12,186 research outputs found
A NOTE ON PARITY CONSTRAINED ORIENTATIONS
This note extends a result of Frank, Jordan, and Szigeti [3] on parity constrained orientations with connectivity requirements. Given a hypergraph H, a non-negative intersecting supermodular set function p, and a preferred in-degree parity for every node, a formula is given on the minimum number of nodes with wrong in-degree parity in an orientation of H covering p
Non-parametric Reconstruction of Cluster Mass Distribution from Strong Lensing: Modelling Abell 370
We describe a new non-parametric technique for reconstructing the mass
distribution in galaxy clusters with strong lensing, i.e., from multiple images
of background galaxies. The observed positions and redshifts of the images are
considered as rigid constraints and through the lens (ray-trace) equation they
provide us with linear constraint equations. These constraints confine the mass
distribution to some allowed region, which is then found by linear programming.
Within this allowed region we study in detail the mass distribution with
minimum mass-to-light variation; also some others, such as the smoothest mass
distribution. The method is applied to the extensively studied cluster Abell
370, which hosts a giant luminous arc and several other multiply imaged
background galaxies. Our mass maps are constrained by the observed positions
and redshifts (spectroscopic or model-inferred by previous authors) of the
giant arc and multiple image systems. The reconstructed maps obtained for \a370
reveal a detailed mass distribution, with substructure quite different from the
light distribution. The method predicts the bimodal nature of the cluster and
that the projected mass distribution is indeed elongated along the axis defined
by the two dominant cD galaxies. But the peaks in the mass distribution appear
to be offset from the centres of the cDs. We also present an estimate for the
total mass of the central region of the cluster. This is in good agreement with
previous mass determinations. The total mass of the central region is
M=(2.0-2.7) 10^14 Msun/h50, depending on the solution chosen.Comment: 14 pages(19 postscript figures), minor corrections, MNRAS in pres
Systematic errors in cosmic microwave background polarization measurements
We investigate the impact of instrumental systematic errors on the potential
of cosmic microwave background polarization experiments targeting primordial
B-modes. To do so, we introduce spin-weighted Muller matrix-valued fields
describing the linear response of the imperfect optical system and receiver,
and give a careful discussion of the behaviour of the induced systematic
effects under rotation of the instrument. We give the correspondence between
the matrix components and known optical and receiver imperfections, and compare
the likely performance of pseudo-correlation receivers and those that modulate
the polarization with a half-wave plate. The latter is shown to have the
significant advantage of not coupling the total intensity into polarization for
perfect optics, but potential effects like optical distortions that may be
introduced by the quasi-optical wave plate warrant further investigation. A
fast method for tolerancing time-invariant systematic effects is presented,
which propagates errors through to power spectra and cosmological parameters.
The method extends previous studies to an arbitrary scan strategy, and
eliminates the need for time-consuming Monte-Carlo simulations in the early
phases of instrument and survey design. We illustrate the method with both
simple parametrized forms for the systematics and with beams based on
physical-optics simulations. Example results are given in the context of
next-generation experiments targeting tensor-to-scalar ratios r ~ 0.01.Comment: 19 pages, 7 figures; Minor changes to match version accepted by MNRA
Ising Anyons in Frustration-Free Majorana-Dimer Models
Dimer models have long been a fruitful playground for understanding
topological physics. Here we introduce a new class - termed Majorana-dimer
models - wherein bosonic dimers are decorated with pairs of Majorana modes. We
find that the simplest examples of such systems realize an intriguing,
intrinsically fermionic phase of matter that can be viewed as the product of a
chiral Ising theory, which hosts deconfined non-Abelian quasiparticles, and a
topological superconductor. While the bulk anyons are described by
a single copy of the Ising theory, the edge remains fully gapped. Consequently,
this phase can arise in exactly solvable, frustration-free models. We describe
two parent Hamiltonians: one generalizes the well-known dimer model on the
triangular lattice, while the other is most naturally understood as a model of
decorated fluctuating loops on a honeycomb lattice. Using modular
transformations, we show that the ground-state manifold of the latter model
unambiguously exhibits all properties of the
theory. We also discuss generalizations with more than one Majorana mode per
site, which realize phases related to Kitaev's 16-fold way in a similar
fashion
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