The Physics and Astronomy Colloquia at the University of San Francisco are talks given by invited research scientists, on topics of current interest. The Physics Colloquium Series has been in place since 1994.

All colloquia typically start at 3:30 p.m. and will have light refreshments served. Colloquia take place in CS 303, unless otherwise noted.

Spring 2015

"Gravitational Lensing and Dark Matter in the Universe"

3:30-5 p.m. in LCSI 210
It was only two decades ago that the reaction to the idea of measuring shape distortions of galaxies by gravitational lensing turned from skepticism to enthusiasm. Today, the technique called weak-lensing is routinely used to study dark matter distribution in galaxy clusters, average mass profiles of galaxies, and the large scale structures in the universe. The next decade will be the most exciting era when we start to collect accurate cosmic shear data from billions of galaxy shapes.

I will provide detailed introduction to gravitational lensing while highlighting some of the key milestones in the field. Ambitious future multi-billion dollar projects, such as LSST, EUCLID, and WFIRST will be discussed. I will emphasize that in order to achieve their proclaimed scientific goals, substantial amounts of concerted efforts are required to overcome systematics. Finally, I will present my most recent results obtained from colliding galaxy clusters, which are often dubbed "cosmic particle accelerators."

James Jee was born in Seoul, Korea. He received his PhD at the Johns Hopkins University in 2005 and is currently working as a senior project scientist at University of California, Davis. His expertise is gravitational lensing. In a recent gravitational lensing accuracy competition called GREAT3, he earned the 1st place award. He will move to Seoul in July 2015 to take an associate professor position at Yonsei University.

"Probing Nanoscale Magnetism with Neutron Scattering at the NCNR"

The ability to probe and control magnetic nanostructures is critical to the development of next generation spin-based electronics. Neutron scattering facilities enable magnetic studies impossible to perform elsewhere, and their unique measurement capabilities are very useful to scientists studying magnetism at the nanoscale. In this talk, I will introduce the research reactor at the NIST Center for Neutron Research and explore the basic concepts of neutron scattering. I will discuss some of my favorite recent experiments using polarized neutron beams to understand nanoscale magnetic interactions. Recently, we have used polarized neutron reflectometry to demonstrate direct electric field control of magnetism confined within a single atomic monolayer in complex oxide thin films. In another recent example, we used polarized small neutron scattering to explore magnetic domain structures in high-density arrays of magnetic nanowires.

Alexander Grutter received his PhD in Materials Science and Engineering from the University of California, Berkeley in 2013. He is currently an NRC postdoctoral fellow at the NIST Center for Neutron Research. Current research interests include electric field control of magnetism at thin film interfaces, emergent magnetic properties in complex oxide heterostructures, the role of defects and structural distortions in determining oxide thin film properties, and probing competing interactions in novel magnetic nanostructures.

"Precision Monte Carlo Event Generation for the LHC"

The Geneva Monte Carlo framework is a next-generation event generator for the LHC capable of combining multiple types of higher-order calculations and parton showering in order to increase the precision that is available to experimentalists using these tools. I outline the basic parts of the Monte Carlo program in particle physics, the kinds of problems one encounters when trying to include higher-order effects, and the way that Geneva seeks to address these. Results for e+e- + jets compared to LEP data and for Drell-Yan production are presented.

Calvin Berggren received his PhD in Particle Physics Theory from the University of California, Berkeley in 2014. He is currently a visiting assistant professor at Texas Lutheran University. His research experience includes advancing the state-of-the-art for Monte Carlo event generators used by experimental collaborations at particle colliders, namely the LHC, through improved accuracy, resumption, improved error estimation, increased domain of applicability, etc; developing theoretically-motivated strategies for insightful analysis of collider data; improving capabilities of parton shower tools.

"How to Make a 3D Map of the Universe (and Why)"

I will describe the process of making a 3D map of the universe and our scientific motivations for doing so. Using a relatively small telescope in New Mexico, every night the eBOSS project observes spectra of thousands of galaxies, stars, and quasars, and over the years we build up a 3D map of the locations and motions of millions of objects. Some of these objects are so distant that the light has been traveling to us for 12 billion years (for comparison, the Big Bang was 13.7 billion years ago). Our next generation project, DESI, will use a larger telescope and 5000 little robots to help position fiber optic cables on our focal plane, so that we can observe 50 million objects to make a more complete map. We use these data to study the expansion of the universe and the underlying physics that describe it.

Dr. Stephen Bailey is a project scientist at Lawrence Berkeley National Laboratory where he leads the software development efforts for current and future galaxy redshift surveys. He enjoys converting raw data into useful data in order to study the history and fate of the universe.

"Testing Quantum Mechanics with Ultra-Cold Strontium Atoms"

Quantum field theory successfully describes three of the four known fundamental physical interactions. Its inability to incorporate gravity, however, compels us to continue to probe for subtle deviations to its predictions.

Here, I discuss our plans to build an experiment to search for small violations in one of the most fundamental pillars in the quantum field theory, the spin-statistics, theorem (SST). THe SST states that all particles with integer spin, such as photons, obey the Bose-Einstein statistics (and are thus dubbed "bosons") and all those with half-integer spin obey Fermi-Dirac statistics (and are thus dubbed "fermions"). Using ultra-cold strontium atoms, we will test if photons can sometimes behave like fermions by looking for a transition between two angular momentum states (J=0 and J'=1)-- a behavior forbidden by the SST. Such a discovery would provide a new direction for the theoretical development of the basic axioms of quantum field theory and reveal an essential ingredient to a more fundamental view of the world.

I received my PhD from UC Berkeley in 2012. For my graduate work, I studied quantum magnetism and quantum phase transitions in rubidium Bose-Einstein condensates. After graduating, I took a post-doctoral position at Sandia National Laboratory in Livermore, California, where my research centered around using ultra-cold atoms as remote sensors. Then in the fall of 2013, I took on a position as an assistant professor in physics at CSU East Bay, where I am currently leading an experiment aimed at searching for small violations in the spin-statistics theorem.

"One Exciton, Two Exciton, Red Exciton, Blue Exciton: Exciton-Exciton Interactions in Graphene Quantum Dots"

As an electron and a proton bind to form a hydrogen atom, a conduction-band electron and a valence-band hole in a crystal bind to form an exciton. As a pair of hydrogen atoms bind to form a hydrogen molecule, so too, a pair of electrons and a pair of holes can bind to form a biexciton. Exciton-exciton interactions are of fundamental importance in understanding many-body effects in semiconductors systems, but they are also important for potential applications ranging from optical gain to solar energy conversion. In nanoscale systems, the binding of biexcitons can be enhanced through quantum confinement and reduced dielectric screening. At the same time, relaxation of momentum conservation and confinement of carriers allow for rapid, non-radiative recombination of biexcitons in nanoscale quantum dots. In this talk, I will discuss my group's collaborations to understand and engineer exciton-exciton interactions in colloidal quantum dots. I will focus on our recent work in graphene quantum dots in which the two-dimensional lattice of light atoms leads to weak screening of the Coulomb interaction and consequently yields strong interactions between excitons. The unique linear dispersion of the Dirac cones in grapheme mean that results from studies of quantum dots may also illuminate open questions about expanded graphene.

John McGuire earned his BS in Biological Sciences and Physics from Stanford University and his PhD in Physics from the University of California, Berkeley, where he worked with Y. Ron Shen on electronic and vibrational dynamics at surfaces ranging from silicon to water. As a postdoctoral fellow with Victor Klimov at Los Alamos National Laboratory, he collaborated on unraveling the controversy over multiple-exciton generation in colloidal semiconductor nanocrystal quantum dots. In 2009, John joined the faculty of Michigan State University, where his group's research is focused on understanding interactions an non-equalibrium states at surfaces and in quantum confined systems through their dynamics and nonlinear optical response.

"Can Evolution be Understood Quantitatively?"

The basic laws of evolution have been known for more than a century and there is overwhelming evidence for the facts of evolution. Yet little is understood quantitatively about the dynamical processes that drive evolution: by physicists' standards the theory of evolution is far from fully-fledged. Huge advances in DNA sequencing technology and laboratory experiments have enabled direct observations of evolution in action and, together with theoretical developments, opened up great opportunities for dramatically advancing our understanding. This talk will focus on framing questions and the challenges to be faced, along with recent progress on addressing some of these.

Daniel S. Fisher is David Starr Jordan Professor of Science at Stanford University. He was educated at Cornell and Harvard, started his career at AT&T Bell Laboratories, and subsequently held faculty positions at Princeton and Harvard. Fisher's research has mostly been in theoretical statistical physics, including superconductivity, spin glasses, and earthquakes; but over the past ten years has shifted to biology, particularly dynamical processes in cells and evolutionary dynamics. He has also been actively involved in public policy, especially arms control and energy.

"Building HERA from PAPERclips and Supercomputers"

Measuring 21 cm hyperfine emission from neutral hydrogen at cosmological distances is one of the most promising techniques for probing our early universe. In the cosmic dawn of our universe, this signal is sensitive to myriad cosmological and astrophysical processes as the first stars and galaxies heat and ionize the intergalactic medium. Recently, the Precision Array to Prob the Epoch of Reionization (PAPER) has overcome the key technical hurdles facing 21cm reionization experiments to place physically constraining upper limits on the cosmological signal. These limits imply a level of heating of the intergalactic medium inconsistent with a rapid decrease in star formation rate density at high redshifts and inconsistent with lower prescriptions relating X-ray luminosity to star formation rate.

Building on these successes, the US community has coalesced around a next-generation experiment for exploring cosmic reionization via 21cm emission. The Hydrogen Epoch of Reionization Array (HERA) will be a large array of zenith-pointing parabolic dishes optimized for power spectral measurements. HERA's considerable collecting area enables it to precisely measure ionization fraction versus redshift, to directly image larger ionization bubbles, and to probe heating in pre-reionization epochs. Phase i of HERA was recently funded and construction has already begun.

Dr. Parsons is currently an Assistant Professor in the Astronomy Department and a member of the UC Berkeley Radio Astronomy Laboratory. He received an A.S. from Colorado Northwestern Community College in 1998 while in high school, and a B.A. from Harvard in 2002 in physics and mathematics working with Paul Horowitz. He worked as a development engineer at the Space Sciences Laboratory from 2002 to 2004 with Dan Werthimer. He received his Ph.D. in 2009 from UC Berkeley working with Don Backer, spending the years from 2007 to 2009 as a predoctoral researcher at Arecibo Observatory. He spent two years as an NSF postdoctoral fellow and honorary Charles Towns fellow at UC Berkeley before joining the faculty at Berkeley in 2011.