31,633 research outputs found
Investigating Multiple Solutions in the Constrained Minimal Supersymmetric Standard Model
Recent work has shown that the Constrained Minimal Supersymmetric Standard
Model (CMSSM) can possess several distinct solutions for certain values of its
parameters. The extra solutions were not previously found by public
supersymmetric spectrum generators because fixed point iteration (the algorithm
used by the generators) is unstable in the neighbourhood of these solutions.
The existence of the additional solutions calls into question the robustness of
exclusion limits derived from collider experiments and cosmological
observations upon the CMSSM, because limits were only placed on one of the
solutions. Here, we map the CMSSM by exploring its multi-dimensional parameter
space using the shooting method, which is not subject to the stability issues
which can plague fixed point iteration. We are able to find multiple solutions
where in all previous literature only one was found. The multiple solutions are
of two distinct classes. One class, close to the border of bad electroweak
symmetry breaking, is disfavoured by LEP2 searches for neutralinos and
charginos. The other class has sparticles that are heavy enough to evade the
LEP2 bounds. Chargino masses may differ by up to around 10% between the
different solutions, whereas other sparticle masses differ at the sub-percent
level. The prediction for the dark matter relic density can vary by a hundred
percent or more between the different solutions, so analyses employing the dark
matter constraint are incomplete without their inclusion.Comment: 30 pages, 12 figures, 2 tables; v2: added discussion on speed of
shooting method, fixed typos, matches published versio
A simple multigrid scheme for solving the Poisson equation with arbitrary domain boundaries
We present a new multigrid scheme for solving the Poisson equation with
Dirichlet boundary conditions on a Cartesian grid with irregular domain
boundaries. This scheme was developed in the context of the Adaptive Mesh
Refinement (AMR) schemes based on a graded-octree data structure. The Poisson
equation is solved on a level-by-level basis, using a "one-way interface"
scheme in which boundary conditions are interpolated from the previous coarser
level solution. Such a scheme is particularly well suited for self-gravitating
astrophysical flows requiring an adaptive time stepping strategy. By
constructing a multigrid hierarchy covering the active cells of each AMR level,
we have designed a memory-efficient algorithm that can benefit fully from the
multigrid acceleration. We present a simple method for capturing the boundary
conditions across the multigrid hierarchy, based on a second-order accurate
reconstruction of the boundaries of the multigrid levels. In case of very
complex boundaries, small scale features become smaller than the discretization
cell size of coarse multigrid levels and convergence problems arise. We propose
a simple solution to address these issues. Using our scheme, the convergence
rate usually depends on the grid size for complex grids, but good linear
convergence is maintained. The proposed method was successfully implemented on
distributed memory architectures in the RAMSES code, for which we present and
discuss convergence and accuracy properties as well as timing performances.Comment: 33 pages, 15 figures, accepted for publication in Journal of
Computational Physic
Recent applications of the transonic wing analysis computer code, TWING
An evaluation of the transonic-wing-analysis computer code TWING is given. TWING utilizes a fully implicit approximate factorization iteration scheme to solve the full potential equation in conservative form. A numerical elliptic-solver grid-generation scheme is used to generate the required finite-difference mesh. Several wing configurations were analyzed, and the limits of applicability of this code was evaluated. Comparisons of computed results were made with available experimental data. Results indicate that the code is robust, accurate (when significant viscous effects are not present), and efficient. TWING generally produces solutions an order of magnitude faster than other conservative full potential codes using successive-line overrelaxation. The present method is applicable to a wide range of isolated wing configurations including high-aspect-ratio transport wings and low-aspect-ratio, high-sweep, fighter configurations
Non-adiabatic transitions in multi-level systems
In a quantum system with a smoothly and slowly varying Hamiltonian, which
approaches a constant operator at times , the transition
probabilities between adiabatic states are exponentially small. They are
characterized by an exponent that depends on a phase integral along a path
around a set of branch points connecting the energy level surfaces in complex
time. Only certain sequences of branch points contribute. We propose that these
sequences are determined by a topological rule involving the Stokes lines
attached to the branch points. Our hypothesis is supported by theoretical
arguments and results of numerical experiments.Comment: 25 pages RevTeX, 9 figures and 4 tables as Postscipt file
PORTA: A three-dimensional multilevel radiative transfer code for modeling the intensity and polarization of spectral lines with massively parallel computers
The interpretation of the intensity and polarization of the spectral line
radiation produced in the atmosphere of the Sun and of other stars requires
solving a radiative transfer problem that can be very complex, especially when
the main interest lies in modeling the spectral line polarization produced by
scattering processes and the Hanle and Zeeman effects. One of the difficulties
is that the plasma of a stellar atmosphere can be highly inhomogeneous and
dynamic, which implies the need to solve the non-equilibrium problem of the
generation and transfer of polarized radiation in realistic three-dimensional
(3D) stellar atmospheric models. Here we present PORTA, an efficient multilevel
radiative transfer code we have developed for the simulation of the spectral
line polarization caused by scattering processes and the Hanle and Zeeman
effects in 3D models of stellar atmospheres. The numerical method of solution
is based on the non-linear multigrid iterative method and on a novel
short-characteristics formal solver of the Stokes-vector transfer equation
which uses monotonic B\'ezier interpolation. Therefore, with PORTA the
computing time needed to obtain at each spatial grid point the self-consistent
values of the atomic density matrix (which quantifies the excitation state of
the atomic system) scales linearly with the total number of grid points.
Another crucial feature of PORTA is its parallelization strategy, which allows
us to speed up the numerical solution of complicated 3D problems by several
orders of magnitude with respect to sequential radiative transfer approaches,
given its excellent linear scaling with the number of available processors. The
PORTA code can also be conveniently applied to solve the simpler 3D radiative
transfer problem of unpolarized radiation in multilevel systems.Comment: 15 pages, 15 figures, to appear in Astronomy and Astrophysic
Structural Analysis and Matrix Interpetive System /SAMIS/ program Technical report, Feb. - Aug. 1966
Development of characteristic equations and error analysis for computer programs contained in structural analysis and matrix interpretive syste
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