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Events at Physics

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Events During the Week of December 13th through December 20th, 2015

Monday, December 14th, 2015

Plasma Physics (Physics/ECE/NE 922) Seminar
Plasma Microinstability Saturation in Toroidal Plasmas
Time: 12:00 pm
Place: 2241 Physics Bldg
Speaker: Paul Terry, UW Madison
Abstract: The question of how microinstabilities saturate in toroidal plasmas is often overlooked or
given insufficient attention. This is particularly true in comprehensive numerical
simulations, like gyrokinetics, where beyond ascertaining that there is a physically
meaningful saturated state, most of the analysis is best described as characterization of
fluctuations and the transport they produce. As a result, certain widely held ideas about
saturation do not hold up under closer scrutiny. In this talk we examine new
developments in the understanding of how ion temperature gradient (ITG) turbulence
saturates. We present evidence from gyrokinetic simulations that ITG turbulence
saturates primarily by energy transfer to damped modes within the wavenumber range of
the instability, rather than by a cascade to small scale. Zonal flows catalyze this transfer,
providing an efficient energy transfer channel while absorbing very little of the
transferred energy. Zonal flow shearing is not the primary saturation mechanism, but is a
secondary (weaker) effect at best. We also show that zonal-flow-catalyzed transfer
excites stable modes with tearing parity. These give rise to a stochastic magnetic field at
very low beta values, and electron thermal transport. While small at low beta, this flutterinduced
transport becomes significant at beta approaching 1%. Magnetic fluctuations
created by transfer to damped modes affect zonal flows. Above a critical beta, they
induce irreversible charge loss from rational surfaces, shorting out zonal flow potentials
and leading to a state of very high transport. This phenomenon, called the non zonal
transition (NZT), has long been observed in gyrokinetic simulations, and in experiment
could limit gradients in stiff discharges.
Host: UW Madison
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Tuesday, December 15th, 2015

Theory Seminar (High Energy/Cosmology)
Raymond Stora's last discovery: Hidden U(1)_Y symmetry and new Ward-Takahashi identities in the Abelian Higgs model
Time: 3:30 pm
Place: 5280 Chamberlin
Speaker: Bryan Lynn, Visiting Professor
Abstract: The SSB AHM (the gauge theory of a scalar ϕ∝(H+iπ) and a transverse vector A) has a massless pseudo-scalar π in Lorenz gauge. Physical states have a conserved global current and Goldstone theorem (GT), but no global charge. π contains a Nambu-Goldstone boson (NGB). Slavnov-Taylor identities force on-shell T-matrix elements to be independent of anomaly-free gauge, and global, U(1) transformations, yielding towers of ϕ-sector Ward-Takahashi IDs (WTI), which severely constrain the dynamics, and the effective potential, of external ϕ. Ultraviolet quadratic divergences (UVQD) contribute only to the pseudo-NGB mass, forced by the GT to 0 in the SSB mode, so all UVQD vanish. Weak-scale renormalized gauge-independent Higgs pole-mass and VEV are therefore not technical-fine-tuned. The NGB is "eaten" and decouples as usual, hiding the U(1) WTI from observable particle physics. Our regularization-scheme-independent all-loop-orders results are unchanged by the addition of certain heavy matter fields, as the WTI and GT cause all relevant operators to vanish. We prove 5 new SSB decoupling theorems, illustrating them with two examples: a Heavy (>>Weak) Z2-symmetric singlet real scalar field with 0 VEV; and a heavy singlet right-handed type 1 see-saw Majorana neutrino. Including all loops we prove that these, and certain other heavy degrees of freedom, decouple from the low-energy effective Lagrangian, contributing only irrelevant operators. The NGB again decouples, but our hidden SSB U(1) WTI, and the resultant NGB shift symmetry, protect the low-energy SSB AHM physics from loop contributions of heavy particles! Gauge-independent observable weak-scale Higgs pole mass and VEV <H> are not technical-fine-tuned.
Host: Amol Upadhye
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Wednesday, December 16th, 2015

No events scheduled

Thursday, December 17th, 2015

R. G. Herb Condensed Matter Seminar
Designing Gates for a Spin-Based Quantum Computer
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Daniel Zeuch, Florida State
Abstract: After reviewing the basic notions of quantum computation with electrons trapped in semiconductor quantum dots, I focus on a computing scheme in which qubits are encoded using three spins each and arbitrary quantum gates are carried out by turning on and off, or pulsing, the Heisenberg exchange Hamiltonian. While most work on finding pulse sequences for two-qubit gates has been numerical, I discuss sequences that were designed completely analytically [1,2] and show, in particular, the framework of the derivation [2] of the shortest known sequence for this particular scheme. At the end, I draw parallels between spin-based quantum computation and topological quantum computation with non-Abelian anyons. I show that the basic problem of finding two-qubit quantum gates, addressed in [3], can be understood in similar terms as the problem of finding two-qubit gate pulse sequences.

[1] D. Zeuch, R. Cipri, and N. E. Bonesteel, Phys. Rev. B 90, 045306 (2014).
[2] D. Zeuch and N. E. Bonesteel, arXiv preprint arXiv:1508.07524 (2015).
[3] C. Carnahan, D. Zeuch, and N. E. Bonesteel, arXiv preprint arXiv:1511.00719 (2015).
Host: Coppersmith
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Friday, December 18th, 2015

No events scheduled