1,418 research outputs found
The elliptic model for communication fluxes
In this paper, a model (called the elliptic model) is proposed to estimate the number of social ties between two locations using population data in a similar manner to how transportation research deals with trips. To overcome the asymmetry of transportation models, the new model considers that the number of relationships between two locations is inversely proportional to the population in the ellipse whose foci are in these two locations. The elliptic model is evaluated by considering the anonymous communications patterns of 25 million users from three different countries, where a location has been assigned to each user based on their most used phone tower or billing zip code. With this information, spatial social networks are built at three levels of resolution: tower, city and region for each of the three countries. The elliptic model achieves a similar performance when predicting communication fluxes as transportation models do when predicting trips. This shows that human relationships are influenced at least as much by geography as is human mobility
Neutrino mass textures from F-theory
Experimental data on the neutrino mixing and masses strongly suggest an
underlying approximate symmetry of the relevant Yukawa superpotential terms.
Intensive phenomenological explorations during the last decade indicate that
permutation symmetries such as S_4, A_4 and their subgroups, under certain
assumptions and vacuum alignments, predict neutrino mass textures compatible
with such data. Motivated by these findings, in the present work we analyse the
neutrino properties in F-theory GUT models derived in the framework of the
maximal underlying E_8 symmetry in the elliptic fibration. More specifically,
we consider local F-SU(5) GUT models and study in detail spectral cover
geometries with monodromies associated to the finite symmetries S_4, A_4 and
their transitive subgroups, including the dihedral group D_4 and Z_2 X Z_2. We
discuss various issues that emerge in the implementation of S_4, A_4 neutrino
models in the F-theory context and suggest how these can be resolved. Realistic
models are presented for the case of monodromies based on their transitive
subgroups. We exemplify this procedure with a detailed analysis performed for
the case of Z_2 X Z_2 model.Comment: 37 pages, 3 figures, revised versio
F-GUTs with Mordell-Weil U(1)'s
In this note we study the constraints on F-theory GUTs with extra 's in
the context of elliptic fibrations with rational sections. We consider the
simplest case of one abelian factor (Mordell-Weil rank one) and investigate the
conditions that are induced on the coefficients of its Tate form. Converting
the equation representing the generic hypersurface to this Tate's
form we find that the presence of a U(1), already in this local description, is
consistent with the exceptional and non-abelian
singularities. We briefly comment on a viable effective
F-theory model.Comment: 13 page
T-duality, Quotients and Currents for Non-Geometric Closed Strings
We use the canonical description of T-duality as well as the formulation of
T-duality in terms of chiral currents to investigate the geometric and
non-geometric faces of closed string backgrounds originating from principal
torus bundles with constant H-flux. Employing conformal field theory
techniques, the non-commutative and non-associative structures among
generalized coordinates in the so called Q-flux and R-flux backgrounds emerge
by gauging the Abelian symmetries of an enlarged Rocek-Verlinde sigma-model and
projecting the associated chiral currents of the enlarged theory to the T-dual
coset models carrying non-geometric fluxes.Comment: 56 page
Suzaku Spectroscopy of an X-Ray Reflection Nebula and a New Supernova Remnant Candidate in the Sgr B1 Region
We made a 100 ks observation of the Sagittarius (Sgr) B1 region at (l, b) =
(0.5, -0.1) near to the Galactic center (GC) with the Suzaku/XIS. Emission
lines of S XV, Fe I, Fe XXV, and Fe XXVI were clearly detected in the spectrum.
We found that the Fe XXV and Fe XXVI line emissions smoothly distribute over
the Sgr B1 and B2 regions connecting from the GC. This result suggests that the
GC hot plasma extends at least up to the Sgr B region with a constant
temperature. There are two diffuse X-ray sources in the observed region. One of
the two (G0.42-0.04) is newly discovered, and exhibits a strong S XV Ka
emission line, suggesting a candidate for a supernova remnant located in the GC
region. The other one (M0.51-0.10), having a prominent Fe I Ka emission line
and a strongly absorbed continuum, is likely to be an X-ray reflection nebula.
There is no near source bright enough to irradiate M0.51-0.10. However, the Fe
I Ka emission can be explained if Sgr A* was ~ 10^6 times brighter 300 years
ago, the light travel time for 100 pc to M0.51-0.10, than it is at present.Comment: 10 pages, 10 figure
Non-local first-order modelling of crowd dynamics: a multidimensional framework with applications
In this work a physical modelling framework is presented, describing the
intelligent, non-local, and anisotropic behaviour of pedestrians. Its
phenomenological basics and constitutive elements are detailed, and a
qualitative analysis is provided. Within this common framework, two first-order
mathematical models, along with related numerical solution techniques, are
derived. The models are oriented to specific real world applications: a
one-dimensional model of crowd-structure interaction in footbridges and a
two-dimensional model of pedestrian flow in an underground station with several
obstacles and exits. The noticeable heterogeneity of the applications
demonstrates the significance of the physical framework and its versatility in
addressing different engineering problems. The results of the simulations point
out the key role played by the physiological and psychological features of
human perception on the overall crowd dynamics.Comment: 26 pages, 17 figure
Flow Characteristics in a Crowded Transport Model
The aim of this paper is to discuss the appropriate modelling of in- and
outflow boundary conditions for nonlinear drift-diffusion models for the
transport of particles including size exclusion and their effect on the
behaviour of solutions. We use a derivation from a microscopic asymmetric
exclusion process and its extension to particles entering or leaving on the
boundaries. This leads to specific Robin-type boundary conditions for inflow
and outflow, respectively. For the stationary equation we prove the existence
of solutions in a suitable setup. Moreover, we investigate the flow
characteristics for small diffusion, which yields the occurence of a maximal
current phase in addition to well-known one-sided boundary layer effects for
linear drift-diffusion problems. In a one-dimensional setup we provide rigorous
estimates in terms of , which confirm three different phases.
Finally, we derive a numerical approach to solve the problem also in multiple
dimensions. This provides further insight and allows for the investigation of
more complicated geometric setups
Rarefaction acceleration in magnetized gamma-ray burst jets
Relativistic jets associated with long/soft gamma-ray bursts are formed and
initially propagate in the interior of the progenitor star. Because of the
subsequent loss of their external pressure support after they cross the stellar
surface, these flows can be modeled as moving around a corner. A strong
steady-state rarefaction wave is formed, and the sideways expansion is
accompanied by a rarefaction acceleration. We investigate the efficiency and
the general characteristics of this mechanism by integrating the steady-state,
special relativistic, magnetohydrodynamic equations, using a special set of
partial exact solutions in planar geometry (r self-similar with respect to the
"corner"). We also derive analytical approximate scalings in the
ultrarelativistic cold/magnetized, and hydrodynamic limits. The mechanism is
more effective in magnetized than in purely hydrodynamic flows. It
substantially increases the Lorentz factor without much affecting the opening
of the jet; the resulting values of their product can be much grater than
unity, allowing for possible breaks in the afterglow light curves. These
findings are similar to the ones from numerical simulations of axisymmetric
jets by Komissarov et al and Tchekhovskoy et al, although in our approach we
describe the rarefaction as a steady-state simple wave and self-consistently
calculate the opening of the jet that corresponds to zero external pressure.Comment: 11 page
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