9,584 research outputs found
Distribution functions for a family of axially symmetric galaxy models
We present the derivation of distribution functions for the first four
members of a family of disks, previously obtained in (MNRAS, 371, 1873, 2006),
which represent a family of axially symmetric galaxy models with finite radius
and well behaved surface mass density. In order to do this we employ several
approaches that have been developed starting from the potential-density pair
and, essentially using the method introduced by Kalnajs (Ap. J., 205, 751,
1976) we obtain some distribution functions that depend on the Jacobi integral.
Now, as this method demands that the mass density can be properly expressed as
a function of the gravitational potential, we can do this only for the first
four discs of the family. We also find another kind of distribution functions
by starting with the even part of the previous distribution functions and using
the maximum entropy principle in order to find the odd part and so a new
distribution function, as it was pointed out by Dejonghe (Phys. Rep., 133, 217,
1986). The result is a wide variety of equilibrium states corresponding to
several self-consistent finite flat galaxy models.Comment: 12 pages, 7 figures, updated version, accepted for publication in
Rev. Acad. Colomb. Cienc. Ex. Fis. Na
Geometric Aspects of Holographic Bit Threads
We revisit the recent reformulation of the holographic prescription to
compute entanglement entropy in terms of a convex optimization problem,
introduced by Freedman and Headrick. According to it, the holographic
entanglement entropy associated to a boundary region is given by the maximum
flux of a bounded, divergenceless vector field, through the corresponding
region. Our work leads to two main results: (i) We present a general algorithm
that allows the construction of explicit thread configurations in cases where
the minimal surface is known. We illustrate the method with simple examples:
spheres and strips in vacuum AdS, and strips in a black brane geometry.
Studying more generic bulk metrics, we uncover a sufficient set of conditions
on the geometry and matter fields that must hold to be able to use our
prescription. (ii) Based on the nesting property of holographic entanglement
entropy, we develop a method to construct bit threads that maximize the flux
through a given bulk region. As a byproduct, we are able to construct more
general thread configurations by combining (i) and (ii) in multiple patches. We
apply our methods to study bit threads which simultaneously compute the
entanglement entropy and the entanglement of purification of mixed states and
comment on their interpretation in terms of entanglement distillation. We also
consider the case of disjoint regions for which we can explicitly construct the
so-called multi-commodity flows and show that the monogamy property of mutual
information can be easily illustrated from our constructions.Comment: 48 pages, multiple figures. v3: matches published versio
QCD resummation for the fully differential Drell-Yan cross section
We study an extension of resummation to the fully differential cross section in the Drell-Yan process. This new method extends the Collins-Soper-Sterman formalism to the longitudinal WL and double delta helicity structure function WDeltaDelta, recovering the next-to-leading-order predictions. The new extension also modifies the transverse structure function WT obtained in previous extensions.;The angular coefficients, lambda and nu, used for parametrization of the angular distribution, were studied with the new structure functions. No violation of the Lam-Tung relation was found. A possible solution to explain the difference between theoretical and experimental results is proposed. This solution may also explain the existence of the azimuthal asymmetry.;For completeness, leading-order and next-to-leading-order results are presented. The Collins-Soper-Sterman formalism is also reviewed
Quasinormal modes of the Schwarzchild black hole with a deficit solid angle and quintessence-like matter: Scalar and electromagnetic perturbations
We study the quasinormal modes (QNM) for scalar, and electromagnetic
perturbations in the Schwarzchild black hole with a deficit solid angle and
quintessence-like matter. Using the sixth--order WKB approximation and the
improved asymptotic iteration method (AIM) we can determine the dependence of
the quasinormal modes on the parameters of the black hole and the parameters on
the test fields. The values of the real part and imaginary parts of the
quasi--normal modes increase with the decrease of the values of the deficit
solid angle and density of quintessence-like matter. The quasinormal modes
gotten by these two methods are in good agreement. Using the finite difference
method, we obtain the time evolution profile of such perturbations in this
Black Hole
Effects of quintessence on scattering and absorption sections of black holes
Basing on the ideas used by Kiselev, we study three black holes surrounded by
quintessence and the effects of quintessence on the classical and semiclassical
scattering cross-sections. In contrast, the absorption section is studied with
the sinc approximation in the eikonal limit. For Schwarzschild,
Reissner-Nordstr\"{o}m and Bardeen black holes surrounded by quintessence, the
values critical of charges and the normalization factor are obtained. We also
described the horizons and the extremal condition of the black holes surrounded
by quintessence. By setting for the quintessence state parameter in two the
particular cases w=-2/3 and w=-1/2
Transverse momentum dependence of the angular distribution of the Drell-Yan process
We calculate the transverse momentum Q_{\perp} dependence of the helicity
structure functions for the hadroproduction of a massive pair of leptons with
pair invariant mass Q. These structure functions determine the angular
distribution of the leptons in the pair rest frame. Unphysical behavior in the
region Q_{\perp} --> 0 is seen in the results of calculations done at
fixed-order in QCD perturbation theory. We use current conservation to
demonstrate that the unphysical inverse-power and \ln(Q/Q_{\perp}) logarithmic
divergences in three of the four independent helicity structure functions share
the same origin as the divergent terms in fixed-order calculations of the
angular-integrated cross section. We show that the resummation of these
divergences to all orders in the strong coupling strength \alpha_s can be
reduced to the solved problem of the resummation of the divergences in the
angular-integrated cross section, resulting in well-behaved predictions in the
small Q_{\perp} region. Among other results, we show the resummed part of the
helicity structure functions preserves the Lam-Tung relation between the
longitudinal and double spin-flip structure functions as a function of
Q_{\perp} to all orders in \alpha_s.Comment: 18 pages, 4 figures; typos corrected, references updated, a few
clarifications recommended by the referee. Paper accepted for publication in
Physical Review
Cumulant expansion framework for internal gradient distributions tensors
Magnetic resonance imaging is a powerful, non invasive tool for medical
diagnosis. The low sensitivity for detecting the nuclear spin signals,
typically limits the image resolution to several tens of micrometers in
preclinical systems and millimeters in clinical scanners. Other sources of
information, derived from diffusion processes of intrinsic molecules as water
in the tissues, allow getting morphological information at micrometric and
submicrometric scales as potential biomarkers of several pathologies. Here we
consider extracting this morphological information by probing the distribution
of internal magnetic field gradients induced by the heterogeneous magnetic
susceptibility of the medium. We use a cumulant expansion to derive the
dephasing on the spin signal induced by the molecules that explore these
internal gradients while diffuse. Based on the cumulant expansion, we define
internal gradient distributions tensors (IGDT) and propose modulating gradient
spin echo sequences to probe them. These IGDT contain microstructural
morphological information that characterize porous media and biological
tissues. We evaluate the IGDT effects on the magnetization decay with typical
conditions of brain tissue and show their effects can be experimentally
observed. Our results thus provide a framework for exploiting IGDT as
quantitative diagnostic tools
Efectos de la discretización en la simulación de escorrentía urbana
La urbanización produce un fuerte impacto sobre las respuestas hidrológicas de las cuencas. El incremento de la impermeabilidad aumenta notablemente los escurrimientos superficiales. Para evacuar los excedentes pluviales urbanos, se diseñan y construyen sistemas de drenaje, utilizando modelos
matemáticos que permiten realizar los cálculos de diseño, operación y planificación de tales sistemas. El avance de la informática ha generalizado la aplicación de la modelación distribuida, lo que supone una mejora de la descripción de los fenómenos que participan en la transformación lluvia-escorrentía. Sin embargo, incorpora una incertidumbre relacionada con la elección del tamaño de la discretización superficial apropiada para la simulación. Este trabajo examina los efectos de la discretización espacial sobre la simulación del escurrimiento en una red de conductos pluviales, analiza la variación del parámetro de calibración W para diferentes escalas espaciales de una cuenca urbana y propone criterios para elegir la mayor escala espacial que satisfaga una precisión deseada en los resultados. Para ello se realizaron ensayos numéricos con el
modelo SWMM sobre una cuenca urbana teórica y sobre una cuenca urbana experimental. A partir de los resultados obtenidos, se observa que la escala espacial influye en los resultados de la simulación con el modelo SWMM. La red de drenaje adiciona almacenamiento al sistema, atenuando y retardando los caudales pico. A medida que aumenta la escala una parte de la red es removida y en consecuencia se empuntan los hidrogramas y se anticipan los picos. Para que el modelo represente, a una escala mayor, una función de respuesta similar a la obtenida con la escala de detalle, es necesario compensar la pérdida de almacenamiento. Para ello, se debe reducir
el ancho total de la cuenca, es decir, aumentar la longitud de escurrimiento. Para aplicaciones del modelo SWMM en cuencas similares a la del estudio, una vez discretizada la cuenca y si no se dispone de información pluvio-hidrométrica, se puede estimar el valor medio del parámetro W a partir de la relación ancho medio de escorrentía - área media de subcuencas. Para la cuenca experimental estudiada la escala espacial más grande que conserva la precisión admisible de los hidrogramas a la salida y de niveles de agua en nodos de interés es la meso escala.Peer Reviewe
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