11,759 research outputs found
Neutron-Deuteron System and Photon Polarization Parameter at Thermal Neutron Energies
Effective Field Theory(EFT) is, the unique, model independent and systematic
low-energy version of QCD for processes involving momenta below the pion mass.
A low-energy photo-nuclear observable in three-body systems, photon
polarization parameter at thermal neutron energies is calculated by using
pionless EFT up to next-to-next to leading order(NLO). In order to make a
comparative study of this model, we compared our results for photon
polarization parameter with the realistic Argonne two-nucleon and
Urbana IX or Tucson-Melbourne three-nucleon interactions. Three-body currents
give small but significant contributions to some of the observables in the
neutron-deuteron radiative capture cross section at thermal neutron energies.
In this formalism the three-nucleon forces are needed up to NLO for cut-off
independent results. Our result converges order by order in low energy
expansion and also cut-off independent at this order.Comment: 10 pages, 3 figures, 2 table
Dynamical Cusp Regeneration
After being destroyed by a binary supermassive black hole, a stellar density
cusp can regrow at the center of a galaxy via energy exchange between stars
moving in the gravitational field of the single, coalesced hole. We illustrate
this process via high-accuracy N-body simulations. Regeneration requires
roughly one relaxation time and the new cusp extends to a distance of roughly
one-fifth the black hole's influence radius, with density rho ~ r^{-7/4}; the
mass in the cusp is of order 10% the mass of the black hole. Growth of the cusp
is preceded by a stage in which the stellar velocity dispersion evolves toward
isotropy and away from the tangentially-anisotropic state induced by the
binary. We show that density profiles similar to those observed at the center
of the Milky Way and M32 can regenerate themselves in several Gyr following
infall of a second black hole; the presence of density cusps at the centers of
these galaxies can therefore not be used to infer that no merger has occurred.
We argue that Bahcall-Wolf cusps are ubiquitous in stellar spheroids fainter
than M_V ~ -18.5 that contain supermassive black holes, but the cusps have not
been detected outside of the Local Group since their angular sizes are less
than 0.1". We show that the presence of a cusp implies a lower limit of
\~10^{-4} per year on the rate of stellar tidal disruptions, and discuss the
consequences of the cusps for gravitational lensing and the distribution of
dark matter on sub-parsec scales.Comment: Accepted for publication in The Astrophysical Journa
Research on new techniques for the analysis of manual control systems Progress report, 16 Jun. - 15 Dec. 1968
Parameter estimation for continuous input-output systems with internal sampling in human controller model
Global Dynamics in Galactic Triaxial Systems I
In this paper we present a theoretical analysis of the global dynamics in a
triaxial galactic system using a 3D integrable Hamiltonian as a simple
representation. We include a thorough discussion on the effect of adding a
generic non--integrable perturbation to the global dynamics of the system. We
adopt the triaxial Stackel Hamiltonian as the integrable model and compute its
resonance structure in order to understand its global dynamics when a
perturbation is introduced. Also do we take profit of this example in order to
provide a theoretical discussion about diffussive processes taking place in
phase space.Comment: Accepted A&
Long-Term Evolution of Massive Black Hole Binaries. III. Binary Evolution in Collisional Nuclei
[Abridged] In galactic nuclei with sufficiently short relaxation times,
binary supermassive black holes can evolve beyond their stalling radii via
continued interaction with stars. We study this "collisional" evolutionary
regime using both fully self-consistent N-body integrations and approximate
Fokker-Planck models. The N-body integrations employ particle numbers up to
0.26M and a direct-summation potential solver; close interactions involving the
binary are treated using a new implementation of the Mikkola-Aarseth chain
regularization algorithm. Even at these large values of N, two-body scattering
occurs at high enough rates in the simulations that they can not be simply
scaled to the large-N regime of real galaxies. The Fokker-Planck model is used
to bridge this gap; it includes, for the first time, binary-induced changes in
the stellar density and potential. The Fokker-Planck model is shown to
accurately reproduce the results of the N-body integrations, and is then
extended to the much larger N regime of real galaxies. Analytic expressions are
derived that accurately reproduce the time dependence of the binary semi-major
axis as predicted by the Fokker-Planck model. Gravitational wave coalescence is
shown to occur in <10 Gyr in nuclei with velocity dispersions below about 80
km/s. Formation of a core results from a competition between ejection of stars
by the binary and re-supply of depleted orbits via two-body scattering. Mass
deficits as large as ~4 times the binary mass are produced before coalescence.
After the two black holes coalesce, a Bahcall-Wolf cusp appears around the
single hole in one relaxation time, resulting in a nuclear density profile
consisting of a flat core with an inner, compact cluster, similar to what is
observed at the centers of low-luminosity spheroids.Comment: 21 page
Bias, the Brain, and Student Evaluations of Teaching
Student evaluations of teaching are a common fixture at American law schools, but they harbor surprising biases. Extensive psychology research demonstrates that these assessments respond overwhelmingly to a professor’s appearance and nonverbal behavior; ratings based on just thirty seconds of silent videotape correlate strongly with end-of-semester evaluations. The nonverbal behaviors that influence teaching evaluations are rooted in physiology, culture, and habit, allowing characteristics like race and gender to affect evaluations. The current process of gathering evaluations, moreover, allows social stereotypes to filter students’ perceptions, increasing risks of bias. These distortions are inevitable products of the intuitive, “system one” cognitive processes that the present process taps. The cure for these biases requires schools to design new student evaluation systems, such as ones based on facilitated group discussion, that enable more reflective, deliberative judgments. This article draws upon research in cognitive decision making, both to present the compelling case for reforming the current system of evaluating classroom performance and to illuminate the cognitive processes that underlie many facets of the legal system
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