4,349 research outputs found
A bias to CMB lensing measurements from the bispectrum of large-scale structure
The rapidly improving precision of measurements of gravitational lensing of
the Cosmic Microwave Background (CMB) also requires a corresponding increase in
the precision of theoretical modeling. A commonly made approximation is to
model the CMB deflection angle or lensing potential as a Gaussian random field.
In this paper, however, we analytically quantify the influence of the
non-Gaussianity of large-scale structure lenses, arising from nonlinear
structure formation, on CMB lensing measurements. In particular, evaluating the
impact of the non-zero bispectrum of large-scale structure on the relevant CMB
four-point correlation functions, we find that there is a bias to estimates of
the CMB lensing power spectrum. For temperature-based lensing reconstruction
with CMB Stage-III and Stage-IV experiments, we find that this lensing power
spectrum bias is negative and is of order one percent of the signal. This
corresponds to a shift of multiple standard deviations for these upcoming
experiments. We caution, however, that our numerical calculation only evaluates
two of the largest bias terms and thus only provides an approximate estimate of
the full bias. We conclude that further investigation into lensing biases from
nonlinear structure formation is required and that these biases should be
accounted for in future lensing analyses.Comment: 15+19 pages, 9 figures. Comments welcom
Long-term magnetic field stability of Vega
We present new spectropolarimetric observations of the normal A-type star
Vega, obtained during the summer of 2010 with NARVAL at T\'elescope Bernard
Lyot (Pic du Midi Observatory). This new time-series is constituted of 615
spectra collected over 6 different nights. We use the
Least-Square-Deconvolution technique to compute, from each spectrum, a mean
line profile with a signal-to-noise ratio close to 20,000. After averaging all
615 polarized observations, we detect a circularly polarized Zeeman signature
consistent in shape and amplitude with the signatures previously reported from
our observations of 2008 and 2009. The surface magnetic geometry of the star,
reconstructed using the technique of Zeeman-Doppler Imaging, agrees with the
maps obtained in 2008 and 2009, showing that most recognizable features of the
photospheric field of Vega are only weakly distorted by large-scale surface
flows (differential rotation or meridional circulation).Comment: Proceedings of the conference "Stellar polarimetry: from birth to
death", 2011 Jun 27-30, Madiso
Electroweak radiative corrections to e+e- --> WW --> 4fermions in double-pole approximation -- the RACOONWW approach
We calculate the complete O(alpha) electroweak radiative corrections to e+e-
--> WW --> 4f in the electroweak Standard Model in the double-pole
approximation. We give analytical results for the non-factorizable virtual
corrections and express the factorizable virtual corrections in terms of the
known corrections to on-shell W-pair production and W decay. The calculation of
the bremsstrahlung corrections, i.e. the processes e+e- --> 4f+gamma in lowest
order, is based on the full matrix elements. The matching of soft and collinear
singularities between virtual and real corrections is done alternatively in two
different ways, namely by using a subtraction method and by applying
phase-space slicing. The O(alpha) corrections as well as higher-order
initial-state photon radiation are implemented in the Monte Carlo generator
RACOONWW. Numerical results of this program are presented for the
W-pair-production cross section, angular and W-invariant-mass distributions at
LEP2. We also discuss the intrinsic theoretical uncertainty of our approach.Comment: 51 pages, latex, 28 postscript file
Pre-torsors and Galois comodules over mixed distributive laws
We study comodule functors for comonads arising from mixed distributive laws.
Their Galois property is reformulated in terms of a (so-called) regular arrow
in Street's bicategory of comonads. Between categories possessing equalizers,
we introduce the notion of a regular adjunction. An equivalence is proven
between the category of pre-torsors over two regular adjunctions
and on one hand, and the category of regular comonad arrows
from some equalizer preserving comonad to on
the other. This generalizes a known relationship between pre-torsors over equal
commutative rings and Galois objects of coalgebras.Developing a bi-Galois
theory of comonads, we show that a pre-torsor over regular adjunctions
determines also a second (equalizer preserving) comonad and a
co-regular comonad arrow from to , such that the
comodule categories of and are equivalent.Comment: 34 pages LaTeX file. v2: a few typos correcte
A Simple Approximation of the One-Loop Corrected Cross Section for at LEP 2
Using the SU(2) gauge coupling, , at the high-energy
scale of , defined by the (theoretical value of the) leptonic
W-width, rather than using the low-energy value, defined via the Fermi
coupling, , in the Born approximation, and supplementing with Coulomb
corrections and initial state radiation, errors with respect to the exact
one-loop results for the differential cross section of are
below 1% at LEP 2 energies at all production angles. A similar
procedure is suggested to incorporate leading bosonic loop effects into
four-fermion production in the fermion-loop scheme. The resulting accuracy
below 1% is sufficient for LEP 2 experiments.Comment: 12 pages, LaTeX including 1 postscript figure, submitted to Phys.
Lett.
Serotonin attenuates feedback excitation onto O-LM interneurons
The serotonergic system is a subcortical neuromodulatory center that controls cortical information processing in a state-dependent manner. In the hippocampus, serotonin (5-HT) is released by ascending serotonergic fibers from the midbrain raphe nuclei, thereby mediating numerous modulatory functions on various neuronal subtypes. Here, we focus on the neuromodulatory effects of 5-HT on GABAergic inhibitory oriens lacunosum-moleculare (O-LM) cells in the hippocampal area CA1 of the rat. These interneurons are thought to receive primarily local excitatory input and are, via their axonal projections to stratum lacunosum-moleculare, ideally suited to control entorhinal cortex input. We show that 5-HT reduces excitatory glutamatergic transmission onto O-LM interneurons. By means of paired recordings from synaptically connected CA1 pyramidal cells and O-LM interneurons we reveal that this synapse is modulated by 5-HT. Furthermore, we demonstrate that the reduction of glutamatergic transmission by serotonin is likely to be mediated via a decrease of calcium influx into presynaptic terminals of CA1 pyramidal cells. This modulation of excitatory synaptic transmission onto O-LM interneurons by 5-HT might be a mechanism to vary the activation of O-LM interneurons during ongoing network activity and serve as a brain state-dependent switch gating the efficiency of entorhinal cortex input to CA1 pyramidal neurons
Ruthenium and hafnium abundances in giant and dwarf barium stars
We present abundances for Ru and Hf, compare them to abundances of other
heavy elements, and discuss the problems found in determining Ru and Hf
abundances with laboratory gf-values in the spectra of barium stars. We
determined Ru and Hf abundances in a sample of giant and dwarf barium stars, by
the spectral synthesis of two RuI (4080.574A and 4757.856A) and two HfII
(4080.437A and 4093.155A) transitions. The stellar spectra were observed with
FEROS/ESO, and the stellar atmospheric parameters lie in the range 4300 <
Teff/K < 6500, -1.2 < [Fe/H] <= 0 and 1.4 <= log g < 4.6. The HfII 4080A and
the RuI 4758A observed transitions result in a unreasonably high solar
abundance, given certain known uncertainties, when fitted with laboratory
gf-values. For these two transitions we determined empirical gf-values by
fitting the observed line profiles of the spectra of the Sun and Arcturus. For
the sample stars, this procedure resulted in a good agreement of Ru and Hf
abundances given by the two available lines. The resulting Ru and Hf abundances
were compared to those of Y, Nd, Sm and Eu. In the solar system Ru, Sm and Eu
are dominated by the r-process and Hf, Nd and Y by the s-process, and all of
these elements are enhanced in barium stars since they lie inside the s-process
path. Ru abundances show large scatter when compared to other heavy elements,
whereas Hf abundances show less scatter and closely follow the abundances of Sm
and Nd, in good agreement with theoretical expectations. We also suggest a
possible, unexpected, correlation of Ru and Sm abundances. The observed
behaviour in abundances is probably due to variations in the 13C pocket
efficiency in AGB stars, and, though masked by high uncertainties, hint at a
more complex scenario than proposed by theory.Comment: 11 pages, 7 figures and 7 tables. accepted to A&
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