721 research outputs found
Smectic blue phases: layered systems with high intrinsic curvature
We report on a construction for smectic blue phases, which have quasi-long
range smectic translational order as well as three dimensional crystalline
order. Our proposed structures fill space by adding layers on top of a minimal
surface, introducing either curvature or edge defects as necessary. We find
that for the right range of material parameters, the favorable saddle-splay
energy of these structures can stabilize them against uniform layered
structures. We also consider the nature of curvature frustration between mean
curvature and saddle-splay.Comment: 15 pages, 11 figure
Supersymmetry Changing Bubbles in String Theory
We give examples of string compactifications to 4d Minkowski space with
different amounts of supersymmetry that can be connected by spherical domain
walls. The tension of these domain walls is tunably lower than the 4d Planck
scale. The ``stringy'' description of these walls is known in terms of certain
configurations of wrapped Dirichlet and NS branes. This construction allows us
to connect a variety of vacua with 4d N=4,3,2,1 supersymmetry.Comment: 11 pages, harvmac, no figures, reference added, minor correction
Subset feedback vertex set is fixed parameter tractable
The classical Feedback Vertex Set problem asks, for a given undirected graph
G and an integer k, to find a set of at most k vertices that hits all the
cycles in the graph G. Feedback Vertex Set has attracted a large amount of
research in the parameterized setting, and subsequent kernelization and
fixed-parameter algorithms have been a rich source of ideas in the field.
In this paper we consider a more general and difficult version of the
problem, named Subset Feedback Vertex Set (SUBSET-FVS in short) where an
instance comes additionally with a set S ? V of vertices, and we ask for a set
of at most k vertices that hits all simple cycles passing through S. Because of
its applications in circuit testing and genetic linkage analysis SUBSET-FVS was
studied from the approximation algorithms perspective by Even et al.
[SICOMP'00, SIDMA'00].
The question whether the SUBSET-FVS problem is fixed-parameter tractable was
posed independently by Kawarabayashi and Saurabh in 2009. We answer this
question affirmatively. We begin by showing that this problem is
fixed-parameter tractable when parametrized by |S|. Next we present an
algorithm which reduces the given instance to 2^k n^O(1) instances with the
size of S bounded by O(k^3), using kernelization techniques such as the
2-Expansion Lemma, Menger's theorem and Gallai's theorem. These two facts allow
us to give a 2^O(k log k) n^O(1) time algorithm solving the Subset Feedback
Vertex Set problem, proving that it is indeed fixed-parameter tractable.Comment: full version of a paper presented at ICALP'1
Heterotic Standard Model Moduli
In previous papers, we introduced a heterotic standard model and discussed
its basic properties. The Calabi-Yau threefold has, generically, three Kahler
and three complex structure moduli. The observable sector of this vacuum has
the spectrum of the MSSM with one additional pair of Higgs-Higgs conjugate
fields. The hidden sector has no charged matter in the strongly coupled string
and only minimal matter for weak coupling. Additionally, the spectrum of both
sectors will contain vector bundle moduli. The exact number of such moduli was
conjectured to be small, but was not explicitly computed. In this paper, we
rectify this and present a formalism for computing the number of vector bundle
moduli. Using this formalism, the number of moduli in both the observable and
strongly coupled hidden sectors is explicitly calculated.Comment: 28 pages, LaTeX; v2: typos corrected, references added; v3:
clarifications, references adde
Estimation of Fiber Orientations Using Neighborhood Information
Data from diffusion magnetic resonance imaging (dMRI) can be used to
reconstruct fiber tracts, for example, in muscle and white matter. Estimation
of fiber orientations (FOs) is a crucial step in the reconstruction process and
these estimates can be corrupted by noise. In this paper, a new method called
Fiber Orientation Reconstruction using Neighborhood Information (FORNI) is
described and shown to reduce the effects of noise and improve FO estimation
performance by incorporating spatial consistency. FORNI uses a fixed tensor
basis to model the diffusion weighted signals, which has the advantage of
providing an explicit relationship between the basis vectors and the FOs. FO
spatial coherence is encouraged using weighted l1-norm regularization terms,
which contain the interaction of directional information between neighbor
voxels. Data fidelity is encouraged using a squared error between the observed
and reconstructed diffusion weighted signals. After appropriate weighting of
these competing objectives, the resulting objective function is minimized using
a block coordinate descent algorithm, and a straightforward parallelization
strategy is used to speed up processing. Experiments were performed on a
digital crossing phantom, ex vivo tongue dMRI data, and in vivo brain dMRI data
for both qualitative and quantitative evaluation. The results demonstrate that
FORNI improves the quality of FO estimation over other state of the art
algorithms.Comment: Journal paper accepted in Medical Image Analysis. 35 pages and 16
figure
Developing Antidote Controlled Antiplatelet Therapies By Targeting The Vwf â Gp IbâIxâV Interaction
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106054/1/jth02400.pd
Vacuum Stability in Heterotic M-Theory
The problem of the stabilization of moduli is discussed within the context of
compactified strongly coupled heterotic string theory. It is shown that all
geometric, vector bundle and five-brane moduli are completely fixed, within a
phenomenologically acceptable range, by non-perturbative physics. This result
requires, in addition to the full space of moduli, non-vanishing Neveu-Schwarz
flux, gaugino condensation with threshold corrections and the explicit form of
the Pfaffians in string instanton superpotentials. The stable vacuum presented
here has a negative cosmological constant. The possibility of ``lifting'' this
to a metastable vacuum with positive cosmological constant is briefly
discussed.Comment: 39 pages, minor correction
Constraining Scale-Dependent Non-Gaussianity with Future Large-Scale Structure and the CMB
We forecast combined future constraints from the cosmic microwave background
and large-scale structure on the models of primordial non-Gaussianity. We study
the generalized local model of non-Gaussianity, where the parameter f_NL is
promoted to a function of scale, and present the principal component analysis
applicable to an arbitrary form of f_NL(k). We emphasize the complementarity
between the CMB and LSS by using Planck, DES and BigBOSS surveys as examples,
forecast constraints on the power-law f_NL(k) model, and introduce the figure
of merit for measurements of scale-dependent non-Gaussianity.Comment: 28 pages, 8 figures, 2 tables; v2: references update
N=3 Warped Compactifications
Orientifolds with three-form flux provide some of the simplest string
examples of warped compactification. In this paper we show that some models of
this type have the unusual feature of D=4, N=3 spacetime supersymmetry. We
discuss their construction and low energy physics. Although the local form of
the moduli space is fully determined by supersymmetry, to find its global form
requires a careful study of the BPS spectrum.Comment: 27 pages, v2: 32pp., RevTeX4, fixed factors, slightly improved
sections 3D and 4B, v3: added referenc
Scale Dependence of the Halo Bias in General Local-Type Non-Gaussian Models I: Analytical Predictions and Consistency Relations
We investigate the clustering of halos in cosmological models starting with
general local-type non-Gaussian primordial fluctuations. We employ multiple
Gaussian fields and add local-type non-Gaussian corrections at arbitrary order
to cover a class of models described by frequently-discussed f_nl, g_nl and
\tau_nl parameterization. We derive a general formula for the halo power
spectrum based on the peak-background split formalism. The resultant spectrum
is characterized by only two parameters responsible for the scale-dependent
bias at large scale arising from the primordial non-Gaussianities in addition
to the Gaussian bias factor. We introduce a new inequality for testing
non-Gaussianities originating from multi fields, which is directly accessible
from the observed power spectrum. We show that this inequality is a
generalization of the Suyama-Yamaguchi inequality between f_nl and \tau_nl to
the primordial non-Gaussianities at arbitrary order. We also show that the
amplitude of the scale-dependent bias is useful to distinguish the simplest
quadratic non-Gaussianities (i.e., f_nl-type) from higher-order ones (g_nl and
higher), if one measures it from multiple species of galaxies or clusters of
galaxies. We discuss the validity and limitations of our analytic results by
comparison with numerical simulations in an accompanying paper.Comment: 25 pages, 3 figures, typo corrected, Appendix C updated, submitted to
JCA
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