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This Week at Physics

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Events During the Week of April 13th through April 20th, 2014

Monday, April 14th, 2014

Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
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
Please 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 Le Zhang (lzhang263@wisc.edu)
Host: Peter Timbie
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Plasma Physics (Physics/ECE/NE 922) Seminar
Study of magnetic reconnection in the fluid regime: The variety of environments and outcomes
Time: 12:00 pm
Place: 2241 Chamberlin
Speaker: Dr. Vyacheslav Lukin, Naval Research > Laboratory, Washington, DC
Host: Jan Egedal
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Condensed Matter Theory Group Seminar
Evolution of the magnetic ordering in iron-chalcogenide Fe(1+y)Te as a function of the level of interstitial Fe excess, y Time
Time: 4:30 pm
Place: Chamberlin 5310
Speaker: Samuel Ducatman, University of Wisconsin
Abstract: We studied the electronic and magnetic properties of iron-chalcogenide Fe$_{1+y}$Te compounds based on the multiband model, in which localized spins and itinerant electrons coexist and are coupled by Hund's rule coupling. Integrating out the conduction electrons, we computed additional couplings between localized spins similarly to the conventional Ruderman-Kittel-Kasuya-Yosida (RKKY)theory. We found that resulting RKKY-like interactions are substantial up to the third neighbors, but are negligible beyond this. We computed the magnetic phase diagram of the modified $J_1-J_2-J_3-K$ model and showed it captures the evolution of the magnetic order in Fe$_{1+y}$Te as a function of Fe excess $y$.
Host: Perkins
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Tuesday, April 15th, 2014

Chaos & Complex Systems Seminar
Criticality and information flow in an adaptive system
Time: 12:05 pm
Place: 4274 Chamberlin (refreshments will be served)
Speaker: Bryan Daniels, Wisconsin Institute for Discovery
Abstract: In physical systems, boundaries in parameter space that separate different large-scale behavior correspond to phase transitions, where small changes in microscopic parameters lead to drastic changes in macroscopic observables. We use fine-grained data about conflict in a macaque society to ask whether this social system is located near a phase transition. We find using two models (an equilibrium Ising model and a dynamic branching process model) that the system is near but below a transition, indicating that aggression dissipates quickly enough to avoid becoming typically widespread, but not so quickly that large fights are impossible. A relation between thermodynamics and information theory shows that being near the transition implies that it is easier for an observer of fight sizes to infer changes in individual proclivities to fight. More generally, this points to the possibility of quantifying a system's collective behavior by measuring the degree to which information can percolate among different spatial scales.<br>
Host: Clint Sprott
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"Physics Today" Undergrad Colloquium (Physics 301)
Quantum noise in magnetometers
Time: 1:20 pm
Place: 2223 Chamberlin
Speaker: Thad Walker, UW Madison Department of Physics
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Theory Seminar (High Energy/Cosmology)
Boundaries and Defects of N=4 Supersymmetric Yang Mills with 4 Supercharges
Time: 2:00 pm
Place: 5280 Chamberlin
Speaker: Peter Ouyang, Purdue
Host: Aki Hashimoto
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Wednesday, April 16th, 2014

No events scheduled

Thursday, April 17th, 2014

NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Detection of degree angular scale B-mode polarization with BICEP2/Keck Array
Time: 2:30 pm
Place: 5280 Chamberlin Hall
Speaker: Grant Teply, Caltech
Abstract: The BICEP2 experiment is a cosmic microwave background (CMB) polarimeter specifically designed to search for the signal of inflationary gravitational waves in the B-mode power spectrum around l=80. BICEP2 observed from the South Pole for three seasons from 2010 to 2012. Using its full 3-year CMB data set, we report an excess of B-mode power at the angular scales of interest, inconsistent with the null hypothesis at a significance of >5σ. The observed B-mode power spectrum is well-fit by a lensed-LCDM + tensor theoretical model with tensor/scalar ratio r=0.20+0.07−0.05. We will describe the BICEP2 instrument, observing strategy, data analysis, and results. We will also describe Keck Array, a successor to BICEP2, which is actively collecting sky data at both 100 and 150 GHz at high sensitivity for improved constraints on galactic foregrounds.
Host: Vandenbroucke
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Astronomy Colloquium
The Fermi Bubbles: Possible Nearby Laboratory for AGN Jet Activity
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Karen Yang, University of Michigan
Abstract: One of the most important discoveries of the Fermi Gamma-ray Space Telescope is the detection of two giant bubbles extending 50 degrees above and below the Galactic center (GC). The symmetry about the GC of the Fermi bubbles suggests some episode of energy injection from the GC, possibly related to past jet activity of the central active galactic nuclei (AGN). Thanks to the proximity to the GC, the Fermi Bubbles are excellent laboratories for studying cosmic rays (CRs), Galactic magnetic field, and AGN feedback in general. Using three-dimensional magnetohydrodynamic simulations that include relevant CR physics, I will show how leptonic AGN jets can explain the key characteristics of the Fermi bubbles and the spatially correlated features observed in the X-ray, microwave, and radio wavelengths. I will also discuss how we use our simulations in combination with the multi-wavelength data to obtain constraints on the composition of the Fermi bubbles.
Host: Robert Lindner
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Friday, April 18th, 2014

Theory Seminar (High Energy/Cosmology)
New Limits on Light Hidden Sectors from Fixed-Target Experiments
Time: 2:00 pm
Place: 5280 Chamberlin Hall
Speaker: Andrew Spray, University of Melbourne
Abstract: New physics can be light if it is hidden, coupling very weakly to the Standard Model. In this work we investigate the discovery prospects of Abelian hidden sectors in lower-energy fixed-target and high-precision experiments. We focus on a minimal supersymmetric realization consisting of an Abelian vector multiplet, coupled to hypercharge by kinetic mixing, and a pair of chiral Higgs multiplets. This simple theory can give rise to a broad range of experimental signals, including both commonly-studied patterns of hidden vector decay as well as new and distinctive hidden sector cascades. We find limits from the production of hidden states other than the vector itself. In particular, we show that if the hidden Abelian symmetry is higgsed, and the corresponding hidden Higgs boson has visible decays, it severely restricts the ability of the hidden sector to explain the anomalous muon magnetic moment.
Host: Jordi Salvadó Serra
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Physics Department Colloquium
New Physics from the Sky: Cosmic Rays, Gamma Rays and the Hunt for Dark Matter
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Stefano Profumo , UC Santa Cruz
Abstract: Can we learn about New Physics with astronomical and astro-particle data? Understanding how this is possible is key to unraveling one of the most pressing mysteries at the interface of cosmology and particle physics: the fundamental nature of dark matter. I will discuss some of the recent puzzling findings in cosmic-ray electron-positron data and in gamma-ray observations that might be related to dark matter. I will argue that cosmic-ray data, most notably from the AMS, Pamela and Fermi satellites, indicate that previously unaccounted-for powerful sources in the Galaxy inject high-energy electrons and positrons. Interestingly, this new source class might be related to new fundamental particle physics, and specifically to pair-annihilation or decay of galactic dark matter. This exciting scenario is directly constrained by Fermi gamma-ray observations, which also inform us on astrophysical source counterparts that could be responsible for the high-energy electron-positron excess. Observations of the gamma-ray emission from the central regions of the Galaxy as well as claims about a gamma-ray line at around 130 GeV also recently triggered a wide-spread interest: I will address the question of whether we are really observing signals from dark matter annihilation, how to test this hypothesis, and which astrophysical mechanisms constitute the relevant background.
Host: Chung
Poster: http://www.physics.wisc.edu/twap/posters/2014/3107.pdf
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"This Week at Physics" poster: http://www.physics.wisc.edu/twap/posters/2014/2014-04-14.pdf

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