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This Week at Physics

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Events on Monday, March 5th, 2012

NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Antiparticles in the shadow of the Earth: cosmic-ray positrons with the Fermi Gamma-ray Space Telescope
Time: 11:00 am
Place: 5280 Chamberlin Hall
Speaker: Justin Vandenbroucke, SLAC
Abstract: Cosmic rays are primarily protons and other nuclei, but there is a small flux of electrons and an even smaller flux of positrons. Cosmic-ray positrons can be produced by astrophysical accelerators, by collisions of cosmic-ray protons with interstellar gas, or by dark matter. We used the Fermi Gamma-ray Space Telescope to measure the charge-separated electron and positron energy spectra. Because Fermi does not have an onboard magnet, we used the Earth's magnetic field to distinguish positrons and electrons. We confirmed the PAMELA discovery that the positron fraction is rising with energy between 10 and 100 GeV and measured the positron flux for the first time in the 100-200 GeV range. Explaining the positron excess remains an outstanding question.
Host: Halzen
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Plasma Physics (Physics/ECE/NE 922) Seminar
Using liquid metal experiments to test models of MHD turbulence
Time: 12:00 pm
Place: 2241 Chamberlin
Speaker: Mark Nornberg, UW-Madison
Abstract: Experiments using liquid metals to demonstrate MHD instabilities like the generation of magnetic field through a dynamo or magnetically induced turbulence through the Magnetorotational Instability provide a wealth of information on MHD turbulence. Recent measurements of the correlated velocity and magnetic fluctuations on the Madison Dynamo Experiment reveal that simple mean-field models, assuming homogeneous and isotropic turbulence, describe the measured transport of magnetic field rather well. Turbulence in a rapidly rotating fluid, like that of an experiment designed to study the magnetorotational instability, has a strong anisotropy however which tends to confine the motions of the fluid to two-dimensional, wave-like turbulence. I will present some ideas for experiments using a rapidly rotating liquid metal to study the effects of magnetic fields on these waves and to test mean-field models for describing turbulent transport in these flows.
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Condensed Matter Theory Group Seminar
Collective instabilities of fermions on a hexagonal lattice
Time: 4:30 pm
Place: 5310 Chamberlin
Speaker: Andrey Chubukov, UW-Madison
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