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Fall 2016 Colloquium Series

The Radial Acceleration Law

Date: Thursday, December 1st, 2016

Speaker: Jim Schombert, University of Oregon


A new kinematic law for rotating galaxies is presented that questions the foundation of the cold dark matter (CMD) paradigm.  The radial acceleration relation (RAR) correlates the acceleration expected from just baryons with the observed gravitational acceleration due to, presumingly, dark matter.  The extremely tight correlation is difficult to explain under any current dark matter model and points to either a highly contrived mechanism of galaxy formation or a new physics framework that involves a decoupling of gravitational and inertial kinematics at very low accelerations (a_o < 10^-10 m s^-2).


NANOGrav: A Galactic Scale Gravitational Wave Observatory

Date: Thursday, November 17th, 2016

Speaker:  Joey Key, University of Washington/Bothell


The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is a member of the International Pulsar Timing Array (IPTA), using pulsars to search for gravitational waves. NANOGrav scientists make use of some of the world’s best telescopes and most advanced technology, drawing on physics, computer science, signal processing, and electrical engineering. Gravitational waves span many orders of magnitude in frequency, from the Hubble-length primordial waves that leave their imprint on the cosmic microwave background (CMB), to the gravitational waves with periods of years detectable by pulsar timing arrays (PTAs) like NANOGrav, the hour-long period waves detectable by space-based instruments such as eLISA, and the millisecond period waves detectable by ground-based interferometers like LIGO and Virgo. PTAs thus complement gravitational wave detectors in other frequency bands, accessing the gravitational wave very low frequency (VLF) band to probe supermassive binary black holes (SMBBHs), whose observations can be used to probe the innermost regions of merging galaxies and perhaps reveal the presence of other exotic phenomena.

Host: Jim Brau



Glimpses of Gut Microbes in their Physical World

Date: Thursday, November 10th, 2016

Speaker: Raghu Parthasarathy, University of Oregon


In each of our digestive tracts, trillions of microbes representing hundreds of different species colonize local environments, reproduce, and compete with one another. Little is known about the physical structure and temporal dynamics of gut microbial communities: how they grow, fluctuate, and respond to perturbations. To address this and investigate microbial colonization of the vertebrate gut, my lab applies light sheet fluorescence microscopy to a model system that combines a realistic live environment with a high degree of experimental control: larval zebrafish with defined subsets of commensal bacterial species. Light sheet microscopy enables three-dimensional imaging with high resolution over the entire intestine, providing visualizations that would be difficult or impossible to achieve with other techniques. I will describe this approach and focus especially on experiments in which a colonizing bacterial species is challenged by the invasion of a second species, which leads to the decline of the first group. We find that responses of bacteria to the mechanical contractions of the gut, and to contact-mediated inter-bacterial killing, can dictate apparent competition between microbes, suggesting a major role for physical mechanisms in guiding the composition of the gut microbiota.


Worldline Methods for Computing Casimir Energies

Date: Thursday, November 3rd, 2016

Speaker: Daniel A. Steck, University of Oregon


The development of new, general methods for the computation of Casimir potentials in arbitrary geometries and for arbitrary material properties remains a difficult problem. I will review our recent work on the world-line method, previously developed for scalar fields coupled to background potentials. This is a path-integral Monte-Carlo method for computing vacuum- and thermal-state energies of the field. Our work focuses on the generalization of this method to electromagnetism. I will review our recent results in considering scalar electromagnetic fields coupled to dielectrics, where we are able to reproduce a number of classic Casimir-potential results within a general framework. I will also briefly review the path towards a generalization to a full vector-electromagnetism formalism.


Searching for the Supersymmetric Partner to the Top Quark at ATLAS

Date: Thursday, October 27th, 2016

Speaker: Stephanie Majewski, University of Oregon


Since 2009, the Large Hadron Collider (LHC) has performed tremendously, delivering copious amounts of proton-proton collision data at center-of-mass energies of 7, 8, and 13 TeV. Analyzing these data, the ATLAS experiment has conducted a broad search for Supersymmetry, a compelling theory for physics beyond the Standard Model. No conclusive evidence of a super-partner has yet been seen, but the discovery of the Supersymmetric partner to the top quark (the so-called “stop”) may still be within reach. I will discuss what we’ve ruled out so far, and an outlook for stop searches in Run 2 of the LHC.


Spinning, Swirling, Twisting: Adventures with Structured Electrons

Date: Thursday, October 20th, 2016

Speaker: Ben McMorran, University of Oregon


Electron wavefunctions with sculpted phase and distribution can provide new insights into quantum behaviour, as well as novel capabilities for electron microscopy. A new tool – nanofabricated diffractive optics – can be used to coherently manipulate electron wavefunctions, analogous to optical wavefront engineering using computer-generated holograms. We use fork grating holograms to shape freely propagating electrons into unique topological states, such as quantum vortices  [1,2]. This electron vortex state is remarkable for its helical wavefront structure and quantized orbital angular momentum (OAM). Free electrons with OAM evolve in magnetic fields in unique ways [3], can be used for nanoscale imaging [4], and provide a new probe for angular momentum at the nanoscale [5]. In addition to producing electrons with OAM, we are developing ways to measure OAM in incoherent superpositions of scattered electrons [6,7]. Investigations of topology, angular momentum, and spatial phase of free electrons has recently lead us to observations of magnetic skyrmion, topologically non-trivial distributions of electron spin, within materials [8,9].


[1]      J. Verbeeck, H. Tian, and P. Schattschneider, Nature 467, 301 (2010).

[2]      B. J. McMorran, A. Agrawal, I. M. Anderson, A. A. Herzing, H. J. Lezec, J. J. McClelland, and J. Unguris, Science 331, 192 (2011).

[3]      G. M. Gallatin and B. McMorran, Phys. Rev. A 86, 12701 (2012).

[4]      P. Ercius, T. Harvey, J. Pierce, J. Chess, M. Linck, and B. McMorran, Microsc. Microanal. 20, 84 (2014).

[5]      T. R. Harvey, J. S. Pierce, J. J. Chess, and B. J. McMorran, Phys. Rev. Lett. under review (2015).

[6]      T. R. Harvey and B. J. McMorran, ArXiv160603631 quant-Ph (2016).

[7]      B. J. McMorran, T. R. Harvey, and M. P. J. Lavery, ArXiv160909124 Phys. (2016).

[8]      J. C. T. Lee, J. J. Chess, S. A. Montoya, X. Shi, N. Tamura, S. K. Mishra, P. Fischer, B. J. McMorran, S. K. Sinha, E. E. Fullerton, S. D. Kevan, and S. Roy, Appl. Phys. Lett. 109, 22402 (2016).

[9]      J. J. Chess, S. A. Montoya, T. R. Harvey, C. Ophus, S. Couture, V. Lomakin, E. E. Fullerton, and B. J. McMorran, ArXiv160806000 Cond-Mat (2016).


Theories and Signals of a “Dark” Photon

Date: Thursday, October 13th, 2016

Speaker: Spencer Chang, University of Oregon


Recently, a new force mediated by “Dark” photons has been the subject of intense interest in high energy physics, both theoretical and experimental, leading to a resurgence in their experimental search with several fixed target experiments operating in the near future.  On the other hand, complementary dark photon signals at colliders are model-dependent, requiring theoretical input to determine the most interesting signals.  In this talk, after a review, I describe a class of theories where the mechanism in which the dark photon mixes with the photon predicts a new particle accessible at the LHC.  After describing the theory, I will discuss our analyses on simulated experimental data, which suggest substantial improvements on existing collider searches for dark photons that decay to electrons.


State of the Department and LIGO Update

Date: Thursday, October 6th, 2016

Speaker: Ray Frey, University of Oregon


Optical Nanofibers: A Platform for Quantum Optics

Date: Thursday, September 29th, 2016

Speaker: Luis A. Orozco, University of Maryland


Nanofibers produced by tapering an ordinary single mode optical fiber to diameters of half a micron are interesting optical objects. Evanescent fields, with large gradients, develop as the radius reaches less than the wavelength of light posing puzzles, questions, and opportunities. The geometry of the nanofiber mode allows for study of quantum optical effects in a one-dimensional configuration, with the preferential evanescent mode of fiber accessible when good adiabatic geometry allows high coupling back into the single mode fiber.

Recent experiments with cold Rb atoms around the nanofiber include the modification of the lifetime of the D2 line in the presence of the nanofiber and its relation to the single atom coupling. We find modification of the lifetime that depends on the alignment of the dipole with respect to the nanofiber: parallel or perpendicular. We also explore collective effects in the decay from the atomic excited that depend on the number of atoms interacting in the evanescent mode of the nanofiber. Polarimetry measurements with trapped atoms show dynamics of the trap atoms with distinctive frequencies in the radial and azimuthal direction.

Host: Michael Raymer