1,110,273 research outputs found
Inferring the Origin Locations of Tweets with Quantitative Confidence
Social Internet content plays an increasingly critical role in many domains,
including public health, disaster management, and politics. However, its
utility is limited by missing geographic information; for example, fewer than
1.6% of Twitter messages (tweets) contain a geotag. We propose a scalable,
content-based approach to estimate the location of tweets using a novel yet
simple variant of gaussian mixture models. Further, because real-world
applications depend on quantified uncertainty for such estimates, we propose
novel metrics of accuracy, precision, and calibration, and we evaluate our
approach accordingly. Experiments on 13 million global, comprehensively
multi-lingual tweets show that our approach yields reliable, well-calibrated
results competitive with previous computationally intensive methods. We also
show that a relatively small number of training data are required for good
estimates (roughly 30,000 tweets) and models are quite time-invariant
(effective on tweets many weeks newer than the training set). Finally, we show
that toponyms and languages with small geographic footprint provide the most
useful location signals.Comment: 14 pages, 6 figures. Version 2: Move mathematics to appendix, 2 new
references, various other presentation improvements. Version 3: Various
presentation improvements, accepted at ACM CSCW 201
The Flavour Portal to Dark Matter
We present a class of models in which dark matter (DM) is a fermionic singlet
under the Standard Model (SM) gauge group but is charged under a symmetry of
flavour that acts as well on the SM fermions. Interactions between DM and SM
particles are mediated by the scalar fields that spontaneously break the
flavour symmetry, the so-called flavons. In the case of gauged flavour
symmetries, the interactions are also mediated by the flavour gauge bosons. We
first discuss the construction and the generic features of this class of
models. Then a concrete example with an abelian flavour symmetry is considered.
We compute the complementary constraints from the relic abundance, direct
detection experiments and flavour observables, showing that wide portions of
the parameter space are still viable. Other possibilities like non-abelian
flavour symmetries can be analysed within the same framework.Comment: 7 pages, 2 figures, more detailed presentation of flavour
constraints, version accepted for publication in PR
First Class Constrained Systems and Twisting of Courant Algebroids by a Closed 4-form
We show that in analogy to the introduction of Poisson structures twisted by
a closed 3-form by Park and Klimcik-Strobl, the study of three dimensional
sigma models with Wess-Zumino term leads in a likewise way to twisting of
Courant algebroid structures by closed 4-forms H.
The presentation is kept pedagogical and accessible to physicists as well as
to mathematicians, explaining in detail in particular the interplay of field
transformations in a sigma model with the type of geometrical structures
induced on a target. In fact, as we also show, even if one does not know the
mathematical concept of a Courant algebroid, the study of a rather general
class of 3-dimensional sigma models leads one to that notion by itself.
Courant algebroids became of relevance for mathematical physics lately from
several perspectives - like for example by means of using generalized complex
structures in String Theory. One may expect that their twisting by the
curvature H of some 3-form Ramond-Ramond gauge field will become of relevance
as well.Comment: 25 pages, invited contribution to the Wolfgang Kummer memorial volum
Instability of Turing patterns in reaction-diffusion-ODE systems
The aim of this paper is to contribute to the understanding of the pattern
formation phenomenon in reaction-diffusion equations coupled with ordinary
differential equations. Such systems of equations arise, for example, from
modeling of interactions between cellular processes such as cell growth,
differentiation or transformation and diffusing signaling factors. We focus on
stability analysis of solutions of a prototype model consisting of a single
reaction-diffusion equation coupled to an ordinary differential equation. We
show that such systems are very different from classical reaction-diffusion
models. They exhibit diffusion-driven instability (Turing instability) under a
condition of autocatalysis of non-diffusing component. However, the same
mechanism which destabilizes constant solutions of such models, destabilizes
also all continuous spatially heterogeneous stationary solutions, and
consequently, there exist no stable Turing patterns in such
reaction-diffusion-ODE systems. We provide a rigorous result on the nonlinear
instability, which involves the analysis of a continuous spectrum of a linear
operator induced by the lack of diffusion in the destabilizing equation. These
results are extended to discontinuous patterns for a class of nonlinearities.Comment: This is a new version of the paper. Presentation of results was
essentially revised according to referee suggestion
Naturalness in emergent spacetime
Effective field theories (EFTs) have been widely used as a framework in order
to place constraints on the Planck suppressed Lorentz violations predicted by
various models of quantum gravity. There are however technical problems in the
EFT framework when it comes to ensuring that small Lorentz violations remain
small -- this is the essence of the "naturalness" problem. Herein we present an
"emergent" space-time model, based on the "analogue gravity'' programme, by
investigating a specific condensed-matter system that is in principle capable
of simulating the salient features of an EFT framework with Lorentz violations.
Specifically, we consider the class of two-component BECs subject to
laser-induced transitions between the components, and we show that this model
is an example for Lorentz invariance violation due to ultraviolet physics.
Furthermore our model explicitly avoids the "naturalness problem", and makes
specific suggestions regarding how to construct a physically reasonable quantum
gravity phenomenology.Comment: V1:4 pages, revtex4; V2: slight changes in title, presentation, and
conclusions. This version to appear in Physical Review Letter
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