7,218 research outputs found
Radiation hydrodynamics including irradiation and adaptive mesh refinement with AZEuS. I. Methods
Aims. The importance of radiation to the physical structure of protoplanetary
disks cannot be understated. However, protoplanetary disks evolve with time,
and so to understand disk evolution and by association, disk structure, one
should solve the combined and time-dependent equations of radiation
hydrodynamics.
Methods. We implement a new implicit radiation solver in the AZEuS adaptive
mesh refinement magnetohydrodynamics fluid code. Based on a hybrid approach
that combines frequency-dependent ray-tracing for stellar irradiation with
non-equilibrium flux limited diffusion, we solve the equations of radiation
hydrodynamics while preserving the directionality of the stellar irradiation.
The implementation permits simulations in Cartesian, cylindrical, and spherical
coordinates, on both uniform and adaptive grids.
Results. We present several hydrostatic and hydrodynamic radiation tests
which validate our implementation on uniform and adaptive grids as appropriate,
including benchmarks specifically designed for protoplanetary disks. Our
results demonstrate that the combination of a hybrid radiation algorithm with
AZEuS is an effective tool for radiation hydrodynamics studies, and produces
results which are competitive with other astrophysical radiation hydrodynamics
codes.Comment: 15 pages, 10 figures, accepted for publication in A&
A quantification of hydrodynamical effects on protoplanetary dust growth
Context. The growth process of dust particles in protoplanetary disks can be
modeled via numerical dust coagulation codes. In this approach, physical
effects that dominate the dust growth process often must be implemented in a
parameterized form. Due to a lack of these parameterizations, existing studies
of dust coagulation have ignored the effects a hydrodynamical gas flow can have
on grain growth, even though it is often argued that the flow could
significantly contribute either positively or negatively to the growth process.
Aims. We intend to provide a quantification of hydrodynamical effects on the
growth of dust particles, such that these effects can be parameterized and
implemented in a dust coagulation code.
Methods. We numerically integrate the trajectories of small dust particles in
the flow of disk gas around a proto-planetesimal, sampling a large parameter
space in proto-planetesimal radii, headwind velocities, and dust stopping
times.
Results. The gas flow deflects most particles away from the
proto-planetesimal, such that its effective collisional cross section, and
therefore the mass accretion rate, is reduced. The gas flow however also
reduces the impact velocity of small dust particles onto a proto-planetesimal.
This can be beneficial for its growth, since large impact velocities are known
to lead to erosion. We also demonstrate why such a gas flow does not return
collisional debris to the surface of a proto-planetesimal.
Conclusions. We predict that a laminar hydrodynamical flow around a
proto-planetesimal will have a significant effect on its growth. However, we
cannot easily predict which result, the reduction of the impact velocity or the
sweep-up cross section, will be more important. Therefore, we provide
parameterizations ready for implementation into a dust coagulation code.Comment: 9 pages, 6 figures; accepted for publication in A&A; v2 matches the
manuscript sent to the publisher (very minor changes
Radiative corrections in neutrino-deuterium disintegration
The radiative corrections of order alpha for the charged- and neutral-current neutrino-deuterium disintegration for energies relevant to the SNO experiment are evaluated. Particular attention is paid to the issue of the bremsstrahlung detection threshold. It is shown that the radiative corrections to the total cross section for the charged current reaction are independent of that threshold, as they must be for consistency, and amount to a slowly decreasing function of the neutrino energy E-nu, varying from about 4% at low energies to 3% at the end of the B-8 spectrum. The differential cross section corrections, on the other hand, do depend on the bremsstrahlung detection threshold. Various choices of the threshold are discussed. It is shown that for a realistic choice of the threshold and for the actual electron energy threshold of the SNO detector, the deduced B-8 nu(e) flux should be decreased by about 2%. The radiative corrections to the neutral-current reaction are also evaluated
DISPATCH: A Numerical Simulation Framework for the Exa-scale Era. I. Fundamentals
We introduce a high-performance simulation framework that permits the
semi-independent, task-based solution of sets of partial differential
equations, typically manifesting as updates to a collection of `patches' in
space-time. A hybrid MPI/OpenMP execution model is adopted, where work tasks
are controlled by a rank-local `dispatcher' which selects, from a set of tasks
generally much larger than the number of physical cores (or hardware threads),
tasks that are ready for updating. The definition of a task can vary, for
example, with some solving the equations of ideal magnetohydrodynamics (MHD),
others non-ideal MHD, radiative transfer, or particle motion, and yet others
applying particle-in-cell (PIC) methods. Tasks do not have to be grid-based,
while tasks that are, may use either Cartesian or orthogonal curvilinear
meshes. Patches may be stationary or moving. Mesh refinement can be static or
dynamic. A feature of decisive importance for the overall performance of the
framework is that time steps are determined and applied locally; this allows
potentially large reductions in the total number of updates required in cases
when the signal speed varies greatly across the computational domain, and
therefore a corresponding reduction in computing time. Another feature is a
load balancing algorithm that operates `locally' and aims to simultaneously
minimise load and communication imbalance. The framework generally relies on
already existing solvers, whose performance is augmented when run under the
framework, due to more efficient cache usage, vectorisation, local
time-stepping, plus near-linear and, in principle, unlimited OpenMP and MPI
scaling.Comment: 17 pages, 8 figures. Accepted by MNRA
Hadron Spin Dynamics
Spin effects in exclusive and inclusive reactions provide an essential new
dimension for testing QCD and unraveling hadron structure. Remarkable new
experiments from SLAC, HERMES (DESY), and the Jefferson Laboratory present many
challenges to theory, including measurements at HERMES and SMC of the single
spin asymmetries in pion electroproduction, where the proton is polarized
normal to the scattering plane. This type of single spin asymmetry may be due
to the effects of rescattering of the outgoing quark on the spectators of the
target proton, an effect usually neglected in conventional QCD analyses. Many
aspects of spin, such as single-spin asymmetries and baryon magnetic moments
are sensitive to the dynamics of hadrons at the amplitude level, rather than
probability distributions. I illustrate the novel features of spin dynamics for
relativistic systems by examining the explicit form of the light-front
wavefunctions for the two-particle Fock state of the electron in QED, thus
connecting the Schwinger anomalous magnetic moment to the spin and orbital
momentum carried by its Fock state constituents and providing a transparent
basis for understanding the structure of relativistic composite systems and
their matrix elements in hadronic physics. I also present a survey of
outstanding spin puzzles in QCD, particularly the double transverse spin
asymmetry A_{NN} in elastic proton-proton scattering, the J/psi to rho-pi
puzzle, and J/psi polarization at the Tevatron.Comment: Concluding theory talk presented at SPIN2001, the Third
Circum-Pan-Pacific Symposium on High Energy Physics, October, 2001, Beijin
Constraints on T-Odd, P-Even Interactions from Electric Dipole Moments
We construct the relationship between nonrenormalizable,effective,
time-reversal violating (TV) parity-conserving (PC) interactions of quarks and
gauge bosons and various low-energy TVPC and TV parity-violating (PV)
observables. Using effective field theory methods, we delineate the scenarious
under which experimental limits on permanent electric dipole moments (EDM's) of
the electron, neutron, and neutral atoms as well as limits on TVPC observables
provide the most stringent bounds on new TVPC interactions. Under scenarios in
which parity invariance is restored at short distances, the one-loop EDM of
elementary fermions generate the most severe constraints. The limits derived
from the atomic EDM of Hg are considerably weaker. When parity symmetry
remains broken at short distances, direct TVPC search limits provide the least
ambiguous bounds. The direct limits follow from TVPC interactions between two
quarks.Comment: 43 pages, 9 figure
Magnetic Moments of Dirac Neutrinos
The existence of a neutrino magnetic moment implies contributions to the
neutrino mass via radiative corrections. We derive model-independent
"naturalness" upper bounds on the magnetic moments of Dirac neutrinos,
generated by physics above the electroweak scale. The neutrino mass receives a
contribution from higher order operators, which are renormalized by operators
responsible for the neutrino magnetic moment. This contribution can be
calculated in a model independent way. In the absence of fine-tuning, we find
that current neutrino mass limits imply that Bohr
magnetons. This bound is several orders of magnitude stronger than those
obtained from solar and reactor neutrino data and astrophysical observations.Comment: 3 pages. Talk given at PANIC'0
Neutrinoless Double Beta Decay and Lepton Flavor Violation
We point out that extensions of the Standard Model with low scale (~TeV)
lepton number violation (LNV) generally lead to a pattern of lepton flavor
violation (LFV) experimentally distinguishable from the one implied by models
with GUT scale LNV. As a consequence, muon LFV processes provide a powerful
diagnostic tool to determine whether or not the effective neutrino mass can be
deduced from the rate of neutrinoless double beta decay. We discuss the role of
\mu -> e \gamma and \mu -> e conversion in nuclei, which will be studied with
high sensitivity in forthcoming experiments.Comment: 4 pages, 3 figure
Global Analysis of Nucleon Strange Form Factors at Low
We perform a global analysis of all recent experimental data from elastic
parity-violating electron scattering at low . The values of the electric
and magnetic strange form factors of the nucleon are determined at
GeV/ to be and .Comment: 8 pages, 1 figur
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