Spring 2010 Physics Colloquium Schedule

University of Colorado Department of Physics

Colloquia are Wednesdays at 4 PM, Duane G-1B20 unless otherwise noted

Coffee, tea, and cookies before regular colloquia at 3:45 PM in Duane G-1B31

Upcoming Colloquia:

• April 28
• Doug Tussaint, University of Arizona 
• Host: Tom DeGrand
• Title: Throwing darts to weigh a quark
• Abstract:  Monte Carlo calculations are a way to calculate properties of protons and other hadrons beginning with the fundamental theory of QCD. I will explain the ideas behind these calculations, and describe one particular large program of computations.  To pick an easily understood question, I will focus on finding the masses of the quarks. These quark masses are fundamental parameters of the standard model, which is another way of saying we can't yet explain them. They vary over almost five orders of magnitude, and are not even precisely known. To test any theory which predicts them, or maybe to inspire such a theory, we would like to know these masses. It is up to the audience to find the theory that explains these numbers.

Colloquia that have already occurred:

• January 13
• Charlie Pellerin, NASA
• Host: David Bartlett
• Title: Think You Can Ignore Contexts? Hubble's Original Flawed Mirror Might Wake You UP.
• Abstract: Former NASA's Director for Astrophysics describes the space mission to fix the Hubble Space Telescope's flawed mirror with entertaining stories and ilustrations of the importance of team-building processes based on his book [1]. 
• [1]. Charlie Pellerin. How NASA builds teams: mission critical soft skills for scientists, engineers, and project teams. (John Wiley & Sons, 2009).
• January 20
• Tom Lubensky, University of Pennsylvania
• Host: Ivan Smalyukh
• Title: On Rotational and Translational Brownian Motion
• Abstract: Einstein’s 1905 paper on Brownian motion ushered in a new era in statistical physics. It provided a description of the peculiar random motion of micron-sized particles dispersed in water in terms of collisions with much smaller water molecules and a prescription for determining Avogadro’s number through measurements (later carried out by Jean Perrin) of the mean-square displacement of the dispersed particles.  Perrin’s measurements were instrumental in establishing the molecular nature of matter. In 1906, Einstein introduced the concept of rotational Brownian motion. This talk will discuss recent experiments by the Penn group [1] on coupled translational-rotational Brownian motion of rigid rods and interpret them in terms of a Langevin theory originally developed by Francis Perrin (son of Jean Perrin). It will discuss in particular crossover from anisotropic to isotropic diffusion and non-Gaussian probability distributions.  Finally it will generalize concepts of rotational Brownian motion to treat the dynamics of a non-equilibrium granular gas of chiral objects.
  
  [1] Han, Y.,  Alsayed, A. M., Nobili, M.,  Zhang, J., Lubensky, T. C., and Yodh, A. G., Brownian Motion of an Ellipsoid, Science 314, 626-630 (2006)
• January 27
• Boris Svistunov, University of Massachusetts, Amherst 
• Host: Victor Gurarie
• Title: Supersolidity of Helium-4
• Abstract: A supersolid is a solid that can conduct its own atoms without friction. The supersolidity of He-4 is one of the biggest puzzles in the modern low-temperature physics, and a subject of intensive experimental and theoretical studies during last 6 years, following Kim and Chan’s discovery that solid He-4 decouples from a torsional oscillator. First-principles numeric simulations exclude supersolidity of a perfect He-4 crystal, while suggesting a number of scenarios of disorder-induced supersolidity. In particular, simulations reveal superfluidity in the cores of dislocations, shedding a considerable light on the effect of giant isochoric compressibility recently observed by Hallock and Ray in the experiment on DC supertransport  in solid He-4.  Despite impressive experimental and theoretical progress of recent years, the field still remains highly controversial.
• February 3
• Tim Stelzer, University of Illinois at Urbana-Champaign 
• Host: Michael Dubson
• Title: Multimedia Pre-Lectures: On-line content designed to improve in-class learning  
• Abstract: The Department of Physics at the University of Illinois has a long tradition of innovation in undergraduate education. From PLATO (the first computer-based learning system), to Peer Instruction with i>clickers, we have worked to develop and implement effective instructional practices for large-enrollment introductory physics courses. This talk will focus on our most recent innovation--- prelectures. These short (<20-min) narrated Flash animations are designed to introduce students to the fundamental concepts before they attend lecture, so they are better prepared to participate and learn in lecture. In addition to screening a typical prelecture, results from our published clinical studies, eyetracking studies, and student outcome data from the use of prelectures in our introductory E&M course will be presented.
• February 10
• Daniel Baker, LASP, University of Colorado at Boulder
• Host: Martin Goldman
• Title: The Earth’s Van Allen Radiation Belts: Old Questions and New Missions
• Abstract: This talk will describe the structure and underlying physics of the Earth’s Van Allen radiation belts. It will provide a brief history of the discovery of the Van Allen belts and will discuss the evolution of modern theoretical understanding of the inner and outer radiation zones. Recent experimental observations of trapped particle enhancement events and loss processes will also be presented. Current issues in radiation belt physics such as particle acceleration mechanisms, wave-particle interactions, magnetosphere-atmosphere coupling, and ring current control of magnetic field properties will be addressed. The talk will include a description of the Radiation Belt Storm Probe (RBSP) mission of NASA’s Living With a Star (LWS) program. The design and operation of our new, highly capable Relativistic Electron-Proton Telescope (REPT) instrument being build at CU/LASP for the RBSP mission will be presented. I will summarize with key questions and methods to obtain scientific closure on these outstanding issues. xxx
• February 17
• Anna Hasenfratz, University of Colorado at Boulder
• Host: Mihaly Horanyi
• Title: Is there gold at the end of the rainbow? Looking beyond the Standard Model with Technicolor
• Abstract: The Standard Model is one of the most successful models of elementary particle physics, unifying electromagnetic and weak  interactions. At the heart of the model is electroweak symmetry breaking and the prediction of a scalar boson, the Higgs particle. Despite of the amazing agreement between the model's predictions and experimental observations, we know that the Standard Model is not a stand-alone theory: at high energies new interactions have to emerge, and an elementary Higgs particle is very unlikely. One of the theoretical candidates of the new physics beyond the Standard Model is technicolor. While technicolor models have been around for many years, only recently became possible to study these strongly coupled models from first principles, using numerical simulations. In this talk I will give an elementary introduction to technicolor theories and describe some of the challanges theorists working with these models face.
• February 24
• Subir Sachdev, Harvard University
• Host: Victor Gurarie
• Title: Quantum criticality, the cuprate superconductors, and the AdS/CFT correspondence
• Abstract: I will begin with a simple introduction to the theory of quantum criticality, as applied to experiments on certain insulating antiferromagnets. I will then survey the phenomenology of the cuprate high temperature superconductors, and show how ideas from quantum criticality have helped explain or predict the results of a number of recent experiments. The applications to the cuprates focus attention on key problems associated with the criticality of Fermi surfaces in two dimensions which remain unresolved. I will discuss how these open problems are being addressed by the AdS/CFT correspondence discovered in string theory.
• March 3
• Jack Gosling, LASP, University of Colorado at Boulder
• Host: Mihaly Horanyi
• Title: Magnetic Reconnection as Revealed by Observations in the Solar Wind
• Abstract: Magnetic reconnection is a physical process that changes magnetic field topology in a plasma and ultimately converts magnetic field energy to bulk flow energy and plasma heating. It plays a central role in a wide variety of solar, space, astrophysical and laboratory phenomena. The relatively recent discovery that reconnection commonly occurs at thin current sheets in the solar wind has opened up a new and important laboratory for studying this fundamental plasma process and its after-effects. This talk provides an overview of some of the new insights on magnetic reconnection derived from observations of reconnection exhaust jets in the solar wind. 
• March 10
• Ka Yee Lee, University of Chicago
• Host: Ivan Smalyukh
• Title: Beyond Wrinkles: Stress Relaxation in Lipid Monolayers and Other Elastic Thin Films
• Abstract: Surfactants at air/water interfaces are often subjected to mechanical stresses as the interfaces they occupy are reduced in area. The most well characterized forms of stress relaxation in these systems are first order phase transitions. However, once chemical phase transitions have been exhausted, the monolayer undergoes global mechanical relaxations termed collapse. We have previously demonstrated that for lung surfactants, a mixture of lipids and proteins that coats the alveoli to reduce the work of breathing, collapse manifests itself as protrusions of folds into the subphase. These folds remain attached to the monolayer and reversibly reincorporated upon expansion. By studying different types of monolayers, we have shown that this folding transition in monolayers is not limited to lung surfactant films, but rather represents a much more general type of stress relaxation mechanism. Our study indicates that collapse modes are found most closely linked to in-plane rigidity. We characterize the rigidity of the monolayer by analyzing in-plane morphology on numerous length scales. More rigid monolayers collapse out-of-plane via a hard elastic mode similar to an elastic membrane, with the folded state being the final collapse state, while softer monolayers relax in-plane by shearing. For the hard elastic mode of collapse, we have further demonstrated experimentally and theoretically that the folded state is preceded by a wrinkled state.
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• March 12 (SPECIAL LASP/PHYSICS/ASEN Colloquium)
• Dave Brain, University of California at Berkeley
• Host: Mihaly Horanyi
• Note different time/location: 12 PM on Friday,  March 12, 2010, DUANE D-142
• Title: The Ins and Outs of Martian Mini-Magnetospheres
• Measurements of magnetic fields and charged particles near Mars made over the past four decades have taught us about its plasma environment, upper atmosphere, near-surface radiation environment, subsurface, and deep interior.  The upper atmosphere and plasma environment of Mars are of interest because they are the sites of energy exchange between the planet and its surroundings, dominated by the Sun and solar wind.  For this reason they may have played a critical role in martian climate evolution.  A number of recent spacecraft observations demonstrate that the exchange of particles and energy between the solar wind and atmosphere is particularly dynamic at Mars because strong localized  crustal magnetic fields form mini-magnetospheres that rotate with the planet, influencing the motion of charged particles.  I will discuss two observed influences of crustal fields on particle motion near Mars and their implications: episodic escape of atmospheric particles via detached crustal fields and localized energy deposition characterized by ultraviolet aurora on the Martian night side.

• March 17
• Priscilla Cushman, University of Minnesota
• Host: Alysia Marino
• Title: Elusive Wimps: Searching for Particle Dark Matter
• Abstract: Recently our experiment, the Cryogenic Dark Matter Search (CDMS), found two candidate events in a blind analysis of our final data run. Whether or not we observed the passage of weakly-interacting massive particles (WIMPs) in our detector will have to wait for confirmation from larger installations, such as the now-running SuperCDMS experiment. However, there is compelling indirect evidence for the existence of dark matter. I will review the evidence for dark matter, the direct detection experiments which are looking for it, and our recent results.  
• March 18 (SPECIAL LASP/PHYSICS/ASEN Colloquium)
• Uwe Konopka, Max-Planck-Institut f¨ur extraterrestrische Physik, Germany
• Host: Mihaly Horanyi
• Note different time/location: 4 PM on Thursday, March 18, 2010, DUANE D-142
• Title: Complex Plasmas – From the Laboratory to Experiments on the International Space Station
• Dusty plasmas, or often called ”complex plasmas” have been studied for decades mainly related to plasma processing or astrophysical environments. 1994 an unfamiliar, ordered state of micro particles in a low temperature plasma environment, the so called ”plasma crystal” was discovered. As a result, the investigation of dusty plasmas was strongly intensified. The behavior of the charged particles within any kind of plasma environment, like low temperature rf, dc or atmospheric pressure plasma is now looked at. Especially the collective effects of larger particle systems with millions of fine particles, distributed isotropically in three dimensions gained much interest. Investigations include the study of wave phenomena, instabilities, particle flows, crystal structures as well as phase transitions, to name a few examples. All those phenomena can be studied, due to the nature of the complex plasmas, on the fundamental kinetic scale of individual particles. Compared to colloidal systems, which have a similar nature, complex plasmas cover also the hole dynamic range from over-damped to undamped system, what makes them unique. A significant knowledge in various research fields might be gained as a result of the interdisciplinarity of complex plasmas experiments. However, due to anisotropic forces acting on the negatively charged fine particles under earthbound conditions, it is difficult to establish an isotropic three dimensional system in the laboratory. In general, the fine particles are confined within the plasma volume due to strong electric fields that push the particles towards the plasma bulk out of the plasma sheath region. In balance with the ”strong” gravitational force, large particles tend to settle close to a lower sheath boundary, leading to vertically compressed particle clouds. For particles in the tens of micro meter range gravity can lead to a complete collapse of the particle cloud towards a mono-layer system, which of course is also of fundamental interest and thus studied extensively too. But, to study isotropic homogeneous fine particle plasmas, it is necessary either to use small sub-micro meter particles, which have the disadvantage that in general conditions their dynamic is over-damped, or to avoid the gravitational influence by performing experiments in microgravity environments, as are supported for example by drop tower experiments, parabolic flights or on the International Space Station (ISS). In the course of the talk, I will show examples of earth bound complex plasma experiments as well as some that were performed in parabolic flights or the Space Station experiments PKE-Nefedov or its successor PK-3Plus, that is operational since 2006.
• March 24
• Spring Break, no Colloquium

• March 31
• Francis Robicheaux, Auburn University 
• Host: Scott Robertson
• Title: Making and Holding(?) Anti-Hydrogen: Theory & Experiment
• Abstract: Recently, two groups succeeded in making and detecting the anti-matter version of the hydrogen atom. I will describe the basic features of these experiments and what is known about the properties of the anti-hydrogen atoms. The anti-hydrogen is formed in a peculiar environment: a cold plasma in a very strong magnetic field. We have performed several calculations that give insight into how the anti-hydrogen is formed. We also predict most of the important properties of the atoms. I will finish with a discussion of the next generation experiments, just started at CERN, that will attempt to trap the anti-hydrogen.

• April 1 (SPECIAL LASP/PHYSICS/ASEN Colloquium)
• Sascha Kempf, Max-Planck-Institute for Nuclear Physics, Heidelberg, Germany
• Host: Mihaly Horanyi
• Note different time/location: 4 PM on Thursday April 1, 2010, DUANE  D-142
  
• Title: The Ins and Outs of Martian Mini-Magnetospheres
• In the light of the Cassini mission to Saturn, the moon Enceladus turned out to be one of the most intriguing bodies in the solar system. Data returned by several instruments on the spacecraft provide compelling evidence that this moon is unusually active and is capable of maintaining a pronounced ice volcanism. In particular, measurements of the spatial distribution of the plume particles recorded by Cassini's dust detector CDA provided the first evidence for a local source of ice grains in the moon's south polar terrain. Data returned by the neutral gas spectrometer INMS, the UV camera UVIS, and images by the Cassini camera led to the surprising conclusion that the plume particles are expelled more slowly than the water vapor from the moon's interior although the grains were expected to be tightly bound to the gas flow within the vents.  By assuming that the grains' motion is strongly affected by collisions with the vents' walls, a model proposed by Schmidt et al. matches all plume data available so far. The most important conclusion of this model is that the temperature at the bottom of the cracks must be 260K or higher, suggesting the possibility of a subsurface water reservoir. The discovery of a ring particle  population rich on sodium salts, which can arise only if the plume particles  are formed from liquid water, strongly supports the existence of a large liquid water reservoir in contact with Enceladus' rocky core. 

• April 7
• Joshua Shaevitz, Princeton University 
• Host: Thomas Perkins 
• Title: Self organization in biology: How cells achieve physical goals
• Abstract: At all levels, biological organisms exhibit collective phenomena, where large-scale behaviors arise from the action of local players. Our research focuses on the emergence of cellular- and population-level order in bacteria, the most genetically diverse group of living organisms on our planet. Cells use a number of strategies to produce specific shapes and mechanical properties. These features are essential to a cell's ability to weather a large variety of environmental stresses and they play an important role in how many bacteria move. I will discuss our recent measurements of the interplay between bacterial mechanics, cell shape and motility. I will also touch on our development of new techniques that combine elements of single-molecule biophysics, super-resolution microscopy and force application in live cells.

• April 14
• Roger Stuewer, University of Minnesota 
• Host: Allan Franklin 
• Title: Nuclear Disintegration and the Cambridge-Vienna Controversy  
• Abstract: I trace the origin, development, and surprising resolution of a controversy during 1922-1928 between Ernest Rutherford and James Chadwick at the Cavendish Laboratory in Cambridge and Hans Pettersson and Gerhard Kirsch at the Institute for Radium Research in Vienna. This controversy centered on the questions of which nuclei can be disintegrated by alpha particles with the emission of protons, whether these protons can be observed under particular experimental conditions, and how this disintegration process should be interpreted theoretically. These scientific issues became entangled in a web of personal and institutional rivalries that greatly raised the stakes in the outcome of the controversy, conditions that illustrate how physics functions in an intensely competitive atmosphere.  

• April 21
• Andrea Liu, University of Pennsylvania 
• Host: Ivan Smalyukh
• Title: The Physics of Cell Crawling
• Abstract: When a cells crawls, its shape must change.  This is accomplished primarily via reorganization of the protein actin. During cell crawling, various processes drive the self-assembly of actin from isolated nm-scale monomers into a network of filaments. The same far-from-equilibrium self-assembly process comes into play when the bacterial pathogen Listeria monocytogenes infects a cell. The bacterium hijacks the host cell's actin machinery to create an actin network that propels the bacterium through cells and into neighboring cells. I will discuss recent results from Brownian dynamics simulations that suggest a new picture for the physical mechanism underlying this form of motility.

Colloquium schedules from previous semesters can be found here