1,098 research outputs found
Preventing a Cyber-9/11: How Universal Jurisdiction Could Protect International Aviation in the Digital Age
Fast and precise map-making for massively multi-detector CMB experiments
Future cosmic microwave background (CMB) polarisation experiments aim to
measure an unprecedentedly small signal - the primordial gravity wave component
of the polarisation field B-mode. To achieve this, they will analyse huge
datasets, involving years worth of time-ordered data (TOD) from massively
multi-detector focal planes. This creates the need for fast and precise methods
to complement the M-L approach in analysis pipelines. In this paper, we
investigate fast map-making methods as applied to long duration, massively
multi-detector, ground-based experiments, in the context of the search for
B-modes. We focus on two alternative map-making approaches: destriping and TOD
filtering, comparing their performance on simulated multi-detector polarisation
data. We have written an optimised, parallel destriping code, the DEStriping
CARTographer DESCART, that is generalised for massive focal planes, including
the potential effect of cross-correlated TOD 1/f noise. We also determine the
scaling of computing time for destriping as applied to a simulated full-season
data-set for a realistic experiment. We find that destriping can out-perform
filtering in estimating both the large-scale E and B-mode angular power
spectra. In particular, filtering can produce significant spurious B-mode power
via EB mixing. Whilst this can be removed, it contributes to the variance of
B-mode bandpower estimates at scales near the primordial B-mode peak. For the
experimental configuration we simulate, this has an effect on the possible
detection significance for primordial B-modes. Destriping is a viable
alternative fast method to the full M-L approach that does not cause the
problems associated with filtering, and is flexible enough to fit into both M-L
and Monte-Carlo pseudo-Cl pipelines.Comment: 16 pages, 14 figures. MNRAS accepted. Typos corrected and computing
time/memory requirement orders-of-magnitude numbers in section 4 replaced by
precise number
Fast Pixel Space Convolution for CMB Surveys with Asymmetric Beams and Complex Scan Strategies: FEBeCoP
Precise measurement of the angular power spectrum of the Cosmic Microwave
Background (CMB) temperature and polarization anisotropy can tightly constrain
many cosmological models and parameters. However, accurate measurements can
only be realized in practice provided all major systematic effects have been
taken into account. Beam asymmetry, coupled with the scan strategy, is a major
source of systematic error in scanning CMB experiments such as Planck, the
focus of our current interest. We envision Monte Carlo methods to rigorously
study and account for the systematic effect of beams in CMB analysis. Toward
that goal, we have developed a fast pixel space convolution method that can
simulate sky maps observed by a scanning instrument, taking into account real
beam shapes and scan strategy. The essence is to pre-compute the "effective
beams" using a computer code, "Fast Effective Beam Convolution in Pixel space"
(FEBeCoP), that we have developed for the Planck mission. The code computes
effective beams given the focal plane beam characteristics of the Planck
instrument and the full history of actual satellite pointing, and performs very
fast convolution of sky signals using the effective beams. In this paper, we
describe the algorithm and the computational scheme that has been implemented.
We also outline a few applications of the effective beams in the precision
analysis of Planck data, for characterizing the CMB anisotropy and for
detecting and measuring properties of point sources.Comment: 26 pages, 15 figures. New subsection on beam/PSF statistics, new and
better figures, more explicit algebra for polarized beams, added explanatory
text at many places following referees comments [Accepted for publication in
ApJS
Map-making in small field modulated CMB polarisation experiments: approximating the maximum-likelihood method
Map-making presents a significant computational challenge to the next
generation of kilopixel CMB polarisation experiments. Years worth of time
ordered data (TOD) from thousands of detectors will need to be compressed into
maps of the T, Q and U Stokes parameters. Fundamental to the science goal of
these experiments, the observation of B-modes, is the ability to control noise
and systematics. In this paper, we consider an alternative to the
maximum-likelihood method, called destriping, where the noise is modelled as a
set of discrete offset functions and then subtracted from the time-stream. We
compare our destriping code (Descart: the DEStriping CARTographer) to a full
maximum-likelihood map-maker, applying them to 200 Monte-Carlo simulations of
time-ordered data from a ground based, partial-sky polarisation modulation
experiment. In these simulations, the noise is dominated by either detector or
atmospheric 1/f noise. Using prior information of the power spectrum of this
noise, we produce destriped maps of T, Q and U which are negligibly different
from optimal. The method does not filter the signal or bias the E or B-mode
power spectra. Depending on the length of the destriping baseline, the method
delivers between 5 and 22 times improvement in computation time over the
maximum-likelihood algorithm. We find that, for the specific case of single
detector maps, it is essential to destripe the atmospheric 1/f in order to
detect B-modes, even though the Q and U signals are modulated by a half-wave
plate spinning at 5-Hz.Comment: 18 pages, 17 figures, MNRAS accepted v2: content added (inc: table
2), typos correcte
Quadratic Lagrangians and Topology in Gauge Theory Gravity
We consider topological contributions to the action integral in a gauge
theory formulation of gravity. Two topological invariants are found and are
shown to arise from the scalar and pseudoscalar parts of a single integral.
Neither of these action integrals contribute to the classical field equations.
An identity is found for the invariants that is valid for non-symmetric Riemann
tensors, generalizing the usual GR expression for the topological invariants.
The link with Yang-Mills instantons in Euclidean gravity is also explored. Ten
independent quadratic terms are constructed from the Riemann tensor, and the
topological invariants reduce these to eight possible independent terms for a
quadratic Lagrangian. The resulting field equations for the parity
non-violating terms are presented. Our derivations of these results are
considerably simpler that those found in the literature
Performance of the Xpert HPV assay in women attending for cervical screening
© 2015 The Authors. Objectives: This study evaluated the Xpert HPV Assay in women attending screening in general practice by comparing Xpert with two established HPV tests, cytology and histology. Methods: A prospective study in women aged 20-60 years attending screening in Bristol, Edinburgh and London using residual Preservcyt cytology samples. Sample order was randomised between Roche cobas4800 and Cepheid Xpert assays with Qiagen hc2 third. Results: 3408 cases were included in the primary analysis. Positivity for Xpert was 19.6%, cobas 19.2% and hc2 19.9% with high concordance (kappa=86.8% vs cobas, 81.55 vs hc2). Xpert, cobas and hc2 showed similar sensitivity (98.7%, 97.5%, 98.7%) for CIN2+. All pairwise comparisons had high concordance (Kappa ≥0.78 with any abnormal cytology. Xpert and hc2 were positive for all cases of ≥moderate dyskaryosis ( N=63)), cobas was negative in two. Histology was available for 172 participants. 79 reported CIN2+, 47 CIN3+. All CIN3+ was positive on Xpert and hc2 and one case negative for cobas. One case of CIN2 was negative for all assays. Conclusions: The performance of Xpert HPV Assay in a general screening population is comparable to established HPV tests. It offers simplicity of testing, flexibility with non-batching of individual samples and rapid turnaround time
All-sky convolution for polarimetry experiments
We discuss all-sky convolution of the instrument beam with the sky signal in
polarimetry experiments, such as the Planck mission which will map the
temperature anisotropy and polarization of the cosmic microwave background
(CMB). To account properly for stray light (from e.g. the galaxy, sun, and
planets) in the far side-lobes of such an experiment, it is necessary to
perform the beam convolution over the full sky. We discuss this process in
multipole space for an arbitrary beam response, fully including the effects of
beam asymmetry and cross-polarization. The form of the convolution in multipole
space is such that the Wandelt-Gorski fast technique for all-sky convolution of
scalar signals (e.g. temperature) can be applied with little modification. We
further show that for the special case of a pure co-polarized, axisymmetric
beam the effect of the convolution can be described by spin-weighted window
functions. In the limits of a small angle beam and large Legendre multipoles,
the spin-weight 2 window function for the linear polarization reduces to the
usual scalar window function used in previous analyses of beam effects in CMB
polarimetry experiments. While we focus on the example of polarimetry
experiments in the context of CMB studies, we emphasise that the formalism we
develop is applicable to anisotropic filtering of arbitrary tensor fields on
the sphere.Comment: 8 pages, 1 figure; Minor changes to match version accepted by Phys.
Rev.
Do patients accurately represent their experiences after hip and knee replacements?
Background To investigate discrepancies, if any, between the complications that patients report on the patient-reported outcome measures (PROMs) questionnaire and what is formally recorded in their medical records. Methodology A retrospective analysis of PROMs-reported complications was performed at a single elective center for all patients who had an elective primary total knee or hip replacement between April 2016 and March 2017. Corresponding patient medical records were then analyzed to correlate the PROMs with any documentation of postoperative complications, which similar to the PROMs data were categorized into wound complications, urinary complications, readmission, and further operative procedures. Results A set of 54 complete patient records were compared to the corresponding PROMs data. The combined overall positive predictive value was 0.47 while the overall negative predictive value was 0.91. Concordance between patients and the medical records was 70.4% for wound complication, 66.7% for urinary complications, 83.3% for readmission, and 96.3% for reoperation. Conclusion PROMs data are becoming increasingly important in auditing and planning healthcare provision. This study highlights a significant level of discrepancy between the PROMs-reported complication rates and those documented in the medical records. There is a visible disparity between patient perception and the medical definition of postoperative complications. Further patient education and empowerment are required in preparation for arthroplasty
- …