Graduate Student Research Archive

Below, you will find graduate student research profiles from previous years. Enjoy…

Philip Barbeau

Phillip Barbeau

B.A., University of Chicago 2001 (Physics)
B.A., University of Chicago 2001 (Mathematics)
Ph.D. (2009), Dept. of Physics, Enrico Fermi Institute
Experimental Astroparticle Physics and Nuclear Physics
Awards: GAANN Teaching Fellow, Dept. of Education
Research Advisor: Juan Collar

As a member of Juan Collar's low-background detectors group I have worked to develop detectors that are, for the first time, sensitive enough to measure coherent neutrino-nucleus scattering.  This is an uncontroversial standard model process in which neutrinos scatter coherently off all of the neutrons in a nucleus.  This enhanced neutral current cross-section is proportional to the square of the number of neutrons.  The recoiling nucleus has very little energy, of which only a small fraction is deposited in the form of ionization.  A first measurement will likely be performed at a nuclear power reactor which produces a massive flux of low energy neutrinos (<10 MeV) which can all interact coherently.  A reactor measurement is feasible, with potentially hundreds of recoils per kg per day, provided the radioactive backgrounds are kept under control.  A viable detector technology thus needs to combine three features: a sub-keV energy threshold, a low radioactivity background, and a large mass (~ 1 kg).  Such a device has applications in monitoring against the illicit use of nuclear reactors.  Physics applications include limits on the neutrino magnetic moment, the effective neutrino charge radius, supersymmetric corrections to the weak nuclear charge and non-standard neutrino interactions.  Others include sterile neutrino searches, and a flavor independent measurement of the total neutrino flux from a nearby supernovae.

Several promising detector technologies have been explored in our efforts to develop a capable detector.  Early work focused on micropatterned gaseous detectors mass-produced for the first time by our group in collaboration with 3M.  With a quadruple layered Gas Electron Multiplier (GEM) our group has been able to demonstrate sensitivity to single electrons in a gaseous device.  A second detector technology, cooled Large Area Avalanche Photodiodes (LAAPDs), capable of reaching gains of a few thousand with very low noise has also been tested.  Our group has been able to demonstrate sensitivity to single photons at liquid nitrogen temperatures with a high quantum efficiency.  The most promising technology we have developed is a p-type point contact germanium diode.  The device, for the first time, has the threshold (~ 300 eV), mass (475 g) and projected background required to measure coherent neutrino scattering.  Calibration of the detector for sub-keV nuclear recoils, a region barren of direct measurements, is crucial for any measurement of coherent neutrino scattering.  To this end we have designed, built and characterized a 24 keV neutron beam at the Kansas State University Triga Mark-II research reactor.  With this facility we have for the first time directly measured individual nuclear recoils with sub-keV energies.  Together with its shielding for environmental and atmospheric radiation backgrounds the detector has been deployed 20 meters from the core of the Unit three reactor at the San Onofre Nuclear Generating Station in an attempt to measure coherent neutrino-nucleus scattering. 

This unique germanium detector is ideal for several other experiments in astroparticle physics.  The detector and its shielding have been deployed to an underground location (300 m.w.e.) outside the city of Chicago to further reduce neutron and muon induced neutron backgrounds.  We have obtained dark matter limits for low mass (<10 GeV) WIMPs unachievable with other, higher threshold dark matter experiments.  Other novel Dark Matter experiments include searches for Solar bound WIMPs and non-pointlike Dark Matter.  In addition, the excellent energy resolution at low energies (~160 eV @ 5.9 keV) provide for new limits on Dark Matter Axions interacting via the axioelectric effect, as well as new measurements on the lifetime of the electron. 

We are also members of the Majorana collaboration, an experiment that attempts to observe zero neutrino double beta decay with isotopically enriched germanium-76.  This decay is allowed only if neutrinos are their own antiparticle (Majorana neutrinos).  A measurement of this decay rate is a measurement of the effective Majorana mass of the electron neutrino.  The aforementioned p-type point contact germanium detector has shown itself to be a superior technology choice for this experiment.  The Majorana experiment must keep radioactive backgrounds in the energy region of interest (~ 2 Mev) as low as possible.  Backgrounds from Compton scattered gamma rays often interact in multiple locations whereas a signal from a zero neutrino double beta decay occurs in a single location.  By analyzing the pulse shape of the signal from this detector we can reject radiation events that involve more than one interaction site with much improved accuracy.

Publications

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Chih-Chun Chien Chih-Chun Chien

B.S., National Taiwan University 2001 (Physics)
M.S., National Taiwan University 2003 (Physics)
Ph.D. (2009), Dept. of Physics, James Franck Institute
Theoretical Condensed Matter Physics
Awards: Bloomenthal Research Fellow, Dept. of Physics
Research Advisor: Kathryn Levin

Initially, I joined Prof. Woowon Kang's lab and worked on low temperature physics of semiconductors focused on the Quantum Hall effect. We used dilution refrigerators to cool the sample to 10 mK and applied magnetic field as strong as 15 Tesla. In 2005 I switched to Prof. Kathryn Levin's group and have worked on superfluidity in ultra-cold atoms since then. So far we have studied the BEC-BCS crossover, vortices in Fermi gases, Fermi gases with population imbalance, Fermi gases in optical lattices, and many other topics. Together with Qijin Chen (postdoc) and Yan He (graduate student) we have written 12 papers on various topics in ultra-cold Fermi gases.

I. Two-component attractive Fermi gases with population imbalance. Recently, experiments on cold atoms done by the MIT group led by W. Ketterle (Science 311, 492; Nature 422, 54) and the Rice group led by R. Hulet (Science 311, 503; PRL 97, 190407) demonstrated their capabilities to tune both the magnitude of attractions and the relative populations in the two components of a Fermi gas. We studied experiment results and former theoretical works and investigated possible phases in such systems. There are many interesting phases in different regimes such as the Sarma state (polarized superfluid), the LOFF state (pairing with finite central momenta), the phase separation state (paired and unpaired phases separation in real space), etc. We considered both the tunable attractions and finite-temperature effects and constructed a self-consistent theory. Our theory takes into account the important contributions of non-condensed pairs, which is commonly ignored or treated inconsistently in the literatures. Our results capture most features in experiment data and also gave predictions in certain limits. Feedbacks from experimentalists helped us clarify confusions and improve our models further. We now extend our theories to investigate radio frequency measurements, collective modes, possible bound states at high polarizations, pairing with unequal masses, etc. Here are some selected publications:

II. Fermi gases on optical lattices. We study fermions with attractive interactions on optical lattices. Although the BCS mean-field ground state at zero temperature shows smooth behavior in any filling and magnitude of attraction, pair fluctuations which is important at finite temperatures should be considered consistently. We found a BEC-BCS crossover behavior on lattices at intermediate attraction regime. The weakly interacting effective boson picture from the mean-field theory breaks down  when the system is close to half filling of fermions and the attraction is strong. Perturbation calculations show that in this regime states with local pairs maximizing on-site attractions become a better ground state. We also consider possible d-wave pairing on lattices and show that the regime where superfluidity does not survive at any finite temperature is significantly larger than its counter part in the s-wave case. Here are selected publications:

Selected Publications

Two-component attractive Fermi gases with population imbalance:

  • Y. He, C. C. Chien, Q. Chen, K. Levin, "Radio frequency spectroscopy of trapped Fermi gases with population imbalance," Physical Review A 77, 011602. (Rapid Communication)
  • Y. He, C. C. Chien, Q. Chen, K. Levin, "Thermodynamics and superfluid density in BCS-BEC crossover with and without population imbalance," Physical Review B 76, 224516.
  • C. C. Chien, Q. Chen, Y. He, K. Levin, "Superfluid phase diagrams of trapped Fermi gases with population imbalance," Physical Review Letters 98, 110404 (2007).
  • Y. He, C. C. Chien, Q. Chen, K. Levin, "Single-plane-wave Larkin-Ovchinnikov-Fulde-Ferrell state in BCS--Bose-Einstein condensation crossover," Physical Review A 75, 021602. (Rapid Communication)
  • Q. Chen, Y. He, C. C. Chien, K. Levin, "Theory of Superfluids with Population Imbalance: Finite Temperature and BCS-BEC Crossover Effects," Physical Review B 75, 014521 (2006).
  • Q. Chen, Y. He, C. C. Chien, K. Levin, "Stability conditions and phase diagrams for two component Fermi gases with population imbalance," Physical Review A 74, 063603 (2006).
  • C. C. Chien, Q. Chen, Y. He, K. Levin, "Finite temperature effects in trapped Fermi gases with population imbalance," Physical Review A 74, 021602 (2006). (Rapid Communication)
  • C. C. Chien, Q. Chen, Y. He, K. Levin, "Intermediate temperature superfluidity in an atomic Fermi gas with population imbalance," Physical Review Letters 97, 090402 (2006).

Fermi gases on optical lattices:

  • C. C. Chien, Q. Chen, K. Levin, "Fermions with attractive interactions on optical lattices: BEC-BCS crossover, quantum phase transition, and finite-temperature effects," in preparation.
  • C. C. Chien, Y. He, Q. Chen, K. Levin, "Superfluid-insulator transitions at non-integer filling in optical lattices of fermionic atomms," Physical Review A 77, 011601. (Rapid Communication).

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sam

Samuel Gralla

B.S., Yale University 2005 (Physics)
B.S., Yale University 2005 (Mathematics)
Ph.D. (2011), Dept. of Physics, Enrico Fermi Institute
General Relativity
Awards: NSF Fellow (NSF), GAANN Fellow (Dept. of Education), Bloomenthal Fellow (Physics), Blue Apple Awards (Midwest Relativity)
Research Advisor: Robert Wald

My research concerns the problem of motion in general relativity and related theories. In general relativity, bodies move on geodesics to lowest order--but what are the corrections? Particalarly, what is the influence of a body's own self-field on its motion? Determining this "self-force" or "radiation reaction" correction is more than a theoretical enterprise: knowledge of the motion with such accuracy is necessary for the production of data analysis templates for use with the planned space-based gravitational-wave detector LISA.

In graduate school I have worked primarily on a formalism for deriving the motion of bodies in relativistic theories. Working with the case of general relativity, the basic ideas are as follows. To take a rigorous approach to the perturbative description of a small body in general relativity, one must consider a one-parameter-family of solutions to Einstein's equation that contains a body that shrinks to zero size. But the body must shrink to zero mass as well, or a black hole (which is a finite-size object) will be formed before the zero-size limit can be reached. The body thus completely shrinks and disappears in the limit, but it leaves behind a preferred worldline (the place where it "disappeared to") characterizing its lowest-order motion. We develop a formalism to build this behavior into an assumed one-parameter-family, and then prove that the worldline must be geodesic. We then compute all first-order corrections to that worldline, which include gravitational self-force. We also applied this approach to classical electromagnetism and (at lowest order) to an arbitrary second-order tensor theory that follows from a diffeomorphism-covariant Lagrangian.

An interesting side-project was an analysis of "bobbing" effects in relativistic systems. Numerical simulations of binary black holes with spin show some bizarre behavior: if the spins are (anti)aligned just right, the whole system undergoes an up-and-down "bobbing" motion in phase with the orbit. Then, if the phase at merger is right, the merged black hole receives a tremendous kick in the direction of bobbing! We looked at whether similar effects could be found in analogous--but simpler--systems. We found that the bobbing effect is in fact ubiquitous, occuring whenever two spinning bodies are held in orbit by any sort of force. For example, two spinning balls connected by a string will display this behavior. The kick, however, is more special and can only occur for systems that possess field momentum which can be radiated to infinity. We conclude that bobbing and kicks are basically unrelated phenomena, which can nevertheless appear correlated for spinning black holes because the spin parameter happens to control both the bobbing and the kick.

Publications

  • S.E. Gralla, A.I. Harte, and R.M. Wald, Bobbing and Kicks in Electromagnetism and Gravity, Phys. Rev. D 81, 104012 (2010).
  • S.E. Gralla, Motion of Small Bodies in Classical Field Theory, Phys. Rev. D 81, 084060 (2010).
  • S.E. Gralla, A.I. Harte, and R.M. Wald, A Rigorous Derivation of Electromagnetic Self-force, Phys. Rev. D 80, 024031 (2009).
  • S.E. Gralla and R.M. Wald, A Rigorous Derivation of Gravitational Self-force, Class. Quantum Grav. 25, 205009 (2008).

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Chen-Lung Hung Chen-Lung Hung

B.S., National Taiwan University 2003 (Physics)
M.S., University of Chicago 2006 (Physics)
Ph.D. (2011), Dept. of Physics, James Franck Institute
Experimental Atomic Physics
Awards: Harper Dissertation Fellowship, Physical Sciences Division
Research Advisor: Cheng Chin

In collaboration with Prof. Cheng Chin, I built a new experiment aiming at using cesium Bose-Einstein condensates (BECs) to study the universality of few- and many- body physics and the quantum phase transitions of atomic quantum gases in optical lattices. By changing the atomic interactions using magnetic Feshbach resonance, and applying precisely controlled lattice potentials formed by laser standing waves, we constructed a clean and highly controllable many-body system. This provides exciting opportunities to explore fundamental phenomena traditionally studied in the context of condensed matter or nuclear physics.

My research over the past years focuses on efficient production of cesium Bose-Einstein condensates and the realization of the superfluid to Mott insulator transition of ultracold atoms in optical lattices, which are described below:

Efficient evaporative cooling of cesium atoms to Bose-Einstein condensation: Despite the rich collision properties of ultracold cesium atoms, which allow us to tune atomic interactions over a wide range, making a cesium BEC has been considered difficult due to the inelastic collision losses at high densities. Evaporative cooling in an optical dipole trap has become a necessity for condensing cesium atoms polarized in their lowest hyperfine ground state. We developed a simple scheme to achieve fast and runaway evaporative cooling of atoms to Bose-Einstein condensation by tilting an optical dipole trap with a magnetic force. This technique overcomes speed limitations in conventional dipole trap cooling, and can produce a large number of BEC atoms within 2~4 seconds.

In-situ observation of Superfluid-to-Mott insulator transition in optical lattices: We developed a novel scheme to load BEC atoms into a monolayer two-dimensional optical lattice, and thus realized the superfluid (SF) to Mott insulator (MI) transition in 2D, simulating the Bose-Hubbard model. High resolution imaging normal to the lattice plane provides in-trap density measurements, and sensitive detection of thermal and quantum density fluctuations. We directly observe the "wedding cake" density structure of a trapped gas, reflecting the coexistence of incompressible Mott-insulator, compressible superfluid and normal gas phases in equilibrium. A precise determination of these phase boundaries can be made possible by careful study of the density profile, local compressibility and density fluctuations within the framework of a local density approximation.

A density-profile based thermometry is developed to extract temperature and chemical potential of the atomic sample in the optical lattices throughout the SF-MI transition regime. When a finite temperature superfluid is adiabatically converted into a Mott insulator without strong external compression, entropy conservation suggests a significant cooling of the sample during the loading process. By shortening the lattice ramp rate and increasing initial density of the cloud, we observed evidence of even lower temperature than expected near the Mott core, implying limited entropy flow against the direction of the mass flow.

In addition to studying quantum phase transitions and quantum criticality using in-situ density profiles and fluctuations, our system also provides promising prospects to access exotic quantum phases by controlling atomic interactions, and to explore quantum magnetism by introducing multiple internal states. Our system is also fully capable of studying physics in low dimensions such as Beresinskii-Kosterlitz-Thouless physics in two dimensions and the Tonks-Girardeau gas in one dimension.

Publications

  • Chen-Lung Hung, Xibo Zhang, Nathan Gemelke and Cheng Chin, “Accelerating Evaporative Cooling of atoms into Bose-Einstein Condensation in Optical Traps”, Physical Review A 78, 011604(R) (2008).
  • Nathan Gemelke, Xibo Zhang, Chen-Lung Hung and Cheng Chin “In-situ Observation of Incompressible Mott-Insulating Domains of Ultracold Atomic Gases”, Nature 460, 995-998 (2009).
  • Chen-Lung Hung, Xibo Zhang, Nathan Gemelke and Cheng Chin “Density Profile-Based Thermometry in Optical Lattices across The Superfluid-Mott Insulator Transition”, in preparation.

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nathan Nathan Keim

B.S., Haverford College 2004 (Physics)
Ph.D. (2010), Dept. of Physics, James Franck Institute
Experimental Condensed Matter Physics
Awards: Michelson Fellow, McCormick Fellow, Sachs Fellow, Grainger Fellow, Dept. of Physics
Research Advisor: Sidney Nagel

I am interested in the formation of singularities: points in space or time where one or more physical quantities grow infinitely large. Static and dynamic singularities appear in many branches of physics — for example: relativity (black holes), statistical mechanics (critical phase transitions), astrophysics (star formation), nuclear physics (fission), or non-linear physics (fluid shape changes). Normally one expects that a singularity is so strong that it controls all the dynamics that lead up to it. Thus singularities ought to be universal, in that they should behave the same independent of initial or boundary conditions. In contrast, our experiments have shown that there is another class of singularity where the initial conditions affect the entire evolution.

Specifically, my experiments in Professor Sidney Nagel’s laboratory study the pinch-off of air bubbles from a submerged nozzle. As the air pinches off, the neck connecting the bubble with the nozzle must collapse down to a very small radius until it breaks into two pieces. As the neck radius approaches zero, the speed of the collapse diverges, producing a singularity. By taking videos at over 180,000 frames per second, I observe how the singularity changes when I perturb the bubble in various ways. As shown in the two images, using a nozzle shaped like a slot instead of a circle can produce dramatic results.

second bubblefirst-bubble

Caption: Bubbles pinching off from a submerged nozzle. Left: a bubble slowly emitted from a circular nozzle, in the process of pinching off. The bright spots inside the bubble are due to the back-lighting. Right: a bubble is rapidly ejected from a slot-shaped nozzle. The image is a close-up of the neck region, showing an irregular “tearing” break-up instead of a universal, symmetric singularity.

We have quantified and analyzed these effects and compared them with the theoretical analysis of Professor Wendy Zhang and her former student Laura Schmidt. They predicted that the information about the initial conditions would be encoded via a novel type or vibration on the collapsing neck. The collaboration of our two groups has discovered that a large class of perturbations is remembered as rapid vibrations of the neck shape, which disrupt the universal evolution of the system toward a singularity.

Publications:

  • Nathan C. Keim, Peder Møller, Wendy W. Zhang, Sidney R. Nagel: Breakup of air bubbles in water: "Memory and breakdown of cylindrical symmetry". Physical Review Letters, 97:144503, 2006.
  • Laura E. Schmidt, Nathan C. Keim, Wendy W. Zhang, Sidney R. Nagel: "Memory-encoding vibrations in a disconnecting air bubble". Nature Physics, 5:343–346, 2009.
  • Nathan W. Krapf, Thomas A. Witten, Nathan C. Keim: "Chiral sedimentation of extended objects in viscous media." Physical Review E, 79:056307, 2009.

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ryan Ryan Keisler

B.S., University of Texas at Austin 2005 (Physics)
B.A., University of Texas at Austin 2005 (Plan II)
Ph.D. (2011), Dept. of Physics, Enrico Fermi Institute
Observational Cosmology
Awards: Nathan Sugarman Award (Enrico Fermi Institute)
Research Advisor: John Carlstrom

I work in observational cosmology, a field that is increasingly driven by rich datasets. One important source of data is the cosmic microwave background, or CMB. The CMB is relic light from the early universe, when photons, normal matter, and dark matter were coupled and undergoing oscillations. Eventually the universe expanded, cooled, and became transparent to photons. Today we see these photons as the CMB. Measurements of the statistical properties of the CMB, as exemplified by the WMAP satellite, contain a wealth of information about the composition of the universe. While it's difficult to overstate the impact that WMAP has had on modern cosmology, there is still plenty to learn from the CMB. More specifically, the angular resolution of WMAP is about twenty minutes of arc, roughly twenty times worse than the human eye, and there is much to be learned from higher resolution images of the CMB. Higher resolution requires few-meter diffracting apertures, which are very expensive to launch into space. For this reason there are a number of new, ground-based, large-aperture CMB telescopes. One such telescope is the South Pole Telescope (SPT), a project led by John Carlstrom here at UChicago, and which is the focus of my graduate research.

The SPT is a 10-meter telescope located about 1 km from the geographic South Pole. The South Pole sits upon the high, dry Antarctic plateau and is one of the best sites in the world for millimeter-wave observations. I've been involved with the SPT project since 2005. In 2006 I was part of a team which traveled to the South Pole to deploy the entire instrument over the course of three months, a very exciting time. While I've been back to the South Pole twice to help upgrade the instrument, most of my work is done from Chicago. Broadly speaking, I've helped to monitor and characterize the instrumental performance and have contributed software to our "pipeline": the body of code which converts our raw data into images of the CMB. As a concrete example, I've written software which determines where the telescope is pointing on the celestial sphere at each moment of time.

More recently I've worked on a project to use SPT data to measure the statistical properties --- namely, the angular power spectrum --- of the CMB (in fact this is the focus of my dissertation). This work will characterize the CMB with unprecedented resolution and sensitivity, and will, among other things, likely provide strong evidence that the CMB photons are gravitationally lensed by intervening matter as they travel to us. There are quite a number of ongoing scientific projects using SPT data which I won't describe here, but I should mention that the key project is to characterize the dark energy by discovering massive, distant galaxy clusters using the Sunyaev-Zel'dovich effect. To summarize, my work with the SPT has been an enjoyable mix of hardware, software, and science, and I think this is true for most students that work in observational cosmology.

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nathan Nathan Krapf

B.A., University of Chicago 2005 (Physics)
B.S., University of Chicago 2005 (Mathematics with Specialization in Computer Science)
Graduate Student (2005), Dept. of Physics, James Franck Institute
Theoretical Condensed Matter Physics
Awards: McCormick Fellow, Sachs Fellow, Dept. of Physics; GAANN Teaching Fellow, Dept. of Education
Research Advisor: Tom Witten

I am currently investigating force propagation in granular systems at the jamming transition. At this point, the system is isostatic, meaning one can uniquely solve for all inter-grain contact forces. In such systems, a proposed null-stress condition gives rise to a hyperbolic equation for the stress tensor. This predicts that on average, a point force on a single bead will propagate through the pack much like a light cone through space-time, where going in the "down" direction corresponds to going forward in time. Such behavior has been numerically verified in systems built sequentially from the floor up. However, such systems have a preferred direction throughout their entire creation history. We expect the null stress condition to hold regardless of the details of how the packing is made, but if we create it with no such preferred direction and then add a floor later, how can the system know which way "down" is? That is, can the behavior in the bulk be influenced by what we have done "in the future" at the boundary? We have found exponentially decaying modes with underdetermined forces at the bottoms of such packings and overdetermined modes at the tops, and are trying to learn how these modes affect force distribution in the bulk. In more general terms, we want to know what aspects of the packing creation determine the behavior of the force response.

Before starting my work on granular systems, my advisor and I looked at the low Reynolds number sedimentation of arbitrarily shaped objects. In general such objects twist as they sink, and we can interpret this as an expression of inherent chirality. We showed that in the limit when internal hydrodynamic interactions between different parts of the object are weak it will follow a helical path while rotating at constant angular velocity about a fixed axis. Even though there can be no such chiral response in the absence of hydrodynamic interactions, the angular velocity reaches a fixed nonzero limit as the interaction strength approaches zero. We then empirically characterized how this chirality depends on the shape of the objects and found various scaling laws governing the angular velocity.

In collaboration with some members and graduates of the Booth School of Business, the Harris School of Public Policy, and the University of Chicago Law School, I am looking at intermittency in Illinois wind speeds to evaluate a recently enacted law setting statewide renewable energy standards. I am also helping my advisor continue the work of a former undergraduate student looking at binding energies and singularities in clusters of charged metal nanoparticles.

Publications:

  • Nathan W. Krapf, Thomas A. Witten, Nathan C. Keim: "Chiral sedimentation of extended objects in viscous media." Physical Review E, 79:056307, 2009.

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John Royer John Royer

B.S., Univ. of California-Santa Barbara 2004 (Physics)
M.S., University of Chicago 2005 (Physics)
Ph.D. (2009), Dept. of Physics, James Franck Institute
Experimental Condensed Matter Physics
Awards: Sachs Fellow, Dept. of Physics
Research Advisors: Heinrich Jaeger and Sidney Nagel

I work with Heinrich Jaeger and Sid Nagel experimentally investigating the complex, non-linear dynamics of granular flow. Granular materials, such as sand, gravel and baking soda, are large collections of macroscopic, individually solid particles, or "grains". Far from being simple materials with  simple properties, they display an astounding range of behavior that defies their categorization as a conventional solid, liquid or gas.  

Much of my research has focused on the impact of a solid object into a granular bed. Though the impact of a solid object into a granular bed has been studied since the 18th century, it still contains many surprises. In 2001 Thoroddson and Shen discovered that when a heavy sphere is dropped onto a bed of very loose, fine sand, a remarkable phenomenon occurs: a large, focused jet of sand shoots upwards. We found that unlike liquid jets, the granular jet is is actually composed of two separate components, an initial thin jet formed by the collapse of the cavity left by the impacting object stacked on top of a second, thicker jet which depends strongly on the ambient air pressure. To study the role of air during the impact and subsequent jet formation, we performed high-speed X-ray radiography at the Advanced Photon Source at Argonne National labs. This new technique allowed us to image the dynamics below the surface at speeds up to 6000 frames per second. We found that air  trapped in fine-grained bed plays two distinct roles in the formation of the jet. First, air trapped and compressed between grains prevents compaction, causing the bed to flow like an incompressible fluid and  allowing the impacting object to sink deep into the bed. Second, air trapped and compressed by the collapsing cavity can amplify the jet by directly pushing bed material upwards and creating the thick jet. 

Publications:

  • J. R. Royer, E. I. Corwin, A. Flior, M.-L. Cordero, M. L. Rivers, P. J. Eng, and H. M. Jaeger. "Formation of granular jets observed by high-speed x-ray radiography." Nature Physics, 1:164–167, December 2005.
  • J. R. Royer, E. I. Corwin, P. J. Eng, and H. M. Jaeger. "Gas-mediated impact dynamics in fine-grained granular materials." Phys. Rev. Lett., 99(3):038003–4, 2007.
  • J. R. Royer, E. I. Corwin, B. Conyers, A. Flior, M. L. Rivers, P. J. Eng, and H. M. Jaeger. "Birth and growth of a granular jet." Submitted to Phys. Rev. E (2008).

Andy Andrew Royston

B.S., University of Cincinnati 2002 (Physics with High Honors)
B.A., University of Cincinnati 2003 (Mathematics with Honors)
M.S., University of Chicago 2005 (Physics)
Ph.D. (2010), Dept. of Physics, Enrico Fermi Institute
String Theory
Awards (undergrad): Dean's List, Cincinnatus Foundation Fellowship, Procter & Gamble Co. Scholarship
Awards (graduate): Gregor Wentzel Prize, Dept. of Physics, GAANN Teaching Fellowship, Dept. of Education
Research Advisor: Jeffrey Harvey

I have conducted my Ph.D. research in string theory, focusing on the connections between four-dimensional field theories and branes. String theory is a description of nature in which the fundamental objects are not particles, but tiny one-dimensional strings. The various vibrational modes of the string give rise to objects resembling particles of different masses and spin. It was realized in the mid ‘90’s that string theory also contains membranes, or “branes” for short. These higher dimensional cousins of the string come in many varieties, from the two-dimensional membrane to a nine-dimensional brane that fills all the spatial directions available in string theory. Branes are interesting objects because open strings, i.e. strings with endpoints, must have those endpoints stuck to a brane, and this effectively localizes open strings to the hypersurfaces spanned by branes. This should be contrasted with closed strings, i.e. loops of string without endpoints, which can propagate freely in the ten-dimensional “bulk” spacetime of string theory.

The connection between branes and field theories—in particular gauge theories such as quantum electrodynamics—is the following. At energy scales well below the masses of excited string states, it is reasonable to focus only on the interactions of the lightest string modes, which are usually massless or nearly massless. In this limit it turns out that the theory of open strings reduces to a gauge theory! (In complete analogy, the interactions of closed strings reproduce general relativity, Einstein’s theory of gravity, in the low energy limit). The type of gauge theory obtained from open strings depends on the type of branes involved and how they are embedded in the bulk. There are nearly endless possibilities. A major area of current research is to engineer a brane system that reproduces exactly the Standard Model at low energies. While this is difficult to achieve precisely, it now appears that string theory may have many “standard model-like” vacua. This begs a couple questions.

Firstly, why bother with strings, branes, and ten dimensions if the only goal is to reproduce the standard model, a theory that we’ve understood quite well without all these notions for over thirty years? The answer is that the standard model is only an effective theory; valid at the energy scales we’ve probed thus far. It’s fully expected that we’ll see physics beyond the standard model at the upcoming Large Hadron Collider (LHC) experiments in Geneva, Switzerland. Arguably the most anticipated discovery will be supersymmetry, a hypothesized approximate symmetry of nature, whose main observable consequence is that every particle should have a supersymmetric partner particle. String theory has supersymmetry built in, so the discovery of supersymmetry would be strong evidence in favour of string theory, though it certainly wouldn’t prove it. One way that string theory might be distinguished from other supersymmetric extensions of the standard model is the way in which supersymmetry is broken in the theory. This could lead to testable predictions for string theory at the LHC, but more work needs to be done to understand the mechanisms of supersymmetry breaking in string theory.

A second question concerns the issue of vacuum selection. If string theory contains many states with different brane configurations, why did we end up in one we did, described by our standard model at low energies? A most satisfying, but difficult to establish, answer would be that the dynamics of string theory in the early universe drove us to the vacuum we are in now. This idea is referred to as dynamical vacuum selection.

In research with Professor David Kutasov and collaborators, I addressed the issues of supersymmetry breaking and dynamical vacuum selection is a particular class of brane constructions. The brane configurations we considered may play a role in supersymmetric extensions of the standard model. Within these configurations, we showed how to describe a set of phenomenologically interesting supersymmetry breaking vacua from the low energy field theory point of view, that were previously only understood in the string theory/brane picture. The system contains many other vacua as well, both supersymmetric ones and supersymmetry breaking ones. We found that early universe dynamics naturally drives the system to the most phenomenologically interesting of all these vacua. We are currently trying to adapt the techniques we developed in this class of systems to other systems, which may provide a more natural description of the compactification of string theory’s ten dimensions down to four.

Another area of tremendous activity in string theory goes under the headings “AdS/CFT correspondence” or “gauge/gravity duality.” It began from a conjecture, motivated by studying brane systems in string theory, that certain scale invariant gauge theories known as conformal field theories (CFT) have mathematically equivalent, or “dual” descriptions in terms of string theory in particular geometric backgrounds. (In the prototypical example, the background geometry is Anti-de-Sitter (AdS) space). These backgrounds have one additional dimension beyond the dimensions of the gauge theory. For example, a four-dimensional CFT would be dual to string theory in a particular five-dimensional background. At low energies, the string theory is well described by gravity, hence gauge/gravity duality. There is mounting evidence that the duality is not restricted to CFTs. Even ordinary gauge theories such as quantum chromodynamics—the theory of quarks and gluons—may have a dual string theory description. The usefulness of the duality is that it is a strong/weak relationship. When the gauge theory is strongly coupled (hard to compute with), the string theory will be weakly coupled (easy to compute with) and vice versa.

In previous work with my advisor, Professor Jeff Harvey, I studied gauge/gravity duality in a particular brane system. The system we chose has some features in common with QCD, but has other simplifying features that afforded us analytic control over the analysis. We found significant evidence for the conjectured duality, as well as several novel features. In particular, the field theory itself was in a curved background and the dual string theory in a different curved background. This particular model is fascinating from a theoretical viewpoint, and teaches us about the mathematical workings of the duality, but it is not intended to provide a description of real world phenomena. Currently, with Prof. Harvey and a fellow student, I am working on a more hands-on model, which provides a dual description of the low lying meson spectrum in QCD. This is an energy regime where QCD is strongly coupled and it is impossible to use the field theory description directly. We are using experimentally measured decay rates to constrain the parameters of our model.

Publications

  • D. Kutasov, O. Lunin, J. McOrist and A. B. Royston, “Dynamical Vacuum Selection in String Theory,” [arXiv:0909.3319].
  • A. Giveon, D. Kutasov, J. McOrist and A. B. Royston, “D-terms and Supersymmetry Breaking from Branes,” Nucl. Phys. B822 (2009) 106, [arXiv:0904.0459].
  • J. A. Harvey and A. B. Royston, “Gauge/gravity duality with a chiral N=(0,8) string defect,” JHEP 08 (2008) 006, [arXiv:0804.2854].
  • J. A. Harvey and A. B. Royston, “Localized modes at a D-brane—O-plane intersection and heterotic Alice strings,” JHEP 04 (2008) 018, [arXiv:0709.1482].

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Nausheen Shah Nausheen Shah

B.S., George Mason University 2001 (Physics)
B.S., George Mason University 2001 (Mathematics)
Ph.D. (2009), Dept. of Physics, Enrico Fermi Institute
Theoretical Particle Physics
Adwards: GAANN Teaching Fellow, Dept. of Education, Bloomenthal Research Fellow, Dept. of Physics
Research Advisor: Carlos Wagner

The Standard Model (SM) as we understand it consists of three generations of quarks and leptons, charged under the symmetry groups: SU(3) x SU(2) x U(1) corresponding to the strong, weak and the electromagnetic forces. The quanta for these forces are associated with the gluons, the W and Z bosons and the photon respectively. This model has been thoroughly verified experimentally. In particular, the electroweak sector has been analyzed extensively and for any new physical theory, the theoretical predictions would have to be consistent with these precision electroweak data.

However, even though the SM has been so successful experimentally, there are a lot of unanswered questions: Quantum gravity has yet to be incorporated in a consistent manner; the hierarchy problem: why is the energy scale of the weak force so different from the scale of gravity? Respecting the gauge structure of the SM, it is not possible to write mass terms, and as mentioned above, the SM particle masses have been measured very accurately over the years:  What then generates the known particle masses? Only left handed neutrinos have been observed in nature, which obviously begs the question about right handed neutrinos. Apart from these theoretical questions, we get a hint that physics beyond the SM must exist from cosmology. The observed expansion of the universe can't be explained with the known matter content. Therefore, an unknown particle is hypothesized consisting of "Dark matter".   

I am interested in studying extensions of the Standard Model (SM) using the interplay between high energy physics and cosmology, specifically incorporating extra dimensions. A nice introduction to the ideas and problems associated with extra dimensions is presented in the following article: http://physicsworld.com/cws/article/print/403

One of the simplest extensions of the SM is to add a flat Universal Extra Dimension (UED), meaning that all the particles are allowed to propagate in the extra dimension. A UED leads to the presence of Kaluza-Klein (KK) excitations of the ordinary fermions and bosons of the SM that may be observed at hadron and lepton colliders. The lightest KK particle (LKP), if neutral, becomes a good dark matter particle. In a recent work [1], we showed that the effect of KK gravitons may lead to a relevant modification of the range of KK masses consistent with the observed relic density. Additionally, if evidence for UED is observed experimentally, we found a stringent upper limit on the reheating temperature depending on the mass of the LKP observed.

Adding a warped, rather than flat extra dimension to the SM is a slightly better motivated model. These so called Randall-Sundrum (RS) models can solve the hierarchy problem since the size of the extra dimension is naturally stabilized with kL ~ 30, where k is the curvature of order Planck scale, and L is the scale of the extra dimension. In [2], we studied the implications on inflation and early universe phase transitions due to gravitons in these scenarios.

As mentioned above, the generation of masses is not explained in the SM. Introducing an additional scalar particle, the so called Higss, which has a potential leading to it acquiring a non-zero vacuum expectation value (vev), which in turn causes electroweak symmetry breaking (EWSB), can be used to explain the SM mass structure. This particle is yet to be observed, even though many experiments have placed bounds on its mass.

Assuming that the fermion and gauge fields are allowed to propagate in the bulk of the extra dimension, a larger gauge group, SO(5), implying a larger symmetry structure, could naturally introduce the Higgs as a scalar decomposed from the 5 dimensional gauge fields. The aesthetic beauty of these models is that we don't need to introduce an ad hoc scalar to incorporate the SM masses. In these Gauge-Higgs Unification scenarios, the potential for the Higgs field then leads naturally to EWSB. In [3], we computed this effective potential. We demonstrated that EWSB may be realized, with the proper generation of the top and bottom quark masses for the same region of bulk mass parameters that lead to good agreement with precision EW data in the presence of a light Higgs. We also computed the Higgs mass and showed that for the range of parameters for which the Higgs boson has SM-like properties, the Higgs mass naturally varies in a range between values close to the LEP experimental limit and about 160 GeV.

In a continuation of the above work, [4], we computed the couplings of the SM particles and the first excited states of the gluons, W's, Z gauge bosons, as well as the Higgs, to the SM particles and first excited states of the third generation quarks. Using the parameter space consistent with EW precision tests and radiative EWSB, we numerically studied the dependence of these couplings on the parameters of our model and the associated collider phenomenology. In particular, we concentrated on the possible detection of the first excited KK state of the top, t1, which tends to have a higher mass than the one necessary to detect these particles via regular QCD production processes.

Currently, we are involved in studying the incorporation of right handed neutrinos in these scenarios. We hope that these right handed neutrinos might shed some light on the smallness of the left handed neutrino mass, the dark matter problem as well as on the origin of the matter-antimatter asymmetry via leptogenesis and the observed SM flavor structure. 

With the upcoming CERN LHC experiment, it is a very exciting time to be involved in particle physics. Current and future astrophysical measurements will also provide valuable information. By working at the interface of these fields, I hope to contribute to our understanding of how nature works at high energies.

Publications

  1. N. R. Shah and C. E. M. Wagner, "Gravitons & Dark Matter in Universal Extra Dimensions," Phys. Rev. D 74, 104008 (2006) [arXiv:hep-ph/0608140].
  2. R. U. H. Ansari, C. Delaunay, R. Gwyn, A. Knauf, A. Sellerholm, N. R. Shah and F. R. Urban, "Braneworld Graviton Interactions in Early Universe Phase Transitions," arXiv:hep-ph/0612321.
  3. A. D. Medina, N. R. Shah and C. E. M. Wagner, "Gauge-Higgs Unification & Radiative Electroweak Symmetry Breaking in Warped Extra Dimensions," Phys. Rev. D 76, 095010 (2007) [arXiv:0706.1281 [hep-ph]].
  4. M. Carena, A. D. Medina, B. Panes, N. R. Shah and C. E. M. Wagner, "Collider Phenomenology of Gauge-Higgs Unification Scenarios in Warped Extra Dimensions," Phys. Rev. D 77, 076003 (2008) [arXiv:0712.0095 [hep-ph]].

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