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Events on Monday, February 20th, 2012

NPAC (Nuclear/Particle/Astro/Cosmo) Forum
High-Energy Astrophysics with the High Altitude Water Cherenkov Experiment
Time: 11:00 am
Place: 5280 Chamberlin Hall
Speaker: John Pretz, Los Alamos National Lab
Abstract: The High Altitude Water Cherenkov (HAWC) experiment, under construction at Sierra Negra, Mexico, consists of a 22500 square meter area of water tanks instrumented with light-sensitive photo-multiplier tubes. The experiment detects energetic secondary particles reaching the ground when a high-energy cosmic ray or gamma ray interacts in the atmosphere above the experiment. High-energy gamma ray astronomy provides a probe of some of the most gravitationally and electromagnetically extreme regions in the universe, from neutron stars and supernova remnants to active galactic nuclei and gamma-ray bursts. High-energy gamma ray are also key to understanding the origin of galactic cosmic radiation. I will describe the design of the HAWC instrument, scheduled to be completed in 2014, and discuss the motivation and scientific return the experiment will bring.
Host: Halzen
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Plasma Physics (Physics/ECE/NE 922) Seminar
Impulsive Fast Reconnection via Flux Rope Dynamics
Time: 12:00 pm
Place: 2241 Chamberlin
Speaker: Hantao Ji, Princeton Plasma Physics Laboratory
Abstract: Magnetic reconnection, the efficient release of magnetic energy by topological rearrangement of field lines, is one of the most important and fundamental plasma processes in space, solar and more distant astrophysical plasmas. The modern collisionless models predict that ions exhaust through a thick, ion-scale layer while mobile electrons leave through a thin, electron-scale layer, allowing for efficient release of magnetic energy. While ion layers have been frequently detected in space and studied in detail in the laboratory, the dissipation on the electron scales near the X-line remains largely unknown. The discrepancies [1-3] between the measured thickness of the electron diffusion layer in MRX and best available 2D kinetic simulations suggest that the electron scale dissipation must be 3D in nature. In this talk, the most recent experimental results from MRX and their comparisons with 3D kinetic simulations will be discussed. It is found that impulsive fast reconnection is caused by a disruption of the current channel localized in 3D space, associated with a burst of electromagnetic fluctuations. There exists substantial evidence that these impulsive behaviors are caused by 3D flux rope dynamics [4]. Looking into the future, a new theme of multiple X-line reconnection in a phase diagram [5] possibly provides solutions for fast reconnection in large space and astrophysical systems and for efficient particle acceleration often observed there. Scientific opportunities for a next generation laboratory experiment based on MRX to study magnetic reconnection in such regimes directly relevant to space and astrophysical plasmas will be described.<br>
[1] H. Ji et al., GRL 35, L13106 (2008).<br>
[2] S. Dorfman et al., PoP 15, 102107 (2008).<br>
[3] V. Roytershteyn et al. PoP 17, 055706 (2010).<br>
[4] S. Dorfman, Ph. D. Thesis (2011); to be submitted to PRL (2012).<br>
[5] H. Ji and W. Daughton, PoP 18, 111207 (2011).<br>
Host: Cary Forest
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Condensed Matter Theory Group Seminar
Topology of Quantum Discord
Time: 4:30 pm
Place: 5310 Chamberlin
Speaker: Nga Nguyen, UW-Madison
Abstract: Quantum discord is arguably a more sensitive measure of quantum correlations than quantum entanglement, and may be able to serve as a resource for quantum computation. All quantum correlations are subject to destruction by external noise. The route by which this destruction takes place depends on the shape of the hypersurface of zero discord in the space of generalized Bloch vectors. In the case of 2 qubits, we show that, except at the origin, this hypersurface is a 9-dimensional manifold with boundary embedded in a 15-dimensional background space. This is done by computing the tangent vectors explicitly and verifying that there are no self-intersections. We discuss the implications for the time evolution of discord in physical models, which contrasts sharply with the evolution of entanglement.
Host: Perkins
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