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This Week at Physics

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Events During the Week of April 8th through April 15th, 2018

Monday, April 9th, 2018

Plasma Physics (Physics/ECE/NE 922) Seminar
Mechanisms for onset of the whistler chorus in Earth’s magnetosphere
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Dr. Ge Wang, UW-Madison Engineering Physics
Abstract: We have extended the formalism that describes chirping phenomena in a fusion plasma to model chirping of whistler waves in the magnetosphere, and developed a code that uses realistic physical scales and high phase space resolution near the particle resonance regions to study the onset of a whistler chorus. Pressure anisotropy in the magnetosphere excites convective whistler instability with the most unstable mode at a frequency and then the excited wave packet moves away from the equator at the group velocity. The wave amplitude is spatially linearly amplified, until the nonlinearity of particles near the resonance region sets in. This nonlinearity arises from trapping of the resonance particles, so that the trapped distribution will transport with a resonant velocity toward the equator (opposite to the propagation direction of the wave packet) in accord with the local resonance condition, while the ambient non-trapped distribution is constrained to oscillate about the equilibrium particle orbit with the energy and magnetic moment conserved. This causes a hole to form in the trapped region, with a hole depth, which steadily increases as the hole moves toward the equator with the field frequency still oscillating at the initial frequency, while transferring particle free energy to the waves. Thus a chain of holes move into an environment where the field amplitudes are spatially decreasing until the wave amplitude matches a nonlinear BGK condition, which allows the BGK mode to form at a shifted frequency, that initiates a chirping signal as the the holes move in phase space. The chirping holes then serve as antennas that radiate new whistler frequencies amplified as the group velocities transmit the new frequencies toward a magnetic pole. The simulation is consistent with our conjecture that these mechanisms are what enable the emergence of the observed rising tone whistler chorus in the magnetosphere.
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Tuesday, April 10th, 2018

Chaos & Complex Systems Seminar
Cloud quantum computing
Time: 12:05 pm
Place: 4274 Chamberlin (Refreshments will be served)
Speaker: Maxim Vavilov, UW Department of Physics
Abstract: In this talk I will describe the IBM quantum processor that is open to the public. The processor has only 5 qubits, but is suitable for quantum demonstrations of basic qubit gates, Bell inequality experiments and elements of quantum error correction. I will review the web-based interface for writing programs for the quantum processor. Then, I will demonstrate the execution of several programs and discuss the accuracy of the results obtained from experiments. I will also review recent progress towards a large-scale universal quantum processor.
Host: Clint Sprott
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"Physics Today" Undergrad Colloquium (Physics 301)
X-ray Astronomy from Sounding Rockets
Time: 1:20 pm
Place: 2241 Chamberlin Hall
Speaker: Dan McCammon, UW Madison Department of Physics
Host: Wesley Smith
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Theory Seminar (High Energy/Cosmology)
Disentangling the Top-Higgs Yukawa CP Structure in the Dileptonic tth with M2-?Assisted reconstructions
Time: 3:30 pm
Place: 5280 Chamberlin Hall
Speaker: Jeong Han Kim, University of Kansas
Abstract: The top quark plays a crucial role in the production and the decays of the Higgs boson, and its Yukawa coupling is constrained by various indirect precision measurements but only via loop-effects. Therefore it is imperative to make a direct measurement of the Top quark Yukawa coupling. In particular, an additional source of CP violation in new physics beyond the standard model may induce the pseudo-scalar component in the Top-Higgs interaction and there are many proposals to measure the relative contribution of CP-even and CP-odd interactions. Since the CP admixture property may be best measured in the center-of-mass frame of either tth or tt system, the majority of currently available methods examine the CP nature in either hadronic or semi-leptonic final states. Existing studies in the dilepton final state are performed in the laboratory frame, which provides limited information on this interaction. In this paper, we investigate the Top-Higgs Yukawa interaction in the dilepton final state and attempt full kinematic reconstructions of both top quarks, which allows to Lorentz-boost to any frame of our interest. We first study event reconstructions at parton level and show that the kinematic correlations survive even after inclusion of more realistic effects such as a parton-shower and hadronization. As a result, we present the required luminosity at the HL-LHC, to distinguish the SM Higgs from an arbitrary CP state, based on a binned log-likelihood method.
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Wednesday, April 11th, 2018

Department Meeting
Department Meeting
Time: 12:15 pm
Place: 5310 Chamberlin Hall
Speaker: Sridhara Dasu
Host: Sridhara Dasu
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Thursday, April 12th, 2018

R. G. Herb Condensed Matter Seminar
Photonic analogues of topological superconductors
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Aashish Clerk, McGill University
Abstract:
Interest continues to grow in photonic and phononic analogues of topological electronic phases. In most cases, these systems are non-interacting, and have the same band structure and edge state structure as their fermionic counterparts. In this talk, I’ll discuss recent theory work in my group showing how parametric “two-photon” driving can be used to realize a new class of photonic topological systems that superficially resemble topological superconductors. Unlike standard particle-number conserving models of non-interacting topological phases, these new systems exhibit crucial differences between their bosonic and fermionic versions. Further, one can realize a situation where all bulk states are stable, but where edge states are guaranteed to be unstable. Such a system can form the basis of a useful device: a topologically-protected amplifier which operates close to the fundamental limits set by quantum mechanics. I’ll discuss how these ideas could be realized in a variety of different experimental platforms, including superconducting quantum circuits and optomechanics.
Host: Vavilov
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Astronomy Colloquium
MHD turbulence in the interstellar medium
Time: 3:30 pm
Place: 4421 Sterling Hall, Coffee and Cookies at 3:30 PM. Talk begins at 3:45 PM
Speaker: Siyao XU, Hubble Fellow UW Astronomy Dept
Abstract: Turbulence and magnetic fields are ubiquitous in the Universe. Magnetohydrodynamic (MHD) turbulence is important for all branches of astrophysics involving gas dynamics. The developments of turbulence theories bring a paradigm shift in many fields of astronomy. I will start from an introduction of modern theories of MHD turbulence and then show some examples for their general applicability in understanding important physical processes in the interstellar medium (ISM), including the formation of density filaments commonly seen in both the diffuse ISM and dense molecular clouds, and the interstellar scattering of Galactic pulsars.<br>
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Friday, April 13th, 2018

Physics Department Colloquium
Quantum quivering from dissipation and noise
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Aashish Clerk, McGill University
Abstract: When trying to coax interesting (and possibly useful) quantum behaviour out of a system, we normally view dissipation as a nuisance whose effects should be minimized as much as possible. In this talk, I'll discuss a powerful and seemingly paradoxical approach where dissipation is deliberately harnessed to prepare interesting quantum states and functionalities. I'll focus on recent theory from my group showing how this strategy can be employed in quantum optomechanical systems, where the motion of a “large” mechanical resonator interacts strongly with light via radiation pressure forces. Here, ‘engineered dissipation’ can allow the preparation quantum states of both light and mechanical motion. These ideas have been recently implemented experimentally to prepare non-classical states of nanogram-scale mechanical resonators, with the relevant dissipation produced by microwave photons in a superconducting quantum circuit.
Host: Maxim Vavilov
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