11,983 research outputs found
One-loop Parke-Taylor factors for quadratic propagators from massless scattering equations
In this paper we reconsider the Cachazo-He-Yuan construction (CHY) of the so
called scattering amplitudes at one-loop, in order to obtain quadratic
propagators. In theories with colour ordering the key ingredient is the
redefinition of the Parke-Taylor factors. After classifying all the possible
one-loop CHY-integrands we conjecture a new one-loop amplitude for the massless
Bi-adjoint theory. The prescription directly reproduces the quadratic
propagators from of the traditional Feynman approach.Comment: 43 pages, new appendix added, few typos corrected. Accepted for
publication in JHE
Spatial inhomogeneities in the sedimentation of biogenic particles in ocean flows: analysis in the Benguela region
Sedimentation of particles in the ocean leads to inhomogeneous horizontal
distributions at depth, even if the release process is homogeneous. We study
this phenomenon considering a horizontal sheet of sinking particles immersed in
an oceanic flow, and determine how the particles are distributed when they
sediment on the seabed (or are collected at a given depth). The study is
performed from a Lagrangian viewpoint attending to the properties of the
oceanic flow and the physical characteristics (size and density) of typical
biogenic sinking particles. Two main processes determine the distribution, the
stretching of the sheet caused by the flow and its projection on the surface
where particles accumulate. These mechanisms are checked, besides an analysis
of their relative importance to produce inhomogeneities, with numerical
experiments in the Benguela region. Faster (heavier or larger) sinking
particles distribute more homogeneously than slower ones.Comment: 24 pages, 8 figures. To appear in J. Geophys. Res.-Ocean
Tensor Analysis and Fusion of Multimodal Brain Images
Current high-throughput data acquisition technologies probe dynamical systems
with different imaging modalities, generating massive data sets at different
spatial and temporal resolutions posing challenging problems in multimodal data
fusion. A case in point is the attempt to parse out the brain structures and
networks that underpin human cognitive processes by analysis of different
neuroimaging modalities (functional MRI, EEG, NIRS etc.). We emphasize that the
multimodal, multi-scale nature of neuroimaging data is well reflected by a
multi-way (tensor) structure where the underlying processes can be summarized
by a relatively small number of components or "atoms". We introduce
Markov-Penrose diagrams - an integration of Bayesian DAG and tensor network
notation in order to analyze these models. These diagrams not only clarify
matrix and tensor EEG and fMRI time/frequency analysis and inverse problems,
but also help understand multimodal fusion via Multiway Partial Least Squares
and Coupled Matrix-Tensor Factorization. We show here, for the first time, that
Granger causal analysis of brain networks is a tensor regression problem, thus
allowing the atomic decomposition of brain networks. Analysis of EEG and fMRI
recordings shows the potential of the methods and suggests their use in other
scientific domains.Comment: 23 pages, 15 figures, submitted to Proceedings of the IEE
Worse than a big rip?
We show that a generalised phantom Chaplygin gas can present a future
singularity in a finite future cosmic time. Unlike the big rip singularity,
this singularity happens for a finite scale factor, but like the big rip
singularity, it would also take place at a finite future cosmic time. In
addition, we define a dual of the generalised phantom Chaplygin gas which
satisfies the null energy condition. Then, in a Randall-Sundrum 1 brane-world
scenario, we show that the same kind of singularity at a finite scale factor
arises for a brane filled with a dual of the generalised phantom Chaplygin gas.Comment: 6 pages, 4 figures, RevTeX 4. Discussion expanded and references
added. Version to appear in PL
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