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Events on Friday, April 26th, 2019

Theory Seminar (High Energy/Cosmology)
Systematic expansion of tunneling rates
Time: 2:00 pm - 3:15 pm
Place: 5280 Chamberlin Hall
Speaker: Björn Garbrecht, Technical University Munich
Abstract: I will review the theory of tunneling in quantum mechanics and field theory to one-loop order. Then, I will address how to systematically go beyond this approximation in an expansion based on Green's functions. One phenomenological relevance of this is that a consistent calculation of the tunneling rate in the Standard Model and beyond requires a one-loop resummation. Finally, the standard Euclideanized calculation of tunneling rates is related to a derivation based on real-time amplitudes.
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Physics Department Colloquium
New Science Opportunities at X-ray Free Electron Lasers
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Uwe Bergmann, Stanford PULSE Institute, SLAC National Accelerator Laboratory
Abstract: Over the past century X-rays have revolutionized numerous fields of science. Starting in the 1970s powerful new synchrotrons sources have dramatically advanced the scientific use of X-rays. In the last decade new X-ray free electron lasers, such as the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory, have come to light. These coherent X-ray sources produce ultra-short pulses with a brightness that is ten billion times larger than synchrotron sources. For the first time scientist can study matter not just at the length scale of atoms and molecules, but also at the time scale of molecular motion. We will describe these machines and present some examples of recent X-ray laser research.

One example is our recent work on stimulated x-ray emission spectroscopy. Here, we have created inner shell x-ray fluorescence laser signals from manganese compounds. To achieve this exponential signal gain, we used highly focused ultrashort x-ray pump pulses from LCLS. The observed signal consisted of very short x-ray pulses up to 10^6 times stronger than normal x-ray fluorescence. Remarkably, these x-ray pulses preserved the small differences in fluorescence energy, caused by the different chemical surrounding of the manganese ions. We describe the phenomena and how we plan to apply this and similar approaches to learn more about transition metal systems and their function in catalysis, chemical, and materials science.
Host: Mark Eriksson
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