as a function
of time. We observe different scenarios of evolution depending on the value of
initial uniform density (height) . During the first stage of the
evolution, the density of particles increases linearly. Reaching a critical
density , the system changes its behavior sharply and relaxes
exponentially to the stationary state of the ASM with . We found
numerically that and . Our
observations suggest that the equality holds for more general
initial conditions with non-positive heights. In parallel with the ASM, we
consider the conservative fixed-energy Abelian sandpile model (FES). The
extensive Monte-Carlo simulations for have confirmed that in the
limit of large lattices coincides with the threshold density
of FES. Therefore, can be identified with
if the FES starts its evolution with non-positive uniform height .Comment: 6 pages, 8 figure
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Dynamic Behavior of Precast Concrete Beam-Column Sub-Assemblages with High Performance Connections Subjected to Sudden Column Removal Scenario
Unbonded posttensioned precast concrete (UPPC) structure has shown its excellent aseismic performance in laboratory tests and earthquake investigation. However, the progressive collapse behavior of UPPC subjected to column removal scenario is still unclear. To fill this knowledge gap, two 1/2 scaled UPPC beam-column sub-assemblages were tested under a penultimate column removal scenario. The dynamic test results indicated that UPPC sub-assemblages have desirable load redistribution capacity to mitigate progressive collapse. The failure modes of the sub-assemblages observed in dynamic test were quite similar to that in static counterparts
Mott-Peierls Transition in the extended Peierls-Hubbard model
The one-dimensional extended Peierls-Hubbard model is studied at several band
fillings using the density matrix renormalization group method. Results show
that the ground state evolves from a Mott-Peierls insulator with a correlation
gap at half-filling to a soliton lattice with a small band gap away from
half-filling. It is also confirmed that the ground state of the Peierls-Hubbard
model undergoes a transition to a metallic state at finite doping. These
results show that electronic correlations effects should be taken into account
in theoretical studies of doped polyacetylene. They also show that a
Mott-Peierls theory could explain the insulator-metal transition observed in
this material.Comment: 4 pages with 3 embedded eps figure
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