Events at Physics |
Events During the Week of October 18th through October 25th, 2009
Monday, October 19th, 2009
- Plasma Physics (Physics/ECE/NE 922) Seminar
- Turbulence, Structures, Thermal and Non-thermal Particle Transport in a Simple Toroidal Plasma
- Time: 12:05 pm
- Place: 4274 Chamberlin Hall
- Speaker: Ambrogio Fasoli, EPFL Laussane
Tuesday, October 20th, 2009
- R. G. Herb Condensed Matter Seminar
- Delocalization by disorder: non-metallic transport in layered metals
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Dmitrii Maslov, University of Florida
- Abstract: Electron transport in many layered metals exhibits a number of puzzling anomalies: for example, while the temperature dependence of the in-plane resistivity is metallic, that of the out-of-plane resistivity is insulating or even non-monotonic. Also, it is often the case that the resistivity anisotropy cannot be accounted simply by mass anisotropy, which implies that disorder is strongly anisotropic as well. We show that, contrary to the widely accepted paradigm of "coherent-incoherent crossover", the Boltzmann equation is applicable to layered systems both for elastic and Eliashberg-type (dynamic) inelastic scattering as long as the "good-metal" condition (E_Fτ) is satisfied and disorder is not correlated. Therefore, a model containing only those sources of scattering cannot explain the experiment. We propose a model of two-channel transport, in which electrons propagate across the layers in two ways--coherently and via phonon-assisted tunneling through random resonant centers--and show how the experiment can be explained within this model. We also propose an explanation of anomalously large resistivity anisotropy in a model of two types of disorder: planar defects and isotropic impurities. We solve this model by mapping it onto the exact Berezinskii's solution of 1D localization problem and show that isotropic impurities destroy localization induced by planar defects.
- Host: Andrey Chubukov
- Chaos & Complex Systems Seminar
- High-Energy neutrino astronomy: Towards a kilometer-scale neutrino observatory.
- Time: 12:05 pm
- Place: 4274 Chamberlin (Refreshments will be served)
- Speaker: Francis Halzen, UW Department of Physics
- Abstract: Kilometer-scale neutrino detectors such as IceCube are discovery instruments covering nuclear and particle physics, cosmology and astronomy. Examples of their multidisciplinary missions include the search for the particle nature of dark matter and for additional small dimensions of space. In the end, their conceptual design is very much anchored to the observational fact that Nature produces photons and protons with energies in excess of one hundred and one hundred million Terraelectronvolts, respectively. The cosmic ray connection sets the scale of cosmic neutrino fluxes. The problem has been to develop a robust and affordable technology to build the kilometer-scale neutrino detectors required to detect candidate sources such as supernova remnants and active galxies. The AMANDA telescope transforming ultra-clear deep Antarctic ice into a Cherenkov detector of muons and showers initiated by neutrinos of all three flavors, has met this challenge. Having collected more than 6000 well-reconstructed muon neutrinos of 50 GeV ~ 500 TeV energy, AMANDA represented a proof of concept for the ultimate kilometer-scale neutrino observatory, IceCube, now almost complete and producing results exceeding seven years of AMANDA data in sensitivity.
Wednesday, October 21st, 2009
- No events scheduled
Thursday, October 22nd, 2009
- R. G. Herb Condensed Matter Seminar
- Fluctuations, Response, Entropy, and "Temperature" in Granular Packings
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Bulbul Chakraborty, Brandeis University
- Abstract: To understand the mechanical response of granular materials, one needs a theoretical approach that can bridge the gap between microscopic, grain-level quantities and macroscopic, collective properties. Fluctuations are inherently related to the number of microscopic states available under a given set of macroscopic parameters. In equilibrium thermodynamics, the microcanonical entropy, or its derivatives in other ensembles, is the measure used to calculate fluctuations and response. In disordered systems such as spin glasses, the concept of complexity has been used to understand collective properties. In mean-field models, free-energy minima are separated by barriers that diverge in the thermodynamic limit, and one can in principle count the number of states unambiguously. Can we identify a physical variable in granular materials that is conserved under any local dynamics, and therefore leads to a definition of complexity? For mechanically stable packings, there is a topological conservation law that allows us to define the analog of complexity. In this talk, I will describe a framework for calculating stress fluctuations in frictional and frictionless granular packings, based on a model calculation of complexity, and compare the predictions of our theory to experiments and simulations.
- Host: Susan Coppersmith
- Whitford Lecture
- WMAP and Beyond
- Time: 2:00 pm - 3:30 pm
- Place: 3425 Sterling Hall
- Speaker: Dr. David Spergel, Princeton University
- Abstract: CMB experiments have made an accurate full-sky measurement of the microwave background temperature and polarization fluctuations. These measurements probe both the physics of the very early universe and the basic properties of the universe today. These measurements rigorously test our standard cosmological model and provide an accurate determination of basic cosmological parameters (the curvature of the universe,its matter density and composition).
I will review the results from WMAP and describe recent results from the Atacama Cosmology Telescope (ACT) and look forward to the upcoming results from Planck. ACT small scale measurements not only probe early universe physics but offer a new tool for studying the evolution of clusters and large-scale structure. - Host: Prof Richard Townsend
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- The Search for the Fundamental Nature of Dark Matter
- Time: 4:00 pm
- Place: 4274 Chamberlin
- Speaker: Dan McKinsey, Yale University
- Abstract: Astrophysical evidence on a variety of distance scales clearly shows that we cannot account for a large fraction of the mass of the universe. This matter is "dark", not emitting or absorbing any electromagnetic radiation. A compelling explanation for this missing mass is the existence of Weakly Interacting Massive Particles (WIMPs).
These particles are well motivated by particle physics theories beyond the Standard Model, and the discovery of WIMPs would have enormous impact on both astrophysics and particle physics. WIMPs, if they exist, would occasionally interact with normal matter. With a mass in the range of 1 to 1000 times the mass of the proton, and moving at speeds relative to the Earth on the order of 200 km/s, WIMPs would only deposit a small amount of energy when scattering with nuclei.
Detectors that are low in radioactivity and sensitive to small energy depositions can search for the rare nuclear recoil events predicted by WIMP models. In recent years, several new efforts on direct dark matter detection have begun in which the detection material is a noble liquid. Advantages include: large nuclear recoil signals in both scintillation and ionization channels, good scalability to large target masses, effective discrimination against gamma ray backgrounds, easy purification, and reasonable cost. - Host: Karsten Heeger
- Introductory Graduate Seminar
- Condensed Matter Theory
- Time: 5:30 pm
- Place: 2223 Chamberlin Hall
- Speaker: Bruch, Chubukov, Coppersmith, Joynt, Perkins, Vavilov, University of Wisconsin Department of Physics
Friday, October 23rd, 2009
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- Quantifying the Unknown in Astronomy: A Bayesian Approach
- Time: 2:30 pm
- Place: 5280 Chamberlin
- Speaker: Brian Connolly, University of Pennsylvania
- Abstract: Over the last few years the Bayesian statistics has played an increasingly important role in astronomical data analyses, from classifying quasars to fitting cosmological models to WMAP data. One aspect of Bayesian statistical methods used to classify astronomical objects is that traditionally they assume that the object being classified falls into a finite set of modeled (or known) astronomical objects. However, astronomical research continually reveals new, unexplained phenomena; new large-scale surveys coming up in the next decade (such as the Dark Energy Survey, the Large Synoptic Survey Telescope, the Joint Dark Energy Mission, etc.) are expected to greatly enrich our catalog of understood (or at least modeled) astronomical objects. In my
talk, I will first review the two approaches to statistics, Bayesian and Frequentist. I will describe how and why the Bayesian approach has been so successful in astronomy in general, and in particular how the Bayesian approach can be extended to account for as yet unmodeled objects. I will then show how this method can be used to quantify the differences in the spectra of
Ultra-Luminous Infra-Red Galaxies (ULIRGs) and aid in the identification of Type Ia supernovae, a staple of modern cosmological research. I will also discuss how network diagrams and graph theory can be used in conjunction with these Bayesian methods to enhance our understanding of the evolution of ULRGs
and Type Ia supernovae. - Host: Stefan Westerhoff
- Physics Department Colloquium
- The Good the Bad and the Awful: Scientific Simulation and Prediction
- Time: 4:00 pm
- Place: 2241 Chamberlin Hall (coffee at 3:30 pm)
- Speaker: Leo Kadanoff, University of Chicago
- Abstract: Worthwhile computer simulations are done to explore uncharted territory, resolve a well-posed scientific or technical question, or to make a design choice. Some excellent work is reviewed Some less happy stories are recounted. I then concentrate my attention upon astrophysical simulations, showing how they can explore possible scenarios for stellar explosions.
- Host: Coppersmith