10,008 research outputs found
Cortical Dynamics of Boundary Segmentation and Reset: Persistence, Afterimages, and Residual Traces
Using a neural network model of boundary segmentation and reset, Francis, Grossberg, and Mingolla (1994) linked the percept of persistence to the duration of a boundary segmentation after stimulus offset. In particular, the model simulated the decrease of persistence duration with an increase in stimulus duration and luminance. Thc present article reveals further evidence for the neural mechanisms used by the theory. Simulations show that the model reset signals generate orientational afterimages, such as the MacKay effect, when the reset signals can be grouped by a subsequent boundary segmentation that generates illusory contours through them. Simulations also show that the same mechanisms explain properties of residual traces, which increase in duration with stimulus duration and luminance. The model hereby discloses previously unsuspected mechanistic links between data about persistence and afterimages, and helps to clarify the sometimes controversial issues surrounding distinctions between persistence, residual traces, and afterimages.Air Force Office of Scientific Research (F49620-92-J-0499); Office of Naval Research (N00014-91-J-4100, N00014-92-J-4015
Single Top Quark Production at the LHC: Understanding Spin
We show that the single top quarks produced in the Wg-fusion channel at a
proton-proton collider at a center-of-mass energy sqrt{s}=14 TeV posses a high
degree of polarization in terms of a spin basis which decomposes the top quark
spin in its rest frame along the direction of the spectator jet. A second
useful spin basis is the eta-beamline basis, which decomposes the top quark
spin along one of the two beam directions, depending on which hemisphere
contains the spectator jet. We elucidate the interplay between the two- and
three-body final states contributing to this production cross section in the
context of determining the spin decomposition of the top quarks, and argue that
the zero momentum frame helicity is undefined. We show that the usefulness of
the spectator and eta-beamline spin bases is not adversely affected by the cuts
required to separate the Wg-fusion signal from the background.Comment: 12 pages with 3 figure
Top Polarization at a mu+mu- Collider
The top quark pairs produced at a polarized muon collider are in a (nearly)
pure spin configuration. This result holds for all center-of-mass energies, and
is insensitive to the next-to-leading order QCD radiative corrections. The
decay products of a polarized top quark show strong angular correlations. We
describe an interesting interference effect between the left-handed and
longitudinally polarized W bosons in top quark decay. This effect is easily
observable in the angular distribution of the charged lepton with respect to
the beam axis.Comment: 6 pages, including 6 figures. Talk presented at MUMU9
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