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Events at Physics

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Events on Tuesday, December 8th, 2015

Chaos & Complex Systems Seminar
Imaging around obstacles and into lunar caves using scattered light
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Andreas Velten, UW Department of Molecular Biology
Abstract: The Computational Optics Group at the UW Laboratory for Optical and Computational Instrumentation (LOCI) develops novel imaging systems by combining new hardware designs with novel computational image reconstruction methods. The traditional method of imaging is based on hardware that resembles the human eye to produce images suitable for analysis and pattern recognition by a human viewer. A traditional camera like this only accesses a very small fraction of the information provided by the light field. Most of the light detected by optical systems occurs in the form of multiply scattered photons that can not be used to create an image in this traditional way. In this talk I will show different systems that, using custom methods to selectively capture and control photon time of flight, allow us to capture information inaccessible to a regular camera.<br>
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Our Modular Indirect Remote Imaging System (MIRIS) uses ultrafast illumination and detection to collect and utilize light transport information. A laser is used to direct a pulse train towards one of the visible surfaces in a scene. The light bounces off of this surface and reflects off objects in the scene before heading back towards the visible scene where it is imaged with high time resolution by a camera. Images of the scene are reconstructed from the collected time-encoded information using a modified backprojection algorithm.<br>
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In collaboration with the NASA Jet Propulsion Lab, we are performing studies to apply this method to explore the inside of caves on the moon from a low lunar orbit.
Host: Sprott
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Theory Seminar (High Energy/Cosmology)
Gauge and fermion preheating and the end of axion inflation
Time: 3:30 pm
Place: 5280 Chamberlin Hall
Speaker: Evangelos Sfakianakis, University of Illinois, Urbana-Champaign
Abstract: Axions are attractive candidates for theories of large-field inflation that are capable of generating observable primordial gravitational wave backgrounds. These fields enjoy shift-symmetries that protect their role as inflatons from being spoiled by coupling to unknown UV physics. This symmetry also restricts the couplings of these axion fields to other matter fields. At lowest order, the only allowed interactions are derivative couplings to gauge fields and fermions. These derivative couplings lead to the biased production of fermion and gauge-boson helicity states during and after inflation. I will describe some recent work on preheating in axion-inflation models that are derivatively coupled to Abelian gauge-fields and fermion axial-currents.

For an axion coupled to U(1) gauge fields it is found –analytically and numerically- that preheating is efficient for a wide range of parameters. In certain cases the inflaton is seen to transfer all its energy to the gauge fields within a few oscillations. In most cases, three-dimensional lattice simulations showed that the gauge fields on sub-horizon scales end preheating in an unpolarized state due to the existence of strong rescattering between the inflaton and gauge-field modes.

Coupling an axion to Majorana fermions leads to the biased production of fermion helicity-states which can have interesting phenomenological implications for leptogenesis.
Host: Amol Upadhye
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