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This Week at Physics

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Events During the Week of September 8th through September 15th, 2019

Monday, September 9th, 2019

Plasma Physics (Physics/ECE/NE 922) Seminar
Effect of Magnetic Perturbations on Turbulence-Flow Dynamics at the L-H Transition on DIII-D
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Matt Kriete, UW Madison
Host: John Sarff
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Towards the Atomic Tritium Future of Neutrino Mass Measurement
Time: 4:00 pm
Place: 5310 Chamberlin (if available)
Speaker: Walter Pettus, University of Washington
Abstract: Neutrino flavor oscillations provided the first break in the Standard Model by proving that neutrinos have nonzero mass, but cannot constrain the absolute mass scale. The most sensitive method to directly measure the mass scale is observation of the tritium beta-decay spectrum endpoint and extraction of the electron antineutrino mass. Project 8 is a next-generation experiment based on the novel Cyclotron Radiation Emission Spectroscopy (CRES) technique to perform a radio-frequency-based measurement of the tritium beta spectrum. The goal of the phased program is to reach a mass sensitivity below 40 meV, completely covering the allowed region of the inverted mass hierarchy. I will present studies performed on the mono-energetic conversion electrons of 83mKr and the ongoing tritium data-taking campaign, which is the first use of the CRES technique for a continuous spectrum measurement. In parallel, an R&D program is being executed to to demonstrate critical technologies for scaling CRES to m^3-scale volumes and for delivering a high-intensity atomic tritium source, establishing a pathway for future experiments based on this technology.
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Tuesday, September 10th, 2019

Chaos & Complex Systems Seminar
Do plants feel pain?
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Simon Gilroy, UW Department of Biology
Abstract: When an animal is wounded, a combination of nerve action supported by neurotransmitters rapidly transmits this information throughout the organism. Similarly, in response to herbivory of one leaf, plants trigger pre-emptive defenses in unwounded tissues but in this case there is no nervous system to propagate the information about damage. Using Arabidopsis plants expressing genetically-encoded bio-probes we have visualized the plant-wide dynamics of changes in cellular Ca2+ that trigger system-wide responses to wounding. These rapid (within seconds), long-range (throughout the plant body) changes are disrupted in mutants in the genes for the plant glutamate like receptor channels, homologs of the glutamate receptor channels of the mammalian nervous system. The patterns of wound signaling can be mimicked by application of the amino acid glutamate (a classic mammalian neurotransmitter). These results suggest that a plant-wide Ca2+ signaling network acts to communicate information about damage throughout the plant body and although plants lack a nervous system, glutamate and glutamate receptor-like channels lie at the core of this long-range plant signaling network.
Host: Clint Sprott
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Council Meeting
Physics Council Meeting
Time: 3:00 pm
Place: 2314 Chamberlin Hall
Speaker: Sridhara Dasu, UW-Madison
Host: Sridhara Dasu, Department Chair
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Wednesday, September 11th, 2019

Department Meeting
Department Meeting
Time: 12:15 pm
Place: B343 Sterling Hall
Speaker: Sridhara Dasu, UW-Madison
Host: Department Chair
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Thursday, September 12th, 2019

R. G. Herb Condensed Matter Seminar
Transport properties of a compensated metal: the Lorentz ratio and the absence of mass renormalization near a Pomeranchuk quantum critical point
Time: 11:00 am
Place: 5310 Chamberlin Hall
Speaker: Songci Li , UW-Madison
Abstract: A violation of the Wiedemann-Franz law in a metal can be quantified by comparing the Lorentz ratio, $L=kapparho/T$. We obtain the Lorentz ratio of a clean compensated metal with intercarrier interaction as the dominant scattering mechanism by solving exactly the system of coupled integral Boltzmann equations. The Lorentz ratio is shown to assume a particular simple form in the forward-scattering limit: $L/L_0=overline{Theta^2}/2$, where $Theta$ is the scattering angle. In this limit, $L/L_0$ can be arbitrarily small. We discuss how a strong downward violation of the Wiedemann-Franz law in a type-II Weyl semimetal WP$_2$ can be explained within our model. In the second part of the talk, we discuss the role of mass renormalization in electron transport of a compensated metal near a QCP. According to a naive interpretation of the Drude formula, as electrons get heavier near a QCP, their electrical and thermal conductivities decrease. However, this picture has never been supported by an actual calculation. We employ a model case of a compensated metal near a Pomeranchuk-type QCP. The advantage of this model is that it allows one to treat electrical and thermal conductivities on the same footing, without invoking umklapp scattering or any other channels of momentum relaxation which are extraneous to the electron system. By solving the kinetic equations, we obtain explicit results for the electrical and thermal conductivities of a two-band compensated metal. We show that mass renormalization factors cancel out with the $Z$ factors, which renormalize the scattering probability, so that all the transport quantities contain the bare rather than renormalized electron masses. We also demonstrate how the same conclusion can be drawn by diagrammatically calculating the optical conductivity.
Host: Alex Levchenko
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Cosmology Journal Club
Time: 12:00 pm
Place: 5242 Chamberlin Hall
Abstract: Please visit the following link for more details:
http://cmb.physics.wisc.edu/journal/index.html
Feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Ross Cawthon (cawthon@wisc.edu) and Santanu Das (sdas33@wisc.edu).
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Astronomy Colloquium
The Yin and Yan of Slowly-Pulasting B Stars: Asteroseismology and Angular Momentum Redistribution
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM, Talk begins 3:45 PM
Speaker: Professor Richard Townsend, UW Madison Astronomy Department
Abstract: During their main-sequence evolution, almost all B-type stars will pass through a phase where they are unstable toward oscillation in one or more global internal gravity waves ('g modes'). The g modes, driven by iron and nickel opacity in the stars' outer envelopes, generate surface temperature and velocity changes with periodicities on the order of days.

In the 'Yin' part of my talk, I'll discuss how time-series spectroscopy and photometry of these `slowly-pulsating B' (SPB) stars can be leveraged into asteroseismology --- probing the stars'interiors by careful analysis of their oscillation spectra. I'll highlight in particular how the Kepler mission, together with the MESA stellar evolution code and my GYRE stellar oscillation code, has allowed novel constraints to be established on the internal rotation and mixing physics of SPB stars.

I'll then pivot to the 'Yang' part of my talk. Although we typically regard stellar oscillations as passive tracers of stellar structure, they can also modify this structure. I'll present recent work with Jacqueline Goldstein and Ellen Zweibel, exploring angular momentum redistribution by g modes. Modeling this process in SPB stars, we find that significant modification of internal rotation profiles can occur on timescales as short as centuries. This suggests that the g modes can impact the stars' life trajectories, a possibility that's been hitherto ignored in stellar evolution calculations.
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Friday, September 13th, 2019

Physics Department Colloquium
Neutrino: Chronicles of an Aloof Witness
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Goran Senjanovic, ICTP
Abstract: As you read this, trillions of neutrinos from the sun are passing through every square cm of your body, doing no harm whatsoever. They convey information from the depth of the universe and have been present from its very birth. Neutrinos have captured the imagination of physicists from the time they were first conceived and have repeatedly provided a window into new physics. A question stood out for decades: Are neutrinos massive like their seemingly inseparable electron siblings? It took almost seventy years to obtain the positive answer. I review here past and present efforts to probe the origin of neutrino mass and show that this is deeply related to the fundamental question of left-right symmetry in nature. This ongoing story will take us from the beginnings of elementary particle physics all the way to the frontiers of the Large Hadron Collider.
Host: Vernon Barger
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