53,540 research outputs found
The R-matrix theory
The different facets of the -matrix method are presented pedagogically in
a general framework. Two variants have been developed over the years: The
"calculable" -matrix method is a calculational tool to derive scattering
properties from the Schr\"odinger equation in a large variety of physical
problems. It was developed rather independently in atomic and nuclear physics
with too little mutual influence. The "phenomenological" -matrix
method is a technique to parametrize various types of cross sections. It was
mainly (or uniquely) used in nuclear physics. Both directions are explained by
starting from the simple problem of scattering by a potential. They are
illustrated by simple examples in nuclear and atomic physics. In addition to
elastic scattering, the -matrix formalism is applied to transfer and
radiative-capture reactions. We also present more recent and more ambitious
applications of the theory in nuclear physics.Comment: 93 pages, 26 figures. Rep. Prog. Phys., in pres
Numerical solution of gravitational dynamics in asymptotically anti-de Sitter spacetimes
A variety of gravitational dynamics problems in asymptotically anti-de Sitter
(AdS) spacetime are amenable to efficient numerical solution using a common
approach involving a null slicing of spacetime based on infalling geodesics,
convenient exploitation of the residual diffeomorphism freedom, and use of
spectral methods for discretizing and solving the resulting differential
equations. Relevant issues and choices leading to this approach are discussed
in detail. Three examples, motivated by applications to non-equilibrium
dynamics in strongly coupled gauge theories, are discussed as instructive test
cases. These are gravitational descriptions of homogeneous isotropization,
collisions of planar shocks, and turbulent fluid flows in two spatial
dimensions.Comment: 70 pages, 19 figures; v4: fixed minus sign typo in last term of eqn.
(3.47
New method for extracting quasi-bound states from the continuum
A new parameter-free method is proposed for treatment of single-particle
resonances in the real-energy continuum shell model. This method yields
quasi-bound states embedded in the continuum which provide a natural
generalization of weakly bound single-particle states.Comment: 22 pages, 10 figure
The origin of the negative torque density in disk-satellite interaction
Tidal interaction between a gaseous disk and a massive orbiting perturber is
known to result in angular momentum exchange between them. Understanding
astrophysical manifestations of this coupling such as gap opening by planets in
protoplanetary disks or clearing of gas by binary supermassive black holes
(SMBHs) embedded in accretion disks requires knowledge of the spatial
distribution of the torque exerted on the disk by a perturber. Recent
hydrodynamical simulations by Dong et al (2011) have shown evidence for the
tidal torque density produced in a uniform disk to change sign at the radial
separation of scale heights from the perturber's orbit, in clear
conflict with the previous studies. To clarify this issue we carry out a linear
calculation of the disk-satellite interaction putting special emphasis on
understanding the behavior of the perturbed fluid variables in physical space.
Using analytical as well as numerical methods we confirm the reality of the
negative torque density phenomenon and trace its origin to the overlap of
Lindblad resonances in the vicinity of the perturber's orbit - an effect not
accounted for in previous studies. These results suggest that calculations of
the gap and cavity opening in disks by planets and binary SMBHs should rely on
more realistic torque density prescriptions than the ones used at present.Comment: 18 pages, 6 figures, accepted to Ap
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