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This Week at Physics

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Events During the Week of September 25th through October 2nd, 2016

Monday, September 26th, 2016

Graduate Introductory Seminar
Atomic, Molecular, and Optical Physics
Time: 12:00 pm
Place: 5280 Chamberlin Hall
Speaker: Lin, Lawler, Saffman, Walker, Yavuz
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Plasma Physics (Physics/ECE/NE 922) Seminar
Turbulent Dynamo Amplification of Magnetic Fields in Laser-Produced Plasmas
Time: 12:00 pm
Place: 2241 Chamberlin Hall
Speaker: Petros Tzeferacos, University of Chicago
Abstract: Magnetic fields are ubiquitous in the Universe, as revealed by diffuse radio-synchrotron emission and Faraday rotation observations, with strengths from a few nG to tens of μG. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter in the Universe. The standard model for the origin of these intergalactic magnetic fields is through the amplification of seed fields via turbulent dynamo to the level consistent with current observations. We have conceived and conducted a series of experiments using high-power laser facilities to study the amplification of magnetic fields via turbulence. In these experiments, we characterize the properties of the fluid and the magnetic field turbulence using a comprehensive suite of plasma and magnetic field diagnostics. We describe the large-scale 3D simulations we performed with the radiation-MHD code FLASH on ANL's Mira to help design and interpret the experiments. We then discuss the results of the experiments, which indicate magnetic Reynolds numbers above the expected dynamo threshold are achieved and seed magnetic fields produced by the Biermann battery mechanism are amplified by turbulent dynamo. We relate our findings to processes occurring in galaxy clusters.
Host: Cary Forest
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Astronomy Lunch Talk
Formation of low-mass helium white dwarfs orbiting millisecond pulsars
Time: 12:00 pm
Place: 4421 Sterling
Speaker: Alina Istrate, UW-Milwaukee
Abstract: Millisecond pulsars (MSPs) belong to a class of radio pulsars characterized by high rotational spin rates and low magnetic fields. These neutron stars are believed to be the end-product of binary evolution, i.e. the recycling scenario, in which an old neutron star accretes matter and angular momentum from a close stellar companion for an extended period of time, while being observable as an X-ray binary.

A handful of MSPs are detected in very compact orbits (orbital periods between 2 − 9 hr), around low-mass He WDs with masses below 0.25 Msun and surface gravities 5 < log g < 7, the so-called extremely low-mass helium white dwarfs (ELM WDs).

Today we know of the existence of more than 80 ELM WDs, with the majority of them in a binary system with a more massive CO WD. The increasing number of discovered ELM WDs reveals that they are formed in different environments, from the Galactic disk to open and globular clusters. ELM WDs are most likely the result of binary evolution as they cannot be formed from single stars within a Hubble time. The new wealth of data raises questions regarding the puzzling presence of metals in the atmospheres of young bloated ELM proto-WDs and the newly discovered pulsations in three ELM proto-WDs. The current available evolutionary models fail to explain such properties.
In this talk, I will present the latest theoretical efforts regarding the formation and evolution of ELM WDs.
Host: Astronomy Dept.
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Tuesday, September 27th, 2016

Chaos & Complex Systems Seminar
Forecasting change in U.S. breeding bird distributions
Time: 12:05 pm
Place: 4274 Chamberlin Hall (refreshments will be served)
Speaker: Brooke Bateman, UW Department of Forest and Wildlife Ecology
Abstract: Species are already coping with climate change by shifting their distributions. The rate at which these shifts are occurring is, however, much faster than faced by species in the past. In an uncertain future, we must be able to recognize and forecast how species distributions have and will continue to change. We used the species distribution modeling algorithm Maxent, occurrences, and annual climate and extreme weather covariates to predict breeding bird distributions for nearly 400 breeding bird species in the U.S. from 1950 through 2100, using 19 GCMs and two rcp scenarios. I will highlight how breeding distributions, in relation to annual climate and extreme weather covariates, have changed over the recent past and what change is forecasted for the future. Given the broad scale nature of climate change and widespread modification of the landscape with agriculture and development, we will need to coordinate and implement efforts at broad spatial scales and across many species.
Host: Clint Sprott
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Theory Seminar (High Energy/Cosmology)
Discovering or Falsifying sub-GeV Thermal Dark Matter
Time: 3:30 pm
Place: 5280 Chamberlin
Speaker: Gordan Krnjaic, Fermilab
Abstract: This talk analyzes the present status of sub-GeV thermal dark matter annihilating through Standard Model mixing. In these scenarios, the requirement that dark matter be a thermal relic sets a sharp sensitivity target for terrestrial experiments, and that one of the simplest and best-motivated interactions between the SM and light dark matter (the vector portal) remains viable. Moreover, it is demonstrated that a small set of future experiments can decisively test these models. In particular, the future LDMX experiment can play a key role in most of the remaining parameter space in simple models.
Host: Yang Bai
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Wednesday, September 28th, 2016

No events scheduled

Thursday, September 29th, 2016

Cosmology Journal Club
An Informal discussion about a broad variety of arXiv papers related to Cosmology
Time: 12:15 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 Amol Upadhye (aupadhye@wisc.edu).
Host: Amol Upadhye
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Astronomy Colloquium
Nuclear clusters and (supermassive) black holes
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM, Talk at 3:45 PM
Speaker: Fabio Antonini, Northwestern University
Abstract: Massive stellar clusters are often found at the photometric and kinematic centers of galaxies. Such nuclear clusters are the densest stellar systems observed in the local universe, representing a natural environment where compact object binaries can dynamically form, harden and merge. I will discuss the dynamical processes that lead to the merger of black hole binaries in nuclear clusters in connection to the origin of the binary black hole mergers recently detected by Advanced LIGO. I will show that nuclear clusters can produce a significant population of black hole binaries that merge in the local universe, and that these binaries have clear differences in the statistical distributions of their properties (e.g., mass, eccentricity) when compared to those formed either in globular clusters or through isolated binary evolution. Finally, I will consider the evolution of supermassive black hole binaries that are formed in nuclear clusters during the merger of galaxies. The results of N-body and Monte Carlo simulations show that coalescence times fall in the range from 10^8 yr to a few Gyr. These results constitute a fully stellar-dynamical solution to the ''final-parsec problem'' and imply a high rate of events for planned low-frequency gravitational wave detectors like eLISA.
Host: UW Astronomy Department
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Friday, September 30th, 2016

R. G. Herb Condensed Matter Seminar
Engineering light-matter interactions with atom-like systems
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Jennifer Choy, Draper Laboratory
Abstract: Techniques that utilize photons to probe and manipulate discrete electronic energy levels in atoms have enabled numerous metrology applications, including precise and stable inertial sensors, time and frequency standards, magnetometers, and test-beds of quantum information protocols. Despite the promise of this technology, the electro-optics and vacuum requirements associated with atomic instruments pose a considerable challenge to their implementation outside of the laboratory. This field can greatly benefit from integration with a photonics platform that provides robust and efficient control of photon-atom interactions.
Wide-bandgap semiconductors are candidate material systems for such a photonics platform. Recent advancements in material processing and nanofabrication have made it possible to develop micro- and nano-photonic devices in these “nonstandard” materials. In some cases, crystalline defects can lead to optically active color centers that can be isolated as single quantum systems (“artificial atoms”), with optical and spin properties that are viable for quantum information and quantum sensing.
In this talk, I will provide examples of engineering light-matter interactions with these artificial atoms, focusing on the nitrogen-vacancy center in single-crystal diamond. I will review several devices that enhance color-center emission, including nanowires and gratings that improve excitation and collection efficiencies, as well as resonators that modify spontaneous emission rates. I will summarize the major applications that have been enabled by diamond-based photonic devices. Finally, I will conclude with an overview of other potential photonic platforms (e.g. silicon carbide, titanium dioxide, silicon nitride, III-V materials, and hybrid approaches), and a comparison between photonics with solid-state artificial atoms and real atomic systems.
Host: McDermott
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Physics Department Colloquium
Nuclei and Cosmos
Time: 3:30 pm
Place: 2241 Chamberlin hall
Speaker: Ani Aprahamian, Notre Dame University
Abstract: : The Origin of the Heavy Elements in the Universe was identified as one of the Eleven Unanswered questions in Physics and Astronomy at the beginning of the 21st century. The question in intertwined in the complexities of the cosmos with the properties of nuclei. The universe can and does synthesize the heavy elements but the question regarding the site(s) of the synthesis remains unresolved. My talk will focus on an attempt to use nuclear properties as a tool to perhaps distinguish between two of the popularly proposed sites of compact object mergers and core collapse supernovae. We have just completed a comprehensive sensitivity study of the exotic nuclei that have the largest impact on the rapid neutron capture process (the main mechanism for producing the heavy elements). Our next goal is to measure the masses, decay rates, and neutron capture rates of these very exotic nuclei. Facilities such as FRIB (Facility for Rare Isotope Beams) now in construction at Michigan State University will allow us to reach the presently unreachable nuclei in the laboratory. Studies of nuclei along with the astrophysical messengers from gravitational waves to neutrinos point to a promising solution of at least one of the outstanding open questions.
Host: Baha Balantekin
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