# Physics

## Seminars in Fall 2015

All seminars are held at 4:10 PM in Bio 19, unless otherwise noted.
Refreshments will be served at 4:00 PM.

#### Upcoming Seminar

December 2, 2015
Christopher Lee ‘00, Los Alamos National Laboratory
"Taming Infinities: Resumming Large Logs in Quantum Field Theory"

Quantum chromodynamics (QCD) and the Standard Model are quantum field theories (QFTs) that are beautifully elegant in their formulation and have been enormously successful in predicting virtually all observed phenomena in collider physics experiments in the past several decades. Perturbation theory—computing observables as expansions in small coupling constants g— is a wonderful tool for making precise predictions—when it works. For many observables, standard perturbation theory woefully breaks down and predicts wildly incorrect results. In this lecture I will discuss one such type of breakdown and its resolution. This occurs in cross sections when particles in the final state of a high-energy collisions are confined to be narrowly collimated in bunches or cones calledjets. At every order in perturbation theory g^n, large logarithms of the small size of these jets appear in the coefficients of g^n, ruining the convergence of the expansion. The solution is resummation—reorganizing the series so the logs are resummed to all orders ing, effectively capturing the radiation of arbitrarily many particles such as gluons or photons from jets. I will illustrate the concepts and tools behind the art of resummation in a QFT such as QCD.
Aug 31 Thesis Start-Up Meeting Student summer research talks Student summer research talks Michael Kellman, Institute of Theoretical Science and Department of Chemistry and Biochemistry, University of Oregon"Quantum Thermodynamics, Entropy of the Universe, Free Energy, and the Second Law"We take the view that the standard von Neumann definition, in which the entropy $$S^{vN}$$ of a pure state is zero, is in obvious conflict with the statement of the second law that the entropy of the universe $$S_{univ}$$ increases in spontaneous processes, $$\Delta S_{univ} > 0$$. In a spirit similar to von Neumann himself in lesser-known work, we seek an alternative definition of the entropy of the universe that is in accord with the second law. We perform simulations of time dependent dynamics for a model quantum system becoming entangled with a quantum environment. We test the new definition of the entropy of the system-environment universe against the standard thermodynamic relation $$\Delta F_{sys} = - T \Delta S_{univ}$$, calculating system properties using the reduced density matrix and standard von Neumann entropy. Good agreement is obtained, showing the compatibility of an entropy for a pure state of a universe with the statement of the second law and the concept of free energy. Interesting deviation from microcanonical behavior within the zero order energy shell is observed in a context of effectively microcanonical ergodization within the much larger total basis of the time dependent universe. Shep Doeleman ’86, MIT Haystack Observatory"The Event Horizon Telescope: Imaging and Time-Resolving a Black Hole"A convergence of high bandwidth radio instrumentation and Global mm and submm wavelength astronomical facilities are enabling assembly of the Event Horizon Telescope (EHT): a short-wavelength Very Long Baseline Interferometry (VLBI) array, which can observe the nearest supermassive black holes with Schwarzschild Radius resolution. Initial observations with the EHT have revealed event horizon scale structure in SgrA*, the 4 million solar mass black hole at the Galactic Center, and in the much more luminous and massive black hole at the center of the giant elliptical galaxy VirgoA. Over the next 2-3 years, this international project will add new sites and increase observing bandwidth to focus on astrophysics at the black hole boundary. EHT data products will have an unprecedented combination of sensitivity and resolution with excellent prospects for imaging strong General Relativistic signatures, detecting magnetic field structures through full polarization observations, time-resolving black hole orbits, testing GR, and modeling black hole accretion, outflow and jet production. This talk will describe the project and the latest EHT observations. Student summer research Regina Jorgenson, Department of Physics, Willamette University"From the Shadows to the Light: Revealing the Mysteries of Galaxy Formation and Evolution using Damped Lyman alpha Systems"Since their discovery over 30 years ago, the true nature of damped Lyman alpha systems (DLAs) has remained a mystery. Notoriously difficult to detect directly in emission, DLAs are typically identified by their large equivalent width absorption features in the spectra of background quasars. Thanks to the Sloan Digitized Sky Survey (SDSS), over a thousand DLAs are now known and their absorption line properties well studied. Given their large column densities of neutral hydrogen gas, DLAs are believed to be the reservoirs of neutral gas for star formation across cosmic time. Evolution in DLA metallicity with redshift as well as the inferred star formation rates measured in roughly half of the DLA population indicate the presence of on-going star formation. However, numerous efforts to directly detect the galaxies that host these neutral gas-rich absorbers have failed. I will discuss the difficulties of this endeavor and the recent technological advances, namely Laser Guide Star Adaptive Optics (LGSAO) and the Keck/OSIRIS Integral Field Spectrograph, that have made possible significant strides in this field. I will present the first spatially resolved, direct detection of a DLA host galaxy and discuss the implications for the field of galaxy formation and evolution. Combining the power of absorption line and emission line diagnostics has the potential to yield unprecedented insight into the physics of high redshift galaxy formation. I will conclude by looking ahead to what advances near-future observational facilities such as JWST and ALMA will bring. Fall Break Davide Lazzati, Department of Physics, Oregon State University"Observing Gamma-Ray Bursts with a Computer"Gamma-Ray Bursts are intense flashes of high-energy radiation, up to a sextillion times brighter than our sun at the peak of their emission. They are produced by massive fastly spinning stars at the end of their main sequence evolution, when the dying stellar core collapses into a newly born black hole. Some bursts are also thought to produce powerful neutrino bursts and distinctive chirps in gravitational waves. In this talk I will introduce the physics and observations of Gamma-Ray Bursts, discuss their role as multi-messenger objects, and describe the accomplishments and recent advances in computer simulations of such extreme events. Janet Tate, Department of Physics, Oregon State University"The Search for New Properties: Into Metastable Space"Controlling the properties of semiconductors makes possible most of the technology that underpins the way we live today (for better or worse).  One way to control properties is to combine dissimilar materials into an alloy to achieve new properties, but such efforts can be thwarted if the alloy is unstable. I will discuss a collaborative effort that seeks to understand how to access metastable thin-film semiconductors using the example of two different kinds of semiconductor alloys, Sn1-xCaxS and Mn1-xZnxO.  In the process, I will describe some of the amazingly diverse uses of semiconductors, the interplay between theory and experiment, and some of the other materials we have discovered in my lab. Stephen Henderson, Washington State University - Vancouver"The mechanics of nutrient mixing in a submerged surfzone"Many lakes are beset by elevated nutrient levels, depleted oxygen levels, and algal blooms. Although the surfaces of small lakes often appear calm, submerged high-accuracy instruments reveal a rich variety of flow patterns controlling mixing of nutrients and pollutants. In Lacamas Lake, WA, we measured waves with 12-24 hour period, driven by density differences between surface and bottom waters, propagating upward through the lake. As waves traveled up the sloping lakebed, density contours pitched forward, leading to breaking in an underwater surf zone. The waves transported water upslope, and a compensating downslope mean current developed, analogous to the undertow observed on beaches. This undertow likely increases the importance of neared mixing. Similar density-driven waves are widespread in the ocean, and the dynamics discussed here may be relevant to mixing in a range of environments. Lucas Illing, Department of Physics, Reed College"Coupled Oscillator Dynamics: Amplitude Death and Chimeras"Assemblies of coupled nonlinear oscillators serve as a useful paradigm for the study of collective phenomena in many physical, chemical, and biological systems and have therefore led to a great deal of theoretical and experimental work. In this talk I will focus on two collective phenomena that are of current interest: amplitude death, where coupling leads to the quenching of oscillations, and chimera states, where networks break into clusters of coherent and incoherent oscillators. Thanksgiving Christopher Lee ‘00, Los Alamos National Laboratory"Taming Infinities: Resumming Large Logs in Quantum Field Theory"Quantum chromodynamics (QCD) and the Standard Model are quantum field theories (QFTs) that are beautifully elegant in their formulation and have been enormously successful in predicting virtually all observed phenomena in collider physics experiments in the past several decades. Perturbation theory—computing observables as expansions in small coupling constants g— is a wonderful tool for making precise predictions—when it works. For many observables, standard perturbation theory woefully breaks down and predicts wildly incorrect results. In this lecture I will discuss one such type of breakdown and its resolution. This occurs in cross sections when particles in the final state of a high-energy collisions are confined to be narrowly collimated in bunches or cones calledjets. At every order in perturbation theory g^n, large logarithms of the small size of these jets appear in the coefficients of g^n, ruining the convergence of the expansion. The solution is resummation—reorganizing the series so the logs are resummed to all orders ing, effectively capturing the radiation of arbitrarily many particles such as gluons or photons from jets. I will illustrate the concepts and tools behind the art of resummation in a QFT such as QCD. Thomas Allen, Lowell Observatory"Determining the Physical Properties of Young Stars with Multi-Wavelength Observations" Star formation spans a range of temporal scales from days to millions of years, and spatial scales from stellar sizes to light years.  A variety of physical processes are involved in star formation including photoevaporation, angular momentum transfer, gravitational contraction and magnetic field/plasma interactions.  These processes affect the final stellar system characteristics including the presence, or lack of, and structure of planetary systems.  In order to study these processes, we must observe a statistically significant sample of young stars at various stages in their formation process.  Further, star formation is not an isolated process, most stars form in groups or clusters.  I therefore examine one young cluster in particular, called Cep OB3b, that contains roughly a few thousand young stars.  My observational campaign to characterize this cluster spans the electromagnetic spectrum and I will discuss how I use these observations to determine the properties of the young stars in the sample.  Using this well-characterized young stellar sample I examine the rotational, and hence angular momentum, evolution of young stars, with a particular emphasis on the coupling of a star with its planet-forming circumstellar material.