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Department of Physics and Astronomy
Seminars/Colloquia, Fall 2009

Unless noted otherwise, Tuesday Colloquia are at 4:00 pm
with refreshments served 15 minutes before each colloquium.

Scheduling for additional seminars will vary.

Abstracts of Talks

Marco Cavaglià Department of Physics and Astronomy University of Mississippi Astronomy's New Messengers: Engaging a Young Adult Audience in Science with a Traveling Exhibit The LIGO exhibit “Astronomy's New Messengers: Listening to the Universe with Gravitational Waves” is traveling to colleges, universities, museums and other public institutions throughout the United States. In 2010, an extended version of this exhibit will appear in a New York City venue that is accessible to a large and diverse cross section of the general public. “Astronomy's New Messengers” will primarily communicate with a late adolescent and young adult audience, potentially inspiring them into the field of science. Acknowledging that this audience is traditionally a difficult one to attract, the exhibit will publicly announce itself in a charismatic fashion to reach its principal goals of broadening the community of people interested in science and encouraging interest in science among young people, in particular.

Douglas Safarik Material Science and Technology Division Los Alamos National Laboratory Low-Energy Vibrational Excitations in Amorphous and Crystalline Metals The heat capacity of amorphous solids, when plotted as Cp/T3 vs. T, shows a distinct hump at approximately θDebye/30. This hump, which is ubiquitous in glasses, is often referred to as the “Boson Peak”. A common misconception is that the Boson Peak is a unique signature of the amorphous state. Recently, however, we demonstrated that a hump in Cp/T3 vs. T is not unique to amorphous solids. Indeed, all metallic elements (fcc, bcc, and hcp) show a similar hump in their Cp/T3 vs. T plots, arising from the dispersion of acoustic phonons as the Brillouin zone boundary is approached, and, in hcp elements, from the excitation of optical phonons. For both crystals and glasses, the temperature and the amplitude of the Boson Peak can be explained in terms of the elastic moduli, which are directly related to the properties of the phonon dispersion curves at k = 0. Surprisingly, we find no clear difference between the energy and the number of Boson Peak modes in metallic glasses, and the energy and number of modes in metallic crystals. To further investigate low-energy excitations in glasses and crystals, we have measured the heat capacity, phonon density of states, elastic constants, and thermal expansion for an amorphous alloy and a single crystal having exactly the same composition, Pd40Cu40P20. To our knowledge, Pd40Cu40P20 is the only alloy that can be prepared as a bulk metallic glass and as single crystal, and thus offers a unique opportunity to compare the two. Not surprisingly, the Cp/T3 vs. T hump and the phonon density of states for crystalline Pd40Cu40P20 are similar to those for amorphous Pd40Cu40P20. However, we do identify some differences at ~ 4 meV, which we tentatively ascribe to vibrational states that are unique to the glass. Surprisingly, below ~50 K the elastic constants of the glass show a different temperature dependence than those of the crystal. This elastic anomaly cannot be explained in terms of the ~4 meV modes, and suggests the presence of a different type of low-energy excitation in amorphous solids.

Dava Sobel Celebrating 400 Years of Galileo and His Telescope The Sally McDonnell Barksdale Honors College Fall Convocation features an address by Dava Sobel, who plans to discuss “Celebrating 400 Years of Galileo and His Telescope.” Sobel has spent her career telling the fascinating stories of scientific discoveries and the people who made them. Perhaps her best-known works are “Longitude,” a best-seller that relates the story of the lone genius who solved the greatest scientific problem of the time, determining longitude; and the 2000 Pulitzer Prize finalist “Galileo's Daughter,” which is based on 124 surviving letters to Galileo from his eldest child. A former New York Times science writer, Sobel also has written several other books, articles for national magazines and a stage play about 16th-century astronomer Nicolaus Copernicus.

Peter Sonnek Department of Physics and Astronomy University of Mississippi Physics at the SLAC B Factory The standard model is an effective theory which successfully reduces the zoo of particles to a few fundamental particles and their interactions. With increasing luminosities and energies in particle colliders, the experimental physics community has made several attempts to find deviations of the standard model in their recorded data. Such deviations could mean the discovery of “new physics”. One area of interest is the precise measurement of the CKM matrix elements which describe weak decay processes in the quark sector. The CKM matrix is the source for CP violation in the standard model and is required to be unitary. Precise measurements of its elements could show a deviation from its postulated unitarity and therefore imply effects beyond the standard model. This talk describes a brief theoretical background of the CKM matrix and the necessary theoretical concepts for the measurement of its specific element Vcb. It describes the BaBar experiment at SLAC and closes with a real measurement of Vcb from data.

German Valencia Department of Physics and Astronomy Iowa State University A Couple of Hiccups in Higgs Physics I review existing theoretical and experimental constraints on the Higgs boson mass, emphasizing that very light and very heavy Higgses are still viable, particularly in multi-Higgs models. I discuss a recent HyperCP observation that reignited interest in rare decays as hunting grounds for very light Higgses. I also discuss some unique signatures in top-quark physics at the LHC that can occur with very heavy Higgses.

Deirdre Shoemaker School of Physics Georgia Institute of Technology Lessons on Astrophysics and Gravity from Numerical Relativity Computer simulations of compact object binary systems have become an essential tool to investigate gravitational phenomena in the non-linear regime of gravity. These simulations have unveiled phenomena impacting both gravitational physics and astrophysics. In this talk, I will present examples of computational results that have influenced or changed our understanding of astrophysics and gravity. Included is the role numerical relativity has begun to play in gravitational wave astronomy, recent results from our numerical code of black holes embedded in a gaseous cloud and how our simulations are revealing the characteristics of the end-state of compact object collisions.

Pablo Laguna School of Physics Georgia Institute of Technology Black Holes: Their Birth and Violent Interactions Einstein's theory of general relativity describes gravity using an elegant but complicated set of equations. Simply put, General Relativity is a description in which the effects of gravity are viewed as due to the curvature of spacetime. Black holes are arguably the most interesting and exotic example of how the curvature of spacetime expresses itself, and their study is no longer an academic exercise. Astronomers have found indirect evidence of their existence from observations of the motion of and light from matter captured by these objects. However, a direct detection has evaded us so far. The ultimate proof would be observing distortions in the fabric of spacetime from the dance of two black holes orbiting each other. Such a detection is a formidable undertaking, requiring innovative engineering, powerful data analysis tools and careful computational modeling. I will present an overview of the current understanding of how these exotic objects form and the effort to simulate, with the most powerful supercomputers available, what happens when they violently collide.

Mark Stanley Terrestrial Gamma-Ray Flashes and Lightning Terrestrial Gamma-Ray Flashes (TGFs) were first discovered in the 1990s by the Burst and Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory (CGRO) satellite. They were quickly attributed to lightning, though the details of their production was unknown. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) greatly expanded our knowledge of TGFs with its more sensitive detectors. It soon became clear from analysis of RHESSI data that TGFs were likely produced near the tops of thunderclouds. In this talk, I will show broadband electric field data collected in 2004 and 2005 from multiple sites which shows that most TGFs are correlated with normal polarity intracloud events. Furthermore, in a couple of cases, the altitude of the events could be discerned from ionospheric reflections. Also, one of the TGFs immediately preceded a narrow bipolar event which is known to occur at the start of flashes, suggesting that the TGF itself might have initiated the flash.

Xinmai Yang Mechanical Engineering University of Kansas Photoacoustic Imaging in Biological Tissue Photoacoustic imaging technology is one of the fastest growing areas among all imaging modalities in past ten years. Photoacoustic imaging is highly sensitive to the optical absorption of biological tissue, and combines the advantages of purely optical imaging and ultrasound imaging to provide optical contrast with ultrasound resolution in regions up to 5 cm in soft tissue. In other words, photoacoustic imaging can provide greater spatial resolution than purely optical imaging in deep regions while simultaneously overcoming the disadvantages of ultrasonic imaging regarding both biochemical contrast and speckle artifact. Photoacoustic imaging has been successfully applied to the visualization of different structures in biological tissues. This talk will summarize the application of photoacoustic imaging on small animal brain imaging, including structural imaging, functional imaging, and molecular imaging. Other applications such as photoacoustic imaging on skin cancer and imaging-guided therapy will also be discussed.

Umberto Cannella Department of Theoretical Physics, University of Geneva, Switzerland Extracting the Three- and Four-Graviton Vertices from Binary Pulsars and Gravitational-Wave Observations of Coalescing Binaries We discuss how various tests of general relativity (GR) can be translated into measurements of its non-linearities from a field theory point of view. We reformulate the post-Newtonian approximation to the two-body problem in terms of Feynman diagrams and we try to constrain the strength of the three- and four-graviton vertices. In problems involving only the conservative dynamics of a system, a deviation of the three-graviton vertex from the GR prediction is bounded, to lowest order, by Lunar Laser Ranging experiments, which measure it at the 0.02% level. A modification of the three-graviton vertex also affects the radiative sector of the theory; the timing of the Hulse-Taylor binary pulsar provides a bound on this correction at the 0.1% level. Tests of the four-graviton vertex would require gravitational-wave observations. Preliminary results on coalescing binaries at interferometers suggest that the effects of modified three- and four-graviton vertices are degenerate with other parameters of the system, such as the masses and spins of the compact objects.