293 research outputs found
Quantum discontinuity between zero and infinitesimal graviton mass with a Lambda term
We show that the recently demonstrated absence of the usual discontinuity for
massive spin 2 with a Lambda term is an artifact of the tree approximation, and
that the discontinuity reappears at one loop.Comment: 8 pages, revtex 3.1, title changed (version to appear in Phys. Rev.
Lett.
Twisted topological structures related to M-branes II: Twisted Wu and Wu^c structures
Studying the topological aspects of M-branes in M-theory leads to various
structures related to Wu classes. First we interpret Wu classes themselves as
twisted classes and then define twisted notions of Wu structures. These
generalize many known structures, including Pin^- structures, twisted Spin
structures in the sense of Distler-Freed-Moore, Wu-twisted differential
cocycles appearing in the work of Belov-Moore, as well as ones introduced by
the author, such as twisted Membrane and twisted String^c structures. In
addition, we introduce Wu^c structures, which generalize Pin^c structures, as
well as their twisted versions. We show how these structures generalize and
encode the usual structures defined via Stiefel-Whitney classes.Comment: 20 page
Quantum M^2 -> 2Lambda/3 discontinuity for massive gravity with a Lambda term
In a previous paper we showed that the absence of the van
Dam-Veltman-Zakharov discontinuity as M^2 -> 0 for massive spin-2 with a Lambda
term is an artifact of the tree approximation, and that the discontinuity
reappears at one loop, as a result of going from five degrees of freedom to
two. In this paper we show that a similar classical continuity but quantum
discontinuity arises in the "partially massless" limit M^2 -> 2Lambda/3, as a
result of going from five degrees of freedom to four.Comment: 8 pages, REVTe
Principal infinity-bundles - General theory
The theory of principal bundles makes sense in any infinity-topos, such as
that of topological, of smooth, or of otherwise geometric
infinity-groupoids/infinity-stacks, and more generally in slices of these. It
provides a natural geometric model for structured higher nonabelian cohomology
and controls general fiber bundles in terms of associated bundles. For suitable
choices of structure infinity-group G these G-principal infinity-bundles
reproduce the theories of ordinary principal bundles, of bundle
gerbes/principal 2-bundles and of bundle 2-gerbes and generalize these to their
further higher and equivariant analogs. The induced associated infinity-bundles
subsume the notions of gerbes and higher gerbes in the literature.
We discuss here this general theory of principal infinity-bundles, intimately
related to the axioms of Giraud, Toen-Vezzosi, Rezk and Lurie that characterize
infinity-toposes. We show a natural equivalence between principal
infinity-bundles and intrinsic nonabelian cocycles, implying the classification
of principal infinity-bundles by nonabelian sheaf hyper-cohomology. We observe
that the theory of geometric fiber infinity-bundles associated to principal
infinity-bundles subsumes a theory of infinity-gerbes and of twisted
infinity-bundles, with twists deriving from local coefficient infinity-bundles,
which we define, relate to extensions of principal infinity-bundles and show to
be classified by a corresponding notion of twisted cohomology, identified with
the cohomology of a corresponding slice infinity-topos.
In a companion article [NSSb] we discuss explicit presentations of this
theory in categories of simplicial (pre)sheaves by hyper-Cech cohomology and by
simplicial weakly-principal bundles; and in [NSSc] we discuss various examples
and applications of the theory.Comment: 46 pages, published versio
ZnO:Co Diluted Magnetic Semiconductor or Hybrid Nanostructure for Spintronics?
We have studied the influence of intrinsic and extrinsic defects in the
magnetic and electrical transport properties of Co-doped ZnO thin films. X ray
absorption measurements show that Co substitute Zn in the ZnO structure and it
is in the 2+ oxidation state. Magnetization (M) measurements show that doped
samples are mainly paramagnetic. From M vs. H loops measured at 5 K we found
that the values of the orbital L and spin S numbers are between 1 and 1.3 for L
and S = 3/2, in agreement with the representative values for isolated Co 2+.
The obtained negative values of the Curie-Weiss temperatures indicate the
existence of antiferromagnetic interactions between transition metal atoms.Comment: To be published in Journal of Materials Scienc
Ground-state energy of H-: a critical test of triple basis sets
We report an improved variational upper bound for the ground state energy of H- using Hylleraaslike wave functions in the form of a triple basis set having three distinct distance scales. The extended precision DQFUN of Bailey, allowing for 70 decimal digit arithmetic, is implemented to retain sufficient precision. Our result exceeds the previous record [A. M. Frolov, Euro. J. Phys. D 69, 132 (2015)], indicating that the Hylleraas triple basis set exhibits comparable convergence to the widely used pseudorandom all-exponential basis sets, but the numerical stability against roundoff error is much better. It is argued that the three distance scales have a clear physical interpretation. The new variational bound is -0.527 751 016 544 377 196 590 814 469 a.u
Twisted topological structures related to M-branes
Studying the M-branes leads us naturally to new structures that we call
Membrane-, Membrane^c-, String^K(Z,3)- and Fivebrane^K(Z,4)-structures, which
we show can also have twisted counterparts. We study some of their basic
properties, highlight analogies with structures associated with lower levels of
the Whitehead tower of the orthogonal group, and demonstrate the relations to
M-branes.Comment: 17 pages, title changed on referee's request, minor changes to
improve presentation, typos correcte
Automated Detection of Cortical Lesions in Multiple Sclerosis Patients with 7T MRI
The automated detection of cortical lesions (CLs) in patients with multiple sclerosis (MS) is a challenging task that, despite its clinical relevance, has received very little attention. Accurate detection of the small and scarce lesions requires specialized sequences and high or ultra- high field MRI. For supervised training based on multimodal structural MRI at 7T, two experts generated ground truth segmentation masks of 60 patients with 2014 CLs. We implemented a simplified 3D U-Net with three resolution levels (3D U-Net-). By increasing the complexity of the task (adding brain tissue segmentation), while randomly dropping input channels during training, we improved the performance compared to the baseline. Considering a minimum lesion size of 0.75 μL, we achieved a lesion-wise cortical lesion detection rate of 67% and a false positive rate of 42%. However, 393 (24%) of the lesions reported as false positives were post-hoc confirmed as potential or definite lesions by an expert. This indicates the potential of the proposed method to support experts in the tedious process of CL manual segmentation
Complementarity of the Maldacena and Karch-Randall Pictures
We perform a one-loop test of the holographic interpretation of the
Karch-Randall model, whereby a massive graviton appears on an AdS_4 brane in an
AdS_5 bulk. Within the AdS/CFT framework, we examine the quantum corrections to
the graviton propagator on the brane, and demonstrate that they induce a
graviton mass in exact agreement with the Karch-Randall result. Interestingly
enough, at one loop order, the spin 0, spin 1/2 and spin 1 loops contribute to
the dynamically generated (mass)^2 in the same 1: 3: 12 ratio as enters the
Weyl anomaly and the 1/r^3 corrections to the Newtonian gravitational
potential.Comment: 20 pages, Revtex 3, Discussion on the absence of a scalar ghost
clarified; Additional details on the computation give
- …