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This Week at Physics

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Events During the Week of November 30th through December 6th, 2014

Monday, December 1st, 2014

R. G. Herb Condensed Matter Seminar
Spin bath decoherence of mixed spin qubits
Time: 10:00 am
Place: 5310 Chamberlin Hall
Speaker: Seto Balian, University College London
Abstract: The limiting decoherence mechanism for a wide variety of spin qubits (spin di usion) arises from
coupling to a bath of other spins; for example, 29Si impurities in natural silicon and 13C in diamond.
In addition to o ering fast manipulation times [1], mixed spin systems such as electron-nuclear
qubits (e.g. donors in silicon) can possess `optimal working points' (OWPs) { magnetic elds at
which decoherence arising from both spin bath dynamics and classical eld
uctuations is strongly
suppressed [2{4].
In this talk, I will introduce mixed spin qubits and describe the cluster correlation expansion
(CCE) [5, 6] for calculating dephasing times (T2) of qubits in spin baths associated with strong back-
action and environment-memory e ects. Using the pseudospin model and operating near OWPs, I
will explain the suppression of decoherence driven by pairs of bath spins (lowest order CCE) and
demonstrate enhancement of T2 in quantitative agreement with measurements [2]. Approaching
the OWP, many-body correlations of increasing order are isolated and begin contributing to the
weakening decoherence mechanism. A simple closed-form T2 formula can be derived for nuclear
spin di usion, predicting T2 in excellent agreement with ESR and NMR measurements as well as
CCE simulations across a wide parameter range [2]. The formula also exposes signi cant di erences
between spin bath decoherence and decoherence by classical eld noise.
Finally, I will discuss a plausible decoherence mechanism for 29Si nuclear impurity qubits in
proximity to a donor, based on equivalent atomic sites due to symmetries of the donor electron
wavefunction. This `equivalent pairs' model predicts T2 in the measured timescale of 1 second [7].
Host: Sue Coppersmith
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Plasma Physics (Physics/ECE/NE 922) Seminar
A retrospective on Dalton Schnack's contributions to plasma science
Time: 12:00 pm
Place: 2535 Engineering Hall
Speaker: Carl Sovinec, UW
Host: CPTC
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Tuesday, December 2nd, 2014

Chaos & Complex Systems Seminar
'Real School' : The tension of standard structures and varied social processes in schools
Time: 12:05 pm
Place: 4274 Chamberlin Hall (Refreshments will be served)
Speaker: Mary Metz, School of Education
Abstract: Over the last century and a half many aspects of schooling, especially secondary schooling, have been standardized and their form, despite some changes, has been remarkably resilient. Patterns of legitimate, “real” school are deeply embedded in social expectations. At the same time, we know that effective teachers create routines and atmospheres that vary widely. Some individual students thrive better with some approaches, others with others. Further, (though less well documented) community context and students’ social class and ethnicity have a big effect on what happens in classrooms. Nonetheless, over the last 35 years, there has been increasing societal pressure to standardize the substance and practice of K-12 education yet further. This presentation explores the reasons for the persistent tension between standardization of routines and the need for wide variation and flexibility in actual instruction inside the classroom.
Host: Clint Sprott
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Theory Seminar (High Energy/Cosmology)
Dark Matter in Dilepton Production
Time: 3:00 pm
Place: 5280 Chamberlin Hall
Speaker: Nirmal Raj, University of Oregon
Abstract: At the LHC a standard search channel for new physics is the production of a pair of leptons. New resonances or contact operators can be gleaned from peaks or broad deviations respectively in the invariant mass distribution (mll) of this channel. I'll demonstrate in this talk how hidden sectors can show up in dileptonic events through radiative corrections to the Standard Model process, giving rise to unique features in mll, as well as in leptonic angular distributions. One compelling possibility that can be probed is that of dark matter with scalar messengers coupling it to the quarks and leptons. I will present constraints from dilepton spectrum measurements at the LHC and dark matter experiments, and make predictions for the bounds from the high luminosity 14 TeV LHC as well as a 100 TeV collider.
Host: Ran Lu
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Wednesday, December 3rd, 2014

Theory Seminar (High Energy/Cosmology)
Neutrino oscillations at current and future experiments
Time: 3:00 pm
Place: 5280 Chamberlin Hall
Speaker: Pilar Coloma
Abstract: The large value of $theta_{13}$ has opened a door to determine the ordering of neutrino masses and the amount of CP violation in the leptonic sector. In the case of the neutrino mass ordering, the situation for the upcoming years is particularly promising and a measurement may be achievable in the near future. The determination of the Dirac CP-violating phase $delta$ is more challenging and will depend on several factors, though. In this talk, I will summarize the prospects for the current and future generations of neutrino oscillation experiments for these two observables, and discuss the main challenges that they will have to face.
Host: Ran Lu
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Thursday, December 4th, 2014

NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Particle Physics at the Highest Energies
Time: 2:30 pm
Place: 5280 Chamberlin Hall
Speaker: Peter Denton, Vanderbilt University
Abstract: The highest energy scales accessible in particle physics are seen at the LHC and ultra high energy cosmic ray detectors such as Pierre Auger Observatory and Telescope Array. First, I consider applying a technique known as integral dispersion relations to extend the reach of the LHC. Integral dispersion relations use analyticity to link experimental values at one energy to those at all energies. This can be used to detect new physics in a model independent fashion at and above a machine's operating energy. Next, to probe physics at even higher energies, I consider the open question of ultra high energy cosmic ray anisotropy. In particular, I look at how problems such as low statistics and partial sky coverage affect an experiment's ability to determine anisotropy.
Host: Westerhoff
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Astronomy Colloquium
Cosmic-Ray Ionization Rates in the Galactic ISM
Time: 3:30 pm
Place: 4421 Sterling Hall
Speaker: Nick Indriolo, University of Michigan
Abstract: Cosmic rays play a vital role in initiating the chemistry that occurs in molecular clouds. The ionization of H and H2 begins a network of ion-molecule reactions responsible for generating many of the diatomic and small polyatomic molecules observed in the interstellar medium. A few such species---HCO+, DCO+, OH+, H2O+, and H3+ in particular---are formed and destroyed by rather simple processes, making them powerful probes of the cosmic-ray ionization rate. I will discuss the current status of observations of these molecular ions within our Galaxy, as well as the conclusions that can be drawn from this ever-growing sample.
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Friday, December 5th, 2014

Physics Department Colloquium
Deep, Dark Detection: the DEAP-3600 Dark Matter Experiment
Time: 3:30 pm
Place: 2241 Chamberlin Hall (coffee at 4:30 pm)
Speaker: Art McDonald, SNOLAB
Abstract: A wide variety of astronomical observations appear to indicate the existence of Dark Matter particles making up about 25% of the Universe. However, to date there has been no conclusive direct observation of these particles. The DEAP-3600 Dark Matter Detector will use 3600 kg of liquid argon for a sensitive search for Weakly Interacting Massive Particles (WIMPS) 2 km underground in the SNOLAB international laboratory near Sudbury, Canada. Pulse shape discrimination in the light output will be used to avoid radioactive background from 39Ar beta decay in the liquid argon. Great care in the control of local radioactivity and the reduction in cosmic ray background due to the great depth will enable the use of an almost background-free fiducial volume of 1000 kg. This will provide a sensitivity of about 10-46 cm2 for 100 GeV WIMPS after 3 years of operation, beginning early in 2015. This is substantially better than current limits for the spin-dependent interaction. Details of the detector and its construction status will be presented.
Host: Francis Halzen
Poster: http://www.physics.wisc.edu/twap/posters/2014/3393.pdf
Video: http://www.physics.wisc.edu/vod/2014/12/05.html
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