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Events During the Week of April 9th through April 16th, 2023

Monday, April 10th, 2023

Academic Calendar
Students begin enrolling for Fall term courses according to their appointment times
Abstract: *Note: actual end time may vary.* CONTACT: 262-3811, URL:
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Plasma Physics (Physics/ECE/NE 922) Seminar
Magnetohydrodynamics of Dirac Plasma in Graphene
Time: 12:00 pm
Place: 2241 Chamberlin Hall
Speaker: Alex Levchenko, UW - Madison
Abstract: Interactions between electrons in solid-state systems can lead to collective behavior described by hydrodynamics. One such system is the electron-hole plasma in graphene near the charge-neutrality point, which can form a strongly coupled Dirac fluid. In this talk, I will present results for hydrodynamic electron magnetotransport in graphene devices. In these systems, a distinct mechanism of magnetoresistance appears which is absent in systems with Galilean-invariant electron liquid. The resulting magnetoresistance depends on the intrinsic conductivity and viscosity of the electron liquid, and becomes especially pronounced near charge neutrality. This mechanism is related to Alfven's theorem on frozen-in magnetic field in magneto-hydrodynamics, according to which the motion of a perfectly conducting liquid perpendicular to the magnetic field lines is forbidden. In addition, we will consider thermoelectric phenomena and discuss hydrodynamic paradox in Corbino devices.

This talk is based on:
PHYSICAL REVIEW B 102, 075305 (2020)
PHYSICAL REVIEW B 105, 155307 (2022)
PHYSICAL REVIEW B 106, L201306 (2022
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Tuesday, April 11th, 2023

Network in Neutrinos, Nuclear Astrophysics, and Symmetries (N3AS) Seminar
Light sterile neutrinos in compact binary merger remnants
Time: 2:00 pm
Place: Join Zoom Meeting Meeting ID: 912 3071 4547
Speaker: Irene Tamborra , Niels Bohr Institute, Denmark
Abstract: The coalescence of a neutron star with another neutron star or a black hole leads to the formation of a compact binary merger. The upcoming large number of binary merger remnant observations will offer unprecedented opportunities to characterize the population of binary mergers, the physics of these sources, and also the eventual existence of physics beyond the Standard Model. The existence of extra sterile neutrino species is not ruled out because of persistent experimental anomalies and it could have indirect implications on the merger disk cooling rate, the merger outflows, and related electromagnetic observables. In this talk, we will explore the flavor conversion phenomenology in the presence of sterile neutrinos in compact binary merger remnants.
Host: A. Baha Balantekin
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Graduate Program Event
A(ction) functional gradient descent algorithm for estimating instantons in chemical reaction networks
Time: 4:00 pm
Place: 4274 Chamberlin
Speaker: Praful Gagrani, Physics Graduate Student
Abstract: Chemical reaction networks (CRNs) taken under mass-action kinetics play a central role in the mathematical modeling of chemistry and biology. A key reason for their widespread utility is their capacity to exhibit multiple attractors and capture a wide range of nonlinear phenomena. Computing paths of transition between attractors, or instantons, is a challenging task, not solvable analytically for all but the simplest cases. In our work, we propose an algorithm for numerically estimating instantons for a CRN. The algorithm uses the Hamiltonian description of a stochastic CRN and solves a MinMax problem on the Action functional to converge on the instanton. In this talk, I will present a schematic derivation of the Hamiltonian and Action functional for stochastic CRNs, explain our Action Functional Gradient Descent (AFGD) algorithm, and show computational and practical applications. Finally, I will briefly discuss the unified formalism to which both stochastic and quantum Hamiltonians belong and propose directions for future research. (For details, see
Host: Sridara Dasu
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Plasma Physics (Physics/ECE/NE 922) Seminar
Wisconsin's role in harnessing fusion energy
Time: 7:00 pm - 8:00 pm
Place: Discovery Building, DeLuca Forum and streaming; see for full details
Speaker: Cary Forest , Physics, Realta Fusion
Abstract: Fusion holds promise to provide abundant clean energy, but realizing this promise will require partners from academia, government, community organizations and the private sector. Cary Forest, professor of physics and co-founder of Realta Fusion, will explore recent breakthroughs in fusion energy and highlight Wisconsin’s role. He will also share the story of Realta Fusion, a UW-Madison startup with WARF support that seeks to use fusion to provide industrial process heat.

This event will held at the Discovery Building and on Zoom.

The Capital Times is the media partner for Crossroads.

See for full details and to register
Host: WARF
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Wednesday, April 12th, 2023

Time: 12:15 pm - 1:15 pm
Abstract: GREAT IDEAS Cancelled for this week - see you April 26th!
Host: GMaWiP and Climate and Diversity Committee (contact Jessie Thwaites or R. Sassella with questions)
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Department Meeting
Closed Department Meeting
Time: 12:15 pm - 1:30 pm
Place: VIRTUAL - link will be sent later.
Speaker: Mark Eriksson, UW-Madison
Closed meeting to discuss personnel matters—pursuant to Section 19.85(1)(c) of the Wisconsin Open Meetings Law Closed to all but tenured faculty
Host: Mark Eriksson
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Theory Seminar (High Energy/Cosmology)
Late-time accelerating cosmologies
Time: 1:00 pm - 2:30 pm
Place: Chamberlin 5280
Speaker: Flavio Tonioni, UW Madison
Abstract: In this talk, I will characterize the late-time expansion rate of the universe in scalar-field cosmologies with multi-exponential potentials. To do this, I will take advantage of previously unobserved universal asymptotic features of the solutions to the cosmological equations. This is a critical achievement because it provides a simple diagnostic of whether any given multi-exponential potential holds the necessary conditions for late-time cosmic acceleration. Such potentials have been studied extensively as phenomenological models of quintessence and, moreover, they are ubiquitous in string-theoretic constructions, which further allows one to sharpen several statements on the low-energy signatures of quantum gravity. I will extensively comment on the tension for acceleration posed by the string-theoretic dilaton, which, if present as a rolling scalar in a weakly-coupled theory, makes the potential too steep for acceleration unless negative-definite potentials are present. I will also show that, if the late-time scale factor is of power-law form, the acceleration parameter can be expressed in terms of a directional derivative of the potential.
Host: George Wojcik
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The James Webb Space Telescope - Launching a New Astronomical Era
Time: 6:00 pm - 7:00 pm
Place: Chamberlin 2241
Speaker: Dr. Steph LaMassa, Space Telescope Science Institute
Abstract: The James Webb Space Telescope revolutionized astronomy with its launch in 2021 and first science images in 2022. Already, it has revealed some of the farthest galaxies ever discovered, analyzed the atmospheres of planets orbiting distant suns, and studied some of the largest stars in the universe. Dr. Stephanie LaMassa, branch manager for the Near-Infrared Imager and Slitless Spectrograph (NIRISS) aboard JWST, will teach about the design, launch, and software maintenance of JWST and NIRISS. Being in charge of the instrument, Dr. LaMassa will provide unique insight into how instruments intended for spaceflight are designed, and how students can get involved with instrumentation work on the ground. In addition, she will also talk about some of the exciting uses for the space telescope, including studies of supermassive black holes and the earliest galaxies in the universe.
Host: Astronomy Club at UW - Madison
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Thursday, April 13th, 2023

R. G. Herb Condensed Matter Seminar
Valley and Momentum Polarization in Multilayer Graphene
Time: 10:00 am - 11:00 am
Place: 5310 Chamberlin
Speaker: Jia Leo Li, Brown University
Abstract: Within the flat energy bands of multilayer graphene, the enhanced influence of Coulomb interaction is reflected by the emergence of exchange-driven instabilities, which stabilize isospin-ordered phases by lifting the spin and valley degeneracy. These isospin-ordered phases are ubiquitous in the low-temperature phase space of multilayer graphene, having been observed in Bernal bilayer, rhombohedral trilayer, and a number of twisted graphene heterostructures near the magic-angle. Here, I will discuss a new type of Coulomb-driven instability that is driven by the exchange interaction between trigonal-warping-induced Fermi pockets. It is recently proposed that such instability could induce a spontaneous condensation of charge carriers into one of the Fermi pockets, giving rise to a net polarization in the momentum space. Such exchange-driven instability can be directly probed and characterized using angle-resolved nonlinear transport measurement at the second-harmonic frequency. Beyond the experimental identification of momentum polarization, I will discuss its implication on our understanding of electronic orders in the flat energy band of multilayer graphene.
Host: Alex Levchenko
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Chancellor Mnookin's Investiture Events
Discovery Past, Present, and Future: Black Holes, Neutrinos, and Life in our Galaxy
Time: 1:30 pm - 3:00 pm
Place: Marquee Theater, Union South and virtual; see
Speaker: Francis Halzen, Andrea Ghez, Susanna Widicus Weaver, and Eric Wilcots, Various
Abstract: Featuring Nobel Prize winning astrophysicist UCLA Professor Andrea Ghez as well as UW-Madison luminaries, our panel will discuss the arc of discovery that unveils the secrets of the Milky Way beginning with UW–Madison Karl Jansky’s detection of radio emission from deep space. Professor Ghez will discuss the extraordinary discovery of the black hole at the center of the Milky Way.

Professor Ghez will be joined by UW–Madison Hilldale Professor Francis L. Halzen who will talk about the IceCube Neutrino Observatory and what it tells us about some of the most energetic events in the Universe.

We will wrap up by looking forward to what could be the next great discovery as Susanna Widicus Weaver, Vozza Professor of Chemistry and Astronomy, discusses her work to understand the origin of life in the Galaxy.

The discussion will be moderated by Eric Wilcots, Dean of the College of Letters & Science and Mary C. Jacoby Professor of Astronomy.

This event will be live streamed for those who prefer to attend virtually.

The event is free to attend, but registration is required for both virtual and in-person guests due to limited capacity.
Host: UW–Madison
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
LDMX: The Light Dark Matter eXperiment
Time: 3:30 pm - 4:30 pm
Place: CH 4274
Speaker: Matthew Solt, The University of Virginia
Abstract: The constituents of dark matter are still unknown, and the viable possibilities span a very large mass range. Specific scenarios for the origin of dark matter sharpen the focus on a narrower range of masses: the natural scenario where dark matter originates from thermal contact with familiar matter in the early Universe requires the DM mass to lie within about an MeV to 100 TeV. Considerable experimental attention has been given to exploring Weakly Interacting Massive Particles in the upper end of this range (few GeV – ~TeV), while the region ~MeV to ~GeV is largely unexplored. Most of the stable constituents of known matter have masses in this lower range, tantalizing hints for physics beyond the Standard Model have been found here, and a thermal origin for dark matter works in a simple and predictive manner in this mass range as well. It is therefore a priority to explore. If there is an interaction between light DM and ordinary matter, as there must be in the case of a thermal origin, then there necessarily is a production mechanism in accelerator-based experiments. The most sensitive way, (if the interaction is not electron-phobic) to search for this production is to use a primary electron beam to produce DM in fixed-target collisions. The Light Dark Matter eXperiment (LDMX) is a planned electron-beam fixed-target missing-momentum experiment that has unique sensitivity to light DM in the sub-GeV range. This seminar will give an overview of the theoretical motivation, the main experimental challenges and how they are addressed, as well as projected sensitivities in comparison to other experiments.
Host: Matthew F Herndon
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Astronomy Colloquium
Unveiling Luminous Obscured Black Hole Growth with the Wide-Area Multi-Wavelength Stripe 82X Survey
Time: 3:30 pm - 4:30 pm
Place: 4421 Sterling Hall
Speaker: Dr. Stephanie LaMassa, Space Telescope Science Institute
Abstract: Active galactic nuclei (AGN) serve as signposts of growing supermassive black holes, allowing us to study how these objects evolve from the early Universe to the present day. Surveying patches of the sky uncover representative samples of AGN, with different survey strategies favoring different populations. Rare objects that have a low space density, like high luminosity AGN, can only be adequately sampled via wide-area surveys that probe a large volume of the Universe. "Stripe 82X”, an X-ray survey covering ~30 deg^2 of the rich multi-wavelength Sloan Digital Sky Survey Stripe 82 field, is designed to uncover obscured, luminous black hole growth. In this talk, I will summarize the AGN demographics discovered in Stripe 82X and how they compare with AGN identified from wide-area optical and mid-infrared surveys. I will discuss what we have learned about the host galaxies of these luminous AGN and how AGN properties evolve with redshift and luminosity. Finally, I will mention exciting future prospects for learning more about the cosmic evolution of rare (yet important!) high-redshift, high luminosity obscured AGN.
Host: Ke Zhang
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Friday, April 14th, 2023

Physics Department Colloquium
Knowns and Unknowns in Neutrinos
Time: 3:30 pm - 4:30 pm
Place: 2241 Chamberlin Hall
Speaker: Peter Denton , Brookhaven National Laboratory
Abstract: In particle physics there exist two regions: the Standard Model which is fairly complete and the new physics sector which is completely unknown. In between and overlapping with both of these is neutrino physics. Neutrinos exist within the Standard Model but are not explained by it due to the discovery of neutrino oscillations. In this colloquium I will discuss where we stand with neutrino oscillations, where we might go with them, and how we might learn about the nature of neutrinos.
Host: A. Baha Balantekin
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Saturday, April 15th, 2023

The Physics Fair
Time: 11:00 am - 2:00 pm
Place: Chamberlin Hall
Speaker: Various, Dept of Physics
Abstract: Join us for the annual Physics Fair from 11am-2pm throughout the second floor hallway of Chamberlin. The Ingersoll Physics Museum will be staffed with docents during that time as well.
Host: Sarah Perdue
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The Wonders of Physics show
Time: 11:00 am - 12:15 pm
Place: 2103 Chamberlin
Abstract: Come see an encore performance of the 40th annual The Wonders of Physics show. No tickets needed for this show.
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The Wonders of Physics show
Time: 2:00 pm - 3:10 pm
Place: 2103 Chamberlin
Abstract: Come see an encore performance of the 40th annual The Wonders of Physics show. No tickets needed for this show.
Host: Haddie McLean
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