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This Week at Physics

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Events During the Week of February 7th through February 14th, 2016

Monday, February 8th, 2016

No events scheduled

Tuesday, February 9th, 2016

Chaos & Complex Systems Seminar
Making computer networks work (Part II)
Time: 12:05 pm
Place: 4274 Chamberlin Hall (Refreshments will be served)
Speaker: Aditya Akella, UW Department of Computer Sciences
Abstract: We depend on computer networks for literally every aspect of our daily lives, e.g., work, family, education, socializing, entertainment, and finances. Yet, the quality of experience that we as users derive from these networks is far from satisfactory. We're routinely hit by poor or variable page-load times and download speeds, and even outright unavailability of critical network-accessible services. Researchers and practitioners alike work round the clock to develop fixes, but disruptive applications, protocols, and hardware quickly render them ineffective. <br>
Host: Sprott
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"Physics Today" Undergrad Colloquium (Physics 301)
Magnetic reconnection, a celestial phenomenon in the laboratory
Time: 1:20 pm
Place: 2241 Chamberlin Hall
Speaker: Jan Egedal, UW Madison Department of Physics
Host: Wesley Smith
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Theory Seminar (High Energy/Cosmology)
High-Scale Axions without Isocurvature from Inflationary Dynamics
Time: 3:30 pm
Place: 5280 Chamberlin Hall
Speaker: Dr. Jack Kearney , Fermilab
Abstract: If the PQ-breaking scale f is larger than the inflationary Hubble scale HI, the PQ symmetry is broken during inflation. In the most straightforward models, this gives rise to a light axion field during inflation, which acquires isocurvature fluctuations. Such fluctuations are very stringently constrained by current CMB measurements---in fact, supposing the near-future observation of primordial tensor modes (i.e., a measurement of a non-zero scalar-to-tensor ratio r, indicating a high inflationary scale), these constraints would exclude simple models of QCD axion dark matter in which f is larger than HI. This is particularly problematic for the near-Planckian values of f favored by, for instance, string theory.

A variety of solutions have been proposed to "resurrect" high-scale axions. Many seek to leverage inflationary dynamics to modify the behavior or potential of the PQ field during inflation in order to suppress isocurvature. However, inflation and the axion potential are both very fragile, and readily disrupted by additional interactions or couplings. As such, it is important to carefully consider the viability of influencing the PQ field via inflationary dynamics; in other words, can this really be accomplished without messing up either inflation or the solution to the strong CP problem? In this talk, I'll discuss the variety of issues that can arise in these constructions, and highlight the steps one must take to build a viable model.
Host: Amol Upadhye
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Wednesday, February 10th, 2016

No events scheduled

Thursday, February 11th, 2016

R. G. Herb Condensed Matter Seminar
Origin of the in-plane resistivity anisotropy of the iron pnictides: scattering rate or plasma frequency?
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Michael Schuett, University of Minnesota
Abstract: In-plane resistivity anisotropy has been the prime tool to probe the electronic nematic state of the iron-based high-temperature superconductors. In these correlated phases, the electronic degrees of freedom spontaneously lower the point-group symmetry of the system from tetragonal to orthorhombic. Thus, the elucidation of the origin of the resistivity anisotropy could provide invaluable information about the microscopic nature of the nematic state of the iron pnictides. In general, an anisotropic resistivity anisotropy can be the result of an anisotropic scattering rate (either elastic or inelastic) and/or an anisotropic plasma frequency. To shed light on this problem, here we investigate the impact of spin fluctuations on the anisotropic ac conductivity of the iron pnictides.

We show that spin fluctuations affect both the scattering rate and the effective plasma frequency. Interestingly, the anisotropy in the effective scattering rate is antagonistic to the anisotropy induced in the effective plasma frequency and can become comparable near the nematic transition temperature. As a result, the ac conductivity may seem to be dominated by an effective plasma frequency anisotropy, although the dc conductivity is actually determined solely by the scattering rate anisotropy. Our results agree qualitatively with recent experiments in detwinned iron pnictides, and reveal an unavoidable entanglement between scattering rate anisotropy and plasma frequency anisotropy caused by spin fluctuations.
Host: Alex Levchenko
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
The Atacama B-mode Search: Cosmology at 17,000 Feet
Time: 2:30 pm
Place: 5280 Chamberlin Hall
Speaker: Sara Simon, Princeton
Abstract: The Atacama B-mode Search (ABS) was a crossed-Dragone telescope located at an elevation of 5200 m in the Atacama Desert in Chile that observed the cosmic microwave background (CMB) from February 2012 until October 2014. ABS was a pathfinder experiment that searched for the primordial B-mode polarization signal at large angular scales from multipole moments of l~40 to l~500, where it is expected to peak. The ABS focal plane consisted of 240 pixels sensitive to 145 GHz, each containing two transition-edge sensor bolometers coupled to orthogonal polarizations. Cold optics and an ambient temperature, rapidly-rotating half-wave plate made the ABS instrument unique. I will discuss the ABS instrument and its contributions to the field of CMB cosmology.
Host: Kam Arnold
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
The Next Generation Balloon-borne Large Aperture Submillimeter Telescope (BLAST-TNG)
Time: 3:00 pm
Place: 5280 Chamberlin Hall
Speaker: Brad Dober, University of Pennsylvania
Abstract: After decades of study, the physical processes regulating star formation remain poorly understood. In particular, the role played by magnetic fields in both the formation of molecular cloud structure and the regulation of core collapse is unclear. In many simulations, magnetic fields dramatically affect both the star formation efficiency and lifetime of molecular clouds. However, observationally the strength and morphology of magnetic fields in molecular clouds remain poorly constrained. Submillimeter polarimetry provides an important observational window on magnetic fields in star forming regions. By mapping polarized emission from dust grains aligned with respect to their local magnetic field, the field orientation (projected on the sky) can be traced. The Next-Generation Balloon-borne Large Aperture Submillimeter Telescope (BLAST-TNG) is a 2.5 meter submillimeter polarimeter designed to map magnetic fields. BLAST-TNG utilizes three polarization-sensitive arrays of over 4000 microwave kinetic inductance detectors, centered at 250, 350, and 500 microns. BLAST-TNG will provide an unprecedented number of magnetic field vectors, and will enable a rigorous statistical analysis of the role that magnetic fields play in star formation. I will present the overall design and progress towards deployment of both the detector arrays and readout hardware in anticipation for a December 2016 BLAST-TNG Antarctic flight.
Host: Kam Arnold
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Astronomy Colloquium
Gas dynamics and star formation in dwarf galaxies
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and Cookies at 3:15 PM
Speaker: Dr. Federico Lelli, Case Western Reserve University
Abstract: Dwarf galaxies are the most common types of galaxies in the Universe. They play a key role in understanding the process of star formation and the effect of stellar feedback on galaxy evolution. I will present recent results from interferometric HI observations, focusing on two key types of low-mass galaxies: starburst dwarfs and tidal dwarfs.

Starburst dwarfs in the nearby Universe represent our best analogues to high-z star-forming galaxies. We find that the inner rotation curves of starburst dwarfs rise more steeply than those of typical dwarf irregulars, pointing to a close link between intense star formation and galaxy dynamics (distribution of baryons and dark matter).

Tidal dwarf galaxies (TDGs) are recycled objects that form within the tidal debris around interacting and merging galaxies. TDGs may represent a new, unexplored channel for the formation of dwarf galaxies. We find that putative TDGs are associated with rotating gas disks and seem to be nearly devoid of dark matter, as predicted by numerical simulations in a LCDM context. I will discuss the implications of these results for the formation and evolution of dwarf galaxies.
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Friday, February 12th, 2016

NPAC (Nuclear/Particle/Astro/Cosmo) Forum
High- and intermediate-velocity clouds as a tracer of cosmic rays in the Galactic halo
Time: 1:00 pm
Place: 4274 Chamberlin Hall
Speaker: Luigi Tibaldo, MPIK, Heidelberg
Abstract: I will present the first observational estimate of the large-scale distribution of cosmic-ray (CR) nuclei in the halo of the Milky Way, achieved through gamma-ray observations of high- and intermediate velocity clouds by the Fermi Large Area Telescope. CRs up to at least PeV energies are usually described in the framework of an elementary scenario that involves acceleration by supernova remnants or superbubbles in the Milky Way disk, and then diffusive propagation throughout the Galaxy and its halo. The details of the propagation process are so far mainly constrained indirectly by the composition of CRs in the solar system, and then extrapolated to the whole Galaxy. The densities of CR nuclei in remote locations, on the other hand, can be traced via the gamma rays they produce in inelastic collisions with clouds of interstellar gas. Recently, we performed a search for gamma-ray emission from several high- and intermediate-velocity clouds located in the halo of the Milky Way up to ~ 7 kpc from the disk. The gamma-ray emission rate per hydrogen atom was found to decrease with distance from the disk, which provides direct evidence that CRs at the relevant energies originate therein. Furthermore, I will discuss how the results call for a critical reexamination of propagation models against other direct and indirect observations of CRs.
Host: Justin Vandenbroucke
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Physics Department Colloquium
Deciphering IceCube’s High-Energy Neutrinos
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Marek Kowalski, Humboldt University Berlin & DESY Zeuthen
Abstract: The existence of high energy neutrinos of cosmic origin was discovered by IceCube in 2013, opening a new window to the cosmos. Since then much has been learned about their properties, however, the astrophysical sources of the neutrinos remain unresolved so far. In this talk I will summarize the knowns and unknowns concerning IceCube's neutrinos and how advances in multi-messenger astronomy can help solve the puzzle. Plans for a next-generation IceCube detector, named IceCube-Gen2, will be presented.
Host: Albrecht Karle
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Saturday, February 13th, 2016

Wonders of Physics
Physics of Superheros
Time: 1:00 pm
Place: 2103 Chamberlin
Speaker: Sprott and others
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Wonders of Physics
The Physics of Superheros
Time: 4:00 pm
Place: 2103 Chamberlin Hall
Speaker: Clint Sprott, University of Wisconsin - Madison
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Wonders of Physics
The Physics of Superheros
Time: 7:00 pm
Place: 2103 Chamberlin Hall
Speaker: Clint Sprott, University of Wisconsin - Madison
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Sunday, February 14th, 2016

Wonders of Physics
The Physics of Superheros
Time: 1:00 pm
Place: 2103 Chamberlin Hall
Speaker: Clint Sprott, University of Wisconsin - Madison
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Wonders of Physics
The Physics of Superheros
Time: 4:00 pm
Place: 2103 Chamberlin Hall
Speaker: Clint Sprott, University of Wisconsin - Madison
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