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This Week at Physics

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Events During the Week of September 28th through October 5th, 2014

Monday, September 29th, 2014

Plasma Physics (Physics/ECE/NE 922) Seminar
Coherent structures and magnetic reconnection in collisionless turbulence from the perspective of kinetic particle simulations
Time: 12:00 pm
Place: 2535 Engineering Hall
Speaker: Vadim Roytershteyn, Space Science Institute
Abstract: Plasma turbulence is one of the most ubiquitous processes in laboratory, space, and astrophysical plasmas. Dissipation of turbulent energy is thought to play an important role in the energy balance of many systems, including the solar corona and the solar wind. In contrast to the relatively simple physics of dissipation in fluids and collisional plasmas, numerous microscopic processes can produce effective dissipation in weakly collisional plasmas. The problem of identifying and quantifying these processes has attracted considerable attention in the last decade partially because of its significance for understanding energy partition in the corona and solar wind. In this talk, we will describe some of the recent advances enabled by applications of kinetic particle computer simulations to this problem. Specifically, we will focus on generation of coherent structures by turbulence and on the contribution of magnetic reconnection to the energy dissipation.
Host: Dr Stanislav Boldyrev
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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 Le Zhang (lzhang263@wisc.edu)
Host: Peter Timbie
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Council Meeting
Time: 4:00 pm
Place: 2314 Chamberlin (Chair's Conference Room)
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Tuesday, September 30th, 2014

Chaos & Complex Systems Seminar
The chaos in vocal fold vibration and sound production
Time: 12:05 pm
Place: 4274 Chamberlin Hall (Refreshments will be served)
Speaker: Jack Jiang, UW Department of Surgery
Abstract: Vocal fold vibration is key for human speech and communication. This vibration is driven by airflow and can be regular, irregular, or chaotic. Our study focuses on in what conditions vocal fold vibration will be regular or irregular. Many voice disorders, such as lesions or paralysis, can lead to irregular voice. We can surgically intervene to restore healthy and more regular vibration. We also use acoustic parameters, such as perturbation analysis (jitter and shimmer) and nonlinear dynamic analysis (correlation dimension, second order entropy, and Lyapunov exponents), to describe the irregularity of voice production. Clinically, these chaotic parameters show discriminatory power for pathological voice. Typically, we classify four types of voice: Type 1 (nearly periodic), Type 2 (contains strong modulations or subharmonics), Type 3 (aperiodic), and Type 4 (predominated by stochastic noise characteristics). The challenge is that when the voice has too much turbulence the degree of freedom approaches infinity. Such voice is difficult to quantitatively describe.
Host: Clint Sprott
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Theory Seminar (High Energy/Cosmology)
Hidden GeV-scale interactions of quarks and dark matter
Time: 3:00 pm
Place: 5280 Chamberlin Hall
Speaker: Claudia Frugiuele
Abstract: I will discuss the discovery prospect of light dark matter with the NOVA near detector at Fermilab. I will give first an overview on the current bounds on the quarks-light dark matter interaction and I will then explain why neutrino experiments can improve these bounds. Finally I will focus on the NOVA experiment discussing possible models and parameter space potentially constrained by this experiment.
Host: Ran Lu
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Wednesday, October 1st, 2014

Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
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Thursday, October 2nd, 2014

R. G. Herb Condensed Matter Seminar
The Zeeman Effect in Confined Superconductors: A new probe of absolute spin polarization on the atomic scale
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Christian Ast, Max Planck Institute for Solid State Research
Abstract: When a dimensionally confined superconductor is subjected to a magnetic field, the compensating currents are suppressed such that superconductivity persists to much higher magnetic fields than in the bulk. This effect has been observed first in planar tunnel junctions by Meservey, Tedrow and Fulde [1]. The resulting Zeeman splitting of the quasiparticle density of states can be exploited as a probe for the absolute spin polarization of a spin polarized tunnelling current [2]. We have transferred this concept to the scanning tunnelling microscope (STM) to probe the spin polarization of the tunnelling current on the atomic scale by using a superconducting tip. As the superconducting tip carries no magnetic moment itself, the information in the spin polarized tunnelling current can be attributed to the magnetic properties of the sample. This is a decisive advantage over other spin-polarized STM techniques. First measurements on magnetic cobalt islands on a Cu(111) substrate are in excellent agreement with literature. Measuring the spin polarization of the tunnelling current as a function of tip-island distance, we find that the spin polarization increases by 65% when the distance is increased by only 2.3Å. This can be attributed to the different exponential decay of majority and minority states into the vacuum. This means that we can exploit the vacuum barrier as a tunable spin filter.

References
[1] R. Meservey et al., Phys. Rev. Lett. 25, 1270 (1970).
[2] P. M. Tedrow and R. Meservey, Phys. Rev. Lett. 26, 192 (1971).
Host: Franz Himpsel
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Astrophysical neutrinos, PeV events at IceCube, and the Direct Detection of Dark Matter
Time: 2:30 pm
Place: 5280 Chamberlin
Speaker: Raj Gandhi, HRI, India and Fermi National Accelerator Laboratory
Abstract: Given present day constraints, dark matter (DM) may not comprise of thermal weakly interacting massive particles (WIMPS). Drawing closely upon the fact that neutral current neutrino nucleon interactions are indistinguishable from DM-nucleon interactions at low energies, we study the consequences of extending this feature to high energies for a small, non-thermal but highly energetic population of DM particle χ, created via the decay of a significantly more massive and long-lived non-thermal relic φ, which forms the bulk of DM. If χ interacts with nucleons, it is possible that its cross-section, like the neutrino-nucleus coherent cross-section, may rise sharply with energy leading to deep inelastic scattering, similar to neutral current neutrino-nucleon interactions at high energies. Thus, its direct detection may be possible via cascades in very large neutrino detectors. IceCube (IC) has recently reported three ultra-high energy PeV cascade events clustered around 1-2 PeV. We apply this notion to these events and discuss the features which may help discriminate this scenario from one in which only astrophysical neutrinos constitute the IC event sample.
Host: Barger
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Astronomy Colloquium
Filaments in simulations of Molecular Cloud FOrmation
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Gilberto Gomez, UNAM Mexico
Abstract: In this talk, we report on the characterization of filaments that develop self-consistently in a numerical simulation of cloud formation by colliding flows. The filaments are not in equilibrium at any time, but instead are long-lived flow features. The filaments are long-lived because they accrete from their environment while simultaneously accreting onto the clumps within them; they are essentially the locus where the flow changes from accreting in two dimensions to accreting in one dimension.

In the simulation, two prominent filaments are formed, with lengths ∼15 pc and masses ∼600Msun above density n ∼ 1e3 cm−3. The density profile exhibits a central flattened core of size ∼0.3 pc and an envelope that decays as r^−2.5 in reasonable agreement with observations. Accretion onto the filament reaches a maximum linear density rate of ∼30Msun Myr^−1 pc^−1.
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Friday, October 3rd, 2014

Theory Seminar (High Energy/Cosmology)
The nonexistence of the eternal black hole
Time: 11:30 am
Place: 5280 Chamberlin
Speaker: Samir Mathur, Ohio State
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Physics Department Colloquium
The the Black Hole Information Paradox, and its resolution in string theory
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Samir Mathur, The Ohio State University
Abstract: Some 40 years ago Hawking found a remarkable contradiction: if we accept the standard behavior of gravity in regions of low curvature, then the evolution of black holes will violate quantum mechanics. Resolving this paradox would require a basic change in our understanding of spacetime and/or quantum theory. In recent years the paradox has found an interesting resolution through string theory. While quantum gravity is normally expected to be important only at distances of order planck length, the situation changes when a large number N of particles are involved, as for instance in the situation where we make a large black hole. Then the length scale of quantum gravity effects grows with N, altering the black hole structure to a "fuzzball"; this effect resolves the paradox.
Host: Hashimoto
Poster: http://www.physics.wisc.edu/twap/posters/2014/3377.pdf
Video: http://www.physics.wisc.edu/vod/2014/10/03.html
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