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Physics
Faculty Research Summaries | Chairman's Introduction
Isaac D. AbellaLaser Coherent Transients. Photon echo techniques are used to probe metastable excited states in rare gas mixtures such as helium, neon, and argon. The states are produced in a weakly ionized r.f. plasma discharge, and nitrogen-pumped dye lasers are used to generate the coherent super-position states. Spectroscopy of Rare-Earth Laser Material. Samples of YLF and YAG crystals doped with erbium, thulium, and holmium are being studied with selective laser excitation in the region of 780 nm, the erbium bands. These materials can be efficiently optically pumped by the AlGaAs-GaAs laser diode arrays, but we are using dye laser excitation. We are interested in the energy transfer process: Er to Tm, to Ho, which concentrates energy emission at 2.085 microns at room temperature and at liquid nitrogen. The process is a radiationless, almost resonant transfer of energy between sites and depends on the relative concentrations of the rare earth ions. In particular we are interested in measuring decay rates, excited state absorption, and branching ratios and detailed theories of such processes. Edward C. BlucherMy current research involves studies of CP violation in the neutral kaon system. The origin of CP violation, which is thought to be necessary for understanding the striking asymmetry in the abundance of matter and antimatter in the Universe, is one of the fundamental questions of particle physics. Professors Wah, Winstein, Winston, and I are working with physicists from twelve universities on an experiment (called KTeV) which was designed to be sensitive enough to observe a new form of this phenomenon. This experiment collected data in 1996, 1997, and 1999. Based on an analysis of the first 15% of these data, our group announced that we had established the existence of a new form of CP violation, called direct CP violation. We are currently analyzing the full data sample to make a precise measurement of this new type of CP violation. John E. Carlstrom (See Department of Astronomy and Astrophysics)Sean M. CarrollI am interested in understanding the fundamental laws of physics and how they manifest themselves in the evolution of the universe. This somewhat extravagant ambition is cut down to manageable size by considering problems which lie at the interfaces of particle physics, field theory, cosmology, and general relativity. An example of such a problem is provided by recent observations of distant supernovae (as confirmed by microwave background anisotropies and other data), which indicate that the expansion of the universe is accelerating rather than slowing down. If true, this is a profound discovery which poses a challenge to theories of particle physics and potentially to theories of quantum gravity. The acceleration can be explained by invoking a nonzero vacuum energy, or cosmological constant, but with a magnitude which seems implausible from a field-theory perspective. A variation on this idea is to introduce quintessence, a dynamical field which induces a vacuum energy which gradually decreases with time. I am actively investigating a number of theories of the cosmological constant and dark energy, including models of quintessence, alternative theories of gravity, and mechanisms for suppressing the naive vacuum energy. On a parallel track, I am studying the cosmology of theories with large extra spatial dimensions, in which the particles and fields of the Standard Model are confined to a brane embedded in a higher-dimensional geometry. Extra dimensions can alter the conventional cosmological picture in interesting ways, perhaps even affecting the vacuum energy. Some of my other current interests include unusual types of topological defects in field theories, violations of Lorentz invariance via Planck-scale physics, supersymmetric cosmology, the dynamics of inflationary universes, and the holographic principle in various spacetimes. Philippe CluzelNoise and information in biological systems. Sudden changes of environment and life-cycle phases (growth, cell division, and death) characterize the conditions far from the equilibrium of chemical reactions occurring within individual cells. Yet cells, or more generally, biological systems, are able to process information accurately and to achieve precise tasks. How do biological systems use or circumvent these noisy conditions? Most of the experiments and mathematical models have assumed that the characteristics of the signaling and chemical reactions occurring within individual cells could be inferred from ensemble measurements. This approach, however, masks the temporal fluctuations and the dynamics of biological heterogeneous systems. A more promising alternative is to describe individually and in "real time" these strongly non-linear (living) systems. By developing novel single-molecule techniques, we can revisit canonical biological systems and question the advantage of "noise" for inter/intracellular signaling. Noise in gene expression in bacteria. When few molecules are involved, chemical reactions are subject to stochastic fluctuations. Important steps of the gene expression in bacteria can be modeled by basic chemical reactions. A few molecules in the cell control these reactions. Consequently, the pattern of protein concentration growth is expected to be highly stochastic, exhibiting short bursts of variable numbers of proteins at varying time intervals. We are interested in characterizing experimentally the statistics of those simple chemical reactions that control the gene expression in individual living cells. Spread of genetic material within a bacterial population. We are interested in identifying the geometrical and dynamical conditions that can favor the DNA spread within a bacterial population in aqueous solution. Conjugation is a process mediated by physical contact that promotes the DNA transfer from a donor to a recipient cell. Spatial architecture, cell density, and the manner in which cells achieve physical contact in solution (by Brownian motion or by directed motility) are important parameters for controlling the dynamics of the propagation of such infection. Signaling in chemotaxis. The chemotactic system of E. coli (the network that governs the migration of bacteria towards chemical attractants) is used as a prototype for the study of intracellular signal transduction networks. Our approach is to consider the chemotactic network controlling cellular behavior as a biochemical circuit composed of independent modules. We identify the modules' contribution to the output within single living cells as well as the possible sources of their noise. Juan I. CollarMy main interest is the development of innovative methods of detection for hypothetical astroparticles (WIMPs, axions, magnetic monopoles, any yet-to-be-discovered component of cosmic rays that might constitute a fraction of the "dark matter"). I am also attracted to other exotica such as double-beta decay and some "hard" problems in neutrino detection (coherent neutrino scattering, detection of the relic neutrino sea). The approach I favor relies heavily on an understanding of the condensed-matter aspects of detector development and the interactions between radiation and matter. My concerns are eminently cross-disciplinary. Together with collaborators at Groupe de Physique des Solides (Univ. Paris 7), University of Lisbon, and Pacific Northwest National Laboratory, I have developed large-mass, low-background superheated droplet detectors (SDDs) dedicated to WIMP (Weakly Interacting Massive Particle) searches. This is a promising and cost-effective technique that will be sensitive to a sizeable fraction of the dark matter particles predicted by supersymmetric extensions of the Standard Model. At CERN I am involved in the Axion Solar Telescope (CAST), a search for solar axions using a decommissioned LHC test magnet, an interesting astroparticle spin-off from the Large Hadron Collider effort. More recently, I have been attracted by the application of micro-patterned gas detectors (new technologies initially intended for HEP applications) in non-accelerator particle physics-in particular, use of MICROMEGAS and GEM chambers as WIMP detectors sensitive to recoil directionality. I am also presently working on a new geochemical method sensitive to highly-ionizing massive particles (monopoles, quark nuggets, etc.) by looking at the production of fullerenes in their aftertrack. The possibility of developing a new generation of low-cost, high-sensitivity neutrino detectors profiting from recent advances in fiber optic sensors and optoelectronics has me particularly mesmerized these days. Albert V. CreweDevelopment of high resolution electron microscopes, particularly scanning microscopes. Theoretical and experimental electron optics. Design of magnetic lenses and electron guns. Current emphasis is on the correction of spherical and chromatic aberrations which are the limiting factors in all present high performance instruments. A new type of corrector using magnetically focused electrostatic mirrors is now being tested, and the theory is being extended to full scale high performance systems. In addition we are exploring new concepts which will extend high resolution microscopy into the low energy arena (< 1,000 volts). James W. CroninJames Cronin and University of Leeds professor Alan Watson lead an international project to study the nature and origin of rare but extremely powerful, high-energy (>1019 eV), cosmic rays that periodically bombard Earth. The project includes more than 250 scientists from nineteen nations. The scientists will practice a new form of astronomy rooted in particle physics. Construction of the Pierre Auger Observatory, a giant detector array near the cities of Malargue and San Rafael in Argentina's Mendoza Province, will be completed by 2003, but researchers plan to begin observations as early as 2001. The site will contain 1600 particle detection stations 1.5 kilometers apart, arranged in a giant grid covering 3000 square kilometers, an area about the size of the state of Rhode Island. The Auger Project collaborators hope later to construct a complementary northern hemisphere observatory which, together with the southern observatory, would allow studies of cosmic rays from the entire sky. Dean E. EastmanMy research interests include the development and application of leading edge synchrotron radiation techniques to interfaces, surfaces, thin films, and nanostructures relevant to future microelectronic devices and other novel material systems. Representative materials systems of interest include those relevant to (1) future Si CMOS-based technologies, (2) GaN and other III?V semiconductor films on Si and sapphire, and (3) multilayer magnetic thin film systems. Synchrotron radiation techniques address physical structures; e.g., nanostructures, surface and interface structures, stress/strain distributions, dislocations, defects and doping distributions, as well as chemical and electronic properties. These techniques include various X?ray diffraction, microspectroscopy, microscopy, and other scattering techniques. Various types of photoelectron spectroscopy techniques (including band-mapping) using synchrotron radiation are used to study the electronic structure of surfaces, nanostructures, and solids. In addition to synchrotron radiation techniques, atomic scanning microscopy (STM, scanning tunneling microscope, and AFM, atomic force microscope) and STEM and SEM electron microscopy techniques offer powerful complemental capabilities to study physical and electronic structures. This research utilizes leading edge synchroton radiation facilities (beamlines/end stations) at the Advanced Photon Source (APS), the Advanced Light Source (ALS) and others. It also involves collaborations with leading researchers in industry, at universities, and at government laboratories who provide samples, complementary characterization and processing capabilities as well as knowledge in addressing key questions and opportunities in advancing the state of the art. These advanced sample preparation and processing capabilities also facilitate studies of novel mesophysics phenomena involving nanostructures. Peter G.O. FreundFrom my work on the number-theoretic features of string theory connected with the algebraic geometry of the strings' world-sheets, I have been led to the study of certain two-dimensional integrable models which exhibit similar number-theoretic features. This has yielded new results on scattering processes in two-dimensional integrable quantum field theories. It has long been known that in such theories there is no particle production and the scattering of two particles determines the scattering of three or more particles. In a very large class of such theories, it turns out that even the input two-particle scattering is determined by simple considerations of quantum?geometry. Geometries involving direct products of 4?dimensional anti?de Sitter (AdS) space with a 7?dimensional compact Einstein manifold, and of 7?dimensional AdS space with a 4?dimensional compact Einstein manifold which have appeared in the context of solutions of 11?dimensional supergravity found with M. Rubin, have recently been connected by Maldacena with conformal field theory in 3 and 6 dimensions. For certain cases this connection (and similar connections in other dimensions) have been studied in quite some detail by many authors. I am considering certain solutions of this type involving minimal supersymmetry or no supersymmetry at all. We have constructed gravitational analogs of Born's nonlinear electrodynamics. In a very different vein we studied the implications of discrete scale invariance in certain rupture phenomena such as stock market crashes. Henry J. FrischMy research is focused on the experimental exploration of new phenomena at very high energies. The Tevatron at Fermilab has been the world's highest energy machine, with almost 4 ergs/collision. We have recently upgraded the Collider Detector at Fermilab (CDF) to be able to handle a factor of 20 more luminosity, and are now commissioning the detector and starting the next major data-taking run. The Chicago group has been one of the major players in the development of a new device, the Silicon Vertex Trigger (SVT), which will allow the accumulation of very large samples of hadrons containing b?quarks. The emphasis of my research in the new run will be aggressively looking for new states of matter such as supersymmetric particles, for signs of extra spatial dimensions (a possible solution to which is called "the Hierarchy problem," basically the problem of why gravity is so much weaker than the other forces), for the particles that form dark matter, or new forces and/or symmetries. The last few years I have been developing the idea of "signature-based searches," in which we test our observations in data against the predictions of the Standard Model, rather than test the predictions of a single specific model. This allows us to search for new phenomena in an unbiased fashion. The idea is catching on; however at Chicago we still have a substantial head start due to our work of the last few years. I have the temerity to predict, in writing, based on the hints we have, that we will make a major discovery in this upcoming run. If so, this will may well revolutionize astrophysics as well as high-energy physics. I am actively looking for graduate students now that the data-taking is beginning. This is a great time to join me in CDF. We have a strong group in both hardware and software; I have a philosophy that I train experimentalists, and not just high-energy physicists. The skills you will learn here will serve you well in any experimental field, I strongly believe. Robert P. GerochMy research interests have been in three areas. The first is in the structure of the partial differential equations of physics. It turns out that there is a class of such equations (called first-order, quasilinear, hyperbolic systems) that, apparently, is sufficiently broad to encompass the descriptions of all (classical) physical systems. This class leads to a "universal" formulation of physical field theories. The second is the issue of describing quantum systems via Feynmann path integrals. There has been found a class of evolution operators for which such path integrals do exist, and this class of evolution operators, remarkably enough, includes candidates that are "close," in a suitable sense, to all bounded operators. Thus, this framework provides justification for the path-integral formulation of quantum mechanics. The third-an outgrowth of the second-involves the broad mathematical structure of measure and integration theory. It turns out that there is a framework for this subject-in which, for example, both the measures and the functions to be integrated are valued in abelian topological groups-sufficiently broad to encompass all physical applications of measure and integration, yet sufficiently narrow that the subject can be developed within this framework. David G. GrierMy group is interested in understanding the mechanisms through which microscopic processes give rise to macroscopic properties in condensed matter systems. In particular, we have been studying structural phase transitions in two classes of model systems: colloidal suspensions and flux lattices in Type-II superconductors. Both of these systems consist of discrete "particles" which interact with each other and whose interactions lead to complex collective behavior. In the first case, the particles are polymer microspheres dispersed in a fluid solvent. In the second, they are individual quanta of magnetic flux piercing a superconductor. We have developed new techniques for measuring the tiny interactions between these particles while simultaneously imaging their collective behavior. Such detailed information usually is not available for conventional atomic materials undergoing phase transitions since the constituent atoms are small and rapidly moving. In this sense, our systems are "models" for studying general processes in condensed matter physics. Using our new techniques, we have discovered conditions under which microspheres carrying the same sign charge actually attract each other. This surprising result runs counter to fundamental assumptions in the study of complex fluids and helps to explain some previously mysterious collective behavior in colloidal suspensions. The same techniques should be useful in the near future for measuring interactions between polymers and proteins tethered to the surfaces of microspheres. An equivalent approach recently has led us to the first measurement of the vortex-vortex pair potential in a Type-II superconductor. We use direct imaging techniques to track the motions of the "particles" in our model systems and thus to follow the progress of phase transitions. Recently completed studies have focused on the freezing of supercooled fluids, melting of metastable superheated solids, and phonon-driven martensitic transitions in confined systems. Emphasis in the near future will be placed on phase transitions in two-dimensional and reduced-dimensional systems including flux arrays and colloidal monolayers. Experimental methods used in this work include precision digital video microscopy, optical trapping, optical interferometry, torque magnetometry and Bitter decoration. We also have begun to exploit the spontaneously ordered crystals formed by the microspheres for their beautiful and potentially useful optical properties. Ilya A. GruzbergMy research is focused mainly on electronic systems with quenched disorder. They exhibit many complex phenomena, rich variety of phases and transitions between them. Examples include mesoscopic quantum transport, localization, quantum Hall effects, metal-insulator and superconductor-insulator transitions. To understand some of these phenomena we can neglect electron-electron interactions, and then use the supersymmetry method to treat disorder averages. In other situations both disorder and interactions are important. In the most interesting and challenging cases, strong disorder and/or interactions must be treated non?perturbatively. I am also interested in disordered statistical mechanics models, such as, for example, random bond Ising models and percolation. In my research I primarily use algebraic (symmetry-based) and field-theoretical methods.
Philippe Guyot-Sionnest (See Department of Chemistry)Jeffrey A. HarveyMuch of the success of particle physics is based on situations where there is a small parameter (such as the fine structure constant in QED) and quantities of physical interest can be expanded in a perturbative series in terms of this small parameter. However there are many interesting problems for which this is not the case. These include confinement and chiral symmetry breaking in QCD and probably the questions of supersymmetry breaking and the choice of vacuum in string theory. There has been recent progress in these "non-perturbative" problems by using ideas based on supersymmetry and duality. Duality often allows one to reformulate non-perturbative questions in terms of a dual, weakly coupled description. In gauge theory the dual degrees of freedom are magnetic monopoles. In string theory the dual degrees of freedom are various types of solitonic extended objects (strings, membranes, etc.). Much of my work is focused on understanding duality and its applications both in field theory and in string theory. Particular topics I am currently studying include non-perturbative mechanisms for vacuum selection in string theory, the algebraic structure of BPS states in string theory, and the role of anomalies in understanding the structure of extended objects. I have also gotten interested in solitons in noncommutative field theory. These field theories arise naturally in string theory and generalize conventional field theories by being defined on a space with noncommuting coordinates (like the phase space of quantum mechanics). I am currently exploring a close connection between D?branes and solitons in noncommutative field theory. Roger H. HildebrandHildebrand and his students study interstellar magnetic fields by mean of far-infrared polarimetry. This technique has made it possible to observe field configurations associated with filamentary clouds, rotating clouds, collapsing clouds, shock fronts, and other interstellar environments. It has also made it possible determine the ratio of uniform to turbulent components of the field and to show that this ratio remains approximately constant from dense molecular clouds to the diffuse interstellar medium. Observations with the University of Chicago polarimeter, Hertz, at the Caltech Submillimeter Observatory have led to a wholly unexpected discovery. Comparison of results at 350 microns with earlier results at 60 microns and 100 microns from the U. of C. polarimeter, Stokes, on the Kuiper Airborne Observatory and with recent results from the James Clerk Maxwell Observatory have shown that the degree of polarization depends strongly on the wavelength. From these results we infer that the cloud medium must be heterogeneous. There must be domains at different temperatures with dust grains of different polarizing power. The results on the polarization spectrum and on field configurations are providing a basis for estimating the interference of Galactic dust emission on measurements of the polarized component of the cosmic microwave background. To extend these investigations, Hildebrand and his students are working on the design of a polarimeter for NASA's new airborne Stratospheric Observatory for Infrared Astronomy (SOFIA). Heinrich M. JaegerNanoscale Physics. On size scales below typically one micrometer, metallic, superconducting, and semiconducting structures display properties which differ fundamentally from behavior on larger scales. In this so?called mesoscopic regime, the quantum nature of electrons leads to novel behavior not present in the macroscopic limit. We are investigating the electronic and magnetic properties of metallic and superconducting nanostructures. This research explores the roles of carrier confinement, quantum fluctuations, and disorder. We utilize experimental techniques such as cryogenic and high-magnetic field measurements, scanning probe and electron microscopy, and electron-beam lithography. Current research focuses on mechanisms for guided self-assembly of nanocrystal monolayers and metal-copolymer nanocomposites, and on the emerging novel electronic transport behavior. Another line of research probes the dynamical behavior of vortex matter (quantized magnetic flux bundles with nanometer-size diameters) inside type?II superconductors. Granular Matter. Piles of dry grains of sand display a variety of behaviors that differ strikingly from those of ordinary gases, liquids, or solids. Macroscopic granular materials are typically found far from the most stable ("equilibrium") configurations. Ordinary temperature is irrelevant in driving granular systems; instead, they exhibit dynamic phase transitions in response to applied forces. An example is the transition from solid- to liquid-like behavior at the onset of particle flow and avalanching down the slope of a sandpile. In collaboration with Sidney Nagel, we have been investigating experimentally the complex, non-linear dynamics of granular flow and find that the behavior of granular materials has parallels in many other phenomena including magnetic flux creep in superconductors, charge density waves, and relaxation in spin glasses. Our current research investigates consequences of the inherently inhomogeneous force distributions inside granular matter, the glassy settling behavior as the system explores phase space, spontaneous structure formation and dynamic instabilities under applied stresses, and the propagation of energy and momentum through these highly dissipative materials. We use capacitive probes, high-speed video, magnetic resonance imaging (MRI), and x?ray tomography (at the Advanced Photon Source) to probe non-invasively the behavior of granular matter. Leo P. KadanoffWe work on non-linear systems using the techniques of statistical physics. More specifically, we are studying how turbulent, chaotic, and stochastic behavior arises in dynamical systems, particularly hydrodynamical and biological systems. For example, we have been extensively concerned with the development of simplified models for turbulence, with the nature of mathematical infinities in the flow of fluids and of bacteria, and with models of granular materials. We use both analytical and simulational methods and try to use experimental data whenever possible. Our basic goal is to understand the nature of the complex motion that can arise in even very simple systems. This work has applications to mathematics, astronomy, and chemical engineering. Woowon KangMy research centers on the studies of novel, quantum mechanical effects in low-dimensional condensed matter systems. The collective response of a system consisting of many identical particles differs considerably from its single-particle behavior due to interaction between different particles. Unusual behaviors and phase transitions emerge as a classical system adiabatically enters the quantum regime. Model systems of interest include low-dimensional semiconductors and a class of charge-transfer molecular conductors. These systems serve as table-top testbed to study new types of ground states and excitations. Currently ongoing projects include:
Kwang-Je KimInvestigation of particle and photon beams and their mutual interactions with the goal of developing novel accelerators or radiation devices. Phase-space evolution of synchrotron radiation beams and quantum particle beams. Classical and quantum analysis of self-amplified spontaneous emission for ultra-high brightness x-ray beams. Production of high-brightness electron beams. Radiative laser cooling of electron and ion beams. Optical stochastic cooling. Application of high power lasers to particle and radiation beam techniques. David KutasovThere are many fascinating relations between gauge and string theories. Gauge theories describe the low energy limit of string theories, and both classes of theories have been discovered to enjoy certain strong-weak coupling duality symmetries that allow one to learn a lot about their quantum behavior. Also, it is believed that strongly coupled gauge theories can be described by a string theory, the "QCD String," which has been constructed in some cases. My work in recent years has focused on different aspects of the interface of gauge and string theories, such as the construction and analysis of interesting gauge theories, realized as low energy dynamics on branes, studies of string theories which are equivalent to field theories via a certain strong-weak coupling duality known as the AdS/CFT correspondence, and the construction and study of non-local, non-gravitational theories using the dynamics at singularities in string theory. I have also been involved in analyzing the physics associated with vacuum instabilities ("tachyons") in string theory and the thermodynamics of certain black holes that arise in string theory. Kathryn LevinOur interest is primarily in theoretical studies of exotic superconducting and magnetic systems. We ask questions such as what makes different materials magnetic or superconducting? What is the nature of their superconductivity? The co?operative effects associated with magnetism and superconductivity lead to some of the most exciting and mysterious phenomena within the sub-field of condensed matter physics. Our experimental colleagues continue to discover and create new materials with fascinating properties, such as the high Tc and more recently discovered strontium ruthenate (p?wave) superconductors. What distinguishes our sub-field of physics is the close and timely interaction between theory and experiment, as well as the long range possibility for making technological impact. This close theory/experiment interaction is at the heart of our research agenda. Currently, we (along with the community) are re-examining the applicability of BCS theory-the standard theory for "conventional" superconductors-to exotic superconductors, such as the high Tc materials. Our particular generalization of the BCS approach will help elucidate the relation between Bose condensation (at the heart of superfluidity) and its relation to superconductivity. Riccardo Levi-SettiA Scanning Ion Microprobe (UC?SIM) has been developed that performs Secondary Ion Mass Spectrometry (SIMS) imaging microanalysis of materials in the sub?100 nm range of lateral resolution. The mass analysis of the secondary ions is carried out with a high performance magnetic sector chemical mass spectrometer. This by now renowned instrument continues to find unique application in the study of advanced ceramics, in collaboration with DuPont scientists, and in a range of biological studies. The ability to detect isotopes is of extreme value in cytogenetics, allowing the distribution of labeled nucleosides to be mapped in human metaphase and Drosophila polytene chromosomes. In collaboration with scientists of the Department of Medicine of our University, investigations on the role of divalent cations in the scaffold structure of human metaphase chromosomes are in progress. Systematic studies of bone physiology continue also to be pursued in collaboration with researchers of the Department of Medicine of the University of Rochester, N.Y. Joseph D. LykkenMy interests in particle physics have not varied much since I was in the third grade. My personal and professional goal is enlightenment on the really big questions which only particle physics can answer: What is the universe made of? Where did it come from? How does it work? I am very excited about the coming decade, during which we will have data from a powerful new generation of experiments in particle physics and astrophysics, while at the same time we exploit remarkable new concepts and tools emerging from the theoretical study of string theory, gauge theories, and cosmology. My immediate research interests mostly have to do with supersymmetry and with extra dimensions. Both are predicted by string theory, but string theory (as yet) does not specify if either is involved in the physics of the electroweak scale. Supersymmetry is by far the best developed idea for new physics at the electroweak scale, and there are strong hints from data that SUSY is really there. However it is my opinion that none of the existing detailed models for SUSY are correct; all of them have awkward features which, to me, is a sign that some essential ingredient is missing. What is it? Extra dimensions are probably around, but they are hidden. How are they hidden? There are at least four ways that I know of to hide them, but which ways does Nature use? Gravity sees the extra dimensions, but gravity is hard to study, both theoretically and experimentally. This is a fascinating challenge. Emil J. MartinecMy work aims at uncovering the structural foundation of string theory (now sometimes called M?theory), which attempts to unify the basic forces. The theoretical underpinnings of the subject are finally taking shape, due to the recent discovery of nonperturbative dualities between different descriptions of the theory. The spacetime manifold on which particles and waves propagate is being replaced by a more "stringy" notion of geometry at short distances and/or in strong fields. One issue of interest to me is how string theory incorporates black holes as quantum states. This problem has turned out to be a rather sensitive probe of the theory, and will likely lead us to new notions of space, time, and dynamics. Gene F. MazenkoOur group has been concerned with understanding the growth of order in unstable systems. The simplest example of such a system is a ferromagnet, initially disordered at some high temperature, then subjected to a rapid temperature quench to a temperature below its Curie temperature. The system is now in an environment where it wants to develop a spontaneous magnetization. However, in the absence of any symmetry breaking fields, the system does not know in which direction to order. Each local magnetic moment aligns with its neighbors, and spontaneous local ordering occurs. There is then a competition between the orientations of different local regimes. This competition results in regions of frustration which are relieved by the formation of defects. Depending on the nature of the order parameter, these defects can be domain walls, vortices, monopoles, strings, disclinations, etc. Our work has centered on developing a theoretical approach which can answer questions like: At a time t after a quench, how many vortices are there per unit volume in a system? How many of these vortices decay via vortex-antivortex annihilation during a certain time period? What is the average speed of a vortex? This type of work has applications in cosmology, superconductivity, superfluids, liquid crystals, magnetism, and in the physics of fluids. Frank S. MerrittI am a member of the OPAL collaboration, which has been carrying out high-precision measurements of electroweak physics at the Large Electron Positron accelerator (LEP) at CERN. Measurements near the Z0 resonance (91.2 GeV) over a 5?year period have provided the most precise tests (<0.1%) of the Weinberg-Salam electroweak theory and other physics. The LEP energy has been substantially increased over the last few years, reaching 200 GeV. This has opened up a number of new research areas, including the most sensitive search for the Higgs boson, the high-precision measurement of the W?boson mass through W+W- pair-production events, and searches for supersymmetric particles and other new physics. Our group is now working on W?mass measurements, Higgs searches, other new-particle searches, precision measurements of the tau lepton, and heavy flavor physics. I am also a member of the Chicago/ATLAS group, now developing the hadron calorimeter to be used in the ATLAS experiment at CERN's Large Hadron Collider. This will be by far the highest-energy accelerator in the world, and will take us into new physics beyond the Standard Model: supersymmetric particles, technicolor, and/or unexplained new phenomena. The Chicago group's major software effort focused initially on analysis of calorimeter test-beam data and now on development of the analysis system and code for ATLAS data analysis. Stephan Meyer (See Department of Astronomy and Astrophysics)Dietrich MüllerOur research is directed towards several areas of high energy astrophysics: Origin of high energy cosmic rays. Particle acceleration is an ubiquitous process in nature that occurs in all astronomical settings, from the surface of the sun to exotic, distant galaxies. In our galaxy, shock fronts from supernova explosions appear to be the sites where cosmic rays are accelerated over a large range of energies. However, the details of these processes are still poorly understood, and at the highest energies, above 1015 eV, other and as yet unknown sources of cosmic rays must exist. We aim to obtain observational constraints on the various models proposed for the origin of cosmic ray particles through precise measurements of the elemental composition and energy spectra of the individual cosmic ray components. Based on a series of investigations on high-altitude balloons and on the Space Shuttle, we have constructed a new and very large instrument, TRACER, that uses transition radiation detectors to permit measurements in the presently unexplored energy region between 1014 and 1015 eV. This detector exhibits a large sensitive area of nearly 5 m2, weighs about 3 tons, and is carried by stratospheric balloons above the earth's atmosphere, at a height of nearly 40 km above ground. A first balloon flight was conducted in 1999, and a long-duration flight to circle the Northern Hemisphere for about two weeks will be launched in 2002. TRACER also serves as a prototype for an instrument that may become part of the ACCESS mission which is now under consideration by NASA. ACCESS will be the largest cosmic ray detector ever flown in space, and will be mounted on the International Space Station for a duration of 3?4 years. Search for anti-particles in the cosmic rays. Positrons and anti-protons are the only antiparticle species detected thus far in the cosmic radiation. They are generated subsequent to nuclear interactions in interstellar space and therefore provide an interesting probe on the structure of the interstellar medium and on the propagation of particles through the galaxy. However, there might also be more exotic contributions to the observed intensities of these particles, for instance, from the decay of hypothetical dark matter particles. These issues are studied with a balloon borne detector system, the High-Energy Antimatter Telescope (HEAT), which includes a superconducting magnet spectrometer. This instrument has been successfully flown in 1994 and 1995 to measure the flux of positrons and electrons over a wide energy range. At present, a modified detector system is used for the observation of high-energy antiprotons in balloon flights in 2000 and 2001. This work is conducted jointly with Simon Swordy and collaborators at several other institutions. Gamma-ray astronomy. A major frontier area in gamma ray astronomy is the region of high energies, from about 100 GeV to several thousand GeV. In this energy region, supernova remnants in our galaxy could be identified, demonstrating that these are indeed the sites where cosmic rays are generated, and exciting discoveries wait to be made with the observation of extragalactic objects. Detectors in space do not have sufficient sensitivity for these observations, but arrays of Cherenkov telescopes on the ground can observe the gamma-ray showers generated in the Earth's atmosphere. We participate in the construction of the VERITAS (Very Energetic Radiation Imaging Telescope Array System) installation in Arizona which has just begun and should lead to a 7?telescope array in 3?4 years. This work is conducted jointly with Simon Swordy, and in collaboration with a number of institutions in the US and in Europe. Sidney R. NagelMany complex phenomena are so familiar that we forget to ask whether or not they are understood. Indeed, some effects are so ubiquitous that we hardly realize that they defy our normal intuition about why they occur. Examples of poorly understood classical physics include the anomalous flow of granular material, the long messy tendrils left by honey spooned from one dish to another, and the pesky rings deposited by spilled coffee on a table after the liquid evaporates. These are all non-linear hydrodynamic phenomena which not only are of technological importance but can also lead the inquisitive into new realms of physics. It is problems such as these which have fueled much of my research effort. Another emphasis of my work can be classified as an attempt to understand the properties of disordered materials. I have worked on several different projects which involve a range of disciplines from conventional solid-state physics to non-linear dynamics with some applications to geophysics. I list here a few of the topics on which my group is currently working: Granular Materials. In collaboration with the group of Heinrich Jaeger, we have been studying the properties of granular media. Despite their ubiquity around us and the simplicity with which we can describe them, we understand very little about how these materials (e.g., sand) behave. In these studies we enter a new area of physics in which we are studying a statistical system of many particles but where the temperature is totally irrelevant. Thus, these systems are unavoidably always out of equilibrium, and we must come up with new concepts in order to understand and predict their properties. Glass Transition. The glass transition has been called "the deepest and most intereresting unsolved problem in solid state theory." We have been studying this transition using a variety of techniques including neutron diffraction, specific heat spectroscopy, computer simulation, dielectric susceptibility, shear modulus. We have managed to produce a master curve onto which all the dielectric data from all samples over 15 decades in frequency can be scaled. Such scaling has important implications for the nature of the glass transition. Jamming. In an effort to deal with diverse phenomena where systems become stuck in a region far from equilibrium (e.g., at the glass transition and in clogged granular materials flowing-unsuccessfully-through a pipe), I have been investigating, along with Andrea Liu at UCLA, whether there can be a more general way of looking at these systems in terms of a "Jamming Phase Diagram." We proposed such a diagram and are currently investigating its utility in connecting a wide range of observed phenomena. Singularities in Free-surface Flows. A drop falling from a faucet is a common example of a liquid fissioning into two or more pieces. The cascade of structure that is produced in this process is of uncommon beauty. As the drop falls, a long neck, connecting two masses of fluid, stretches out and then breaks. What is the shape of the drop at the instant of breaking apart? Something dire must happen to the mathematical description of the liquid at that point since the drop undergoes a topological transition where it starts out as a single, connected fluid and ends up in two or more separate pieces. This is an example of a finite-time singularity since the drop breakup occurs in a short time after the drop becomes unstable and starts to fall. At the transition, a singularity occurs since the radius of the neck holding the drop to the nozzle becomes vanishingly thin. As its radius goes to zero, the curvature diverges and the surface tension forces become infinite. How can such dramatic dynamics occur in something which had such smooth and innocuous initial conditions and forcing terms? Using photographic techniques, we have been studying transitions such as these to understand how the non-linearities in the governing equations (in this case the Navier-Stokes equations) can be tamed and understood. Singularities of this kind occur in many areas of physics from stellar structure to turbulence to bacterial colony growth. This drop breakup problem is one of the simplest places to start an experiment which directly probes the singularity itself. Encapsulation of Biological Cells for Transplantation. Using the fluid experiments of the kind described above, in collaboration of Milan Mrksich in our Chemistry Department we have invented a new procedure for encapsulating small particles. This method may be particularly useful for coating biological cells for transplantation or for coating drugs for controlled time-release. Crumpling. How does a sheet of paper crumple into a small ball? If you squeeze a sheet of paper very hard by hand, nearly 80% of the volume is still filled by air. What gives the crumpled ball its strength? We have been attacking this question experimentally in connection with the theoretical investigations of Tom Witten. We have found surprising time dependence and histeresis in the response. Yoichiro NambuFermion hierarchy problem. The masses of the six generations of quarks and leptons are spread over almost six orders of magnitude for the charged particles, and another six orders if the (not yet well known) neutrinos are included. The pattern of the masses and mixings show no precise regularity. In the highly successful Standard Model, they are mere phenomenological parameters. So far none of the existing theories based on various unified theories have been able to offer a satisfactory account of them. I regard the problem of mass as something that should be looked upon afresh and without prejudice. Specifically I have been trying (1) to uncover some unnoticed regularities in the masses and mixings which might serve as a clue to model building and (2) to search for a general dynamical mechanism for generating hierarchies. I have found one possible new and simple regularity for quarks, but I do not have a theory about it yet. Aharonov-Bohm media. This is a long-standing unsolved mathematical problem which has interested me for twenty years: the behavior of an electron in a medium filled with nonquantized magnetic fluxes. The motivation came from my speculation that it might serve as a model for such theoretical ideas as the quark confinement in a medium of monopoles. If the fluxes are all parallel, the problem reduces to that of solving the Schroedinger equation in two-dimensions in the presence of magnetic charges. Only explicit solutions for a single flux had been known, and its extension to many fluxes has turned out to be a highly nontrivial mathematical problem. But recently I have succeeded in constructing the solutions. Two different types emerge. In one, both charges and fluxes have to be treated as dynamical objects. In the other, the fluxes are fixed objects and a magnetic field is present, but the solution has no zero-field limit. But all the solutions suffer from some unphysical properties. For example, the aysmptotic states are not complete in angular momentum, so a scattering amplitude cannot be contructed. The origin of the difficulty is not clear. Hagedorn-Rumer phenomena. It is well known in hadron multiple production (according to the so-called Hagedorn fireball model) as well as in string theory that the density of states increases exponentially with energy and competes with the Boltzmann factor so that thermodynamic equilibrium cannot be maintained above a certain limiting temperature. I have been interested in this as part of my search for phenomena that defy thermodynamics. The entropy of the black hole is of a similar but more drastic nature, and has been the subject of intense study in string theory lately. In classical physics I have found two simple examples of exponential density of states. One is a particle in a logarithmic potential, which had already been pointed out by Y. Rumer in 1960. The other is a particle under gravity and placed in a vessel whose horizontal cross section grows exponentially upwards. This phenomenon should be observable at room temperature for particles of atomic weight around 107 (e.g., viruses and synthetic microspheres)! There are also indications that small atomic clusters have an exponential density of states, according to S. Berry. I suspect this is a rather general phenomenon which might eventually become relevant to biology and the black hole problem. Reinhard OehmeThe confinement of gluons and quarks is a fundamental problem in non-perturbative quantum chromodynamics. Some time ago, we have obtained results about the phase structure of ordinary and SUSY gauge field theories on the basis of superconvergence relations and the BRST cohomology. Now we find that, for SUSY theories, our conclusions agree with those obtained more recently on the basis of duality. In contrast to duality, our methods are applicable to non?SUSY theories. As a general method of imposing restrictions on quantum field theories with several parameters, we have introduced a theory of reduction of couplings. This method is based upon the renormalization group, and is more general than the imposition of symmetries. There are solutions of the reduction equations which do not correspond to additional symmetries, but may be related to aspects of superstring theories. Our reduction theory is finding a wide range of applications. A comprehensive article for Physics Reports is in preparation. Previously, in connection with the introduction and the proof of dispersion relations for hadron scattering, we have discussed effects of possible violations of locality and Lorentz invariance. With string theory, and in particular with field theories on non-commutative spaces, there are now more explicit models, and we are studying their implications. In addition to the absorptive thresholds and the composite structure cuts, there appear new singularities. Mark J. OregliaI am currently spending most of my time with photons. As a member of the OPAL experiment, I have been working with students and postdoctoral fellows to find the Higgs Boson, both in its "minimal" conventional form and in more exotic decays into photons. This work is carried out at the University and at the Large Electron Positron accelerator (LEP) at the CERN laboratory in Geneva, Switzerland. The OPAL collaboration has been conducting precision tests of the electroweak theory, searching for new particles which could couple to electron-positron collisions, and studying properties of b? and c?quarks and tau leptons. Other new particles for which I am searching are supersymmetric particles and excited quarks. The LEP program finished taking data at the end of 2000, and currently we are finishing the data analyses. There were tantalyzing hints of a Higgs Boson in the LEP data, but unfortunately not conclusive evidence. Therefore, I am also a member of the ATLAS Experiment. I am organizing the searches for non-standard Higgs bosons at the Large Hadron Collider Project (LHC), where we will have a mass reach up to 200 GeV. Indirect evidence from LEP virtually guarantees that new physics will be evident in the LHC mass reach. See Professor Pilcher's research description for more details on ATLAS. For study of particles in the 100-200 GeV mass range and higher, a new accelerator will be necessary in the next decade. I am engaged in research and development of acceleration techniques (together with Professor Kwang?Je Kim) for future accelerators. In particular, I am currently developing beam profile monitors which work at liquid hydrogen temperatures in very intense muon beams. This work requires the use of high-Tc superconductors and bolometric techniques. Photons from the cosmos cover all energy ranges and can be unique signatures of high energy processes in astrophysical objects, such as collapsing massive objects and Active Galactic Nuclei. Photons have an advantage over charged particles in that they do not bend in the intergalactic magnetic fields, and therefore point back to their source. I am a member of the Gamma Ray Large Area Space Telescope (GLAST) collaboration, which hopes to launch a new satellite instrument with state-of-the-art technologies in 2005. I have contributed to the calorimeter for spaceborne mapping of gamma ray point sources in the energy range of 10 MeV-300 GeV. James E. PilcherA group of us from Chicago is studying high energy electron-positron annihilations with the OPAL experiment at the Large Electron Positron accelerator (LEP) at CERN. The work has involved precise measurements of the properties of the Z and W bosons with several tests of the electroweak theory at accuracies of 0.1%. One of the parameters used in the theory's predictions is the Higgs boson mass, so that fitting the measurements with the theory leads to an estimate of this mass. The result is the most quantitative determination available for the Higgs mass. The Higgs boson is believed to be the source of electroweak symmetry breaking, and its interactions with the fermions determines their masses. It plays an extraordinarily important role in determining the nature of the universe. It is exciting to note that the present best fits to the electroweak data point to a mass close to the current upper limit of our direct searches. The LEP facility has operated at a center-of-mass energy of over 200 GeV, and a data sample corresponding to 1000 pb?1 has been collected. Our group is involved in measuring the mass of the W boson, an essential parameter in the electroweak theory. The error on this measurement should be less than 50 MeV. We have also been searching for direct production of the Higgs boson, particularly in final states involving tau leptons or photons. Our current lower limit on its mass is over 110 GeV. Another study in progress is of W?bosons decaying to final states involving charmed quarks. The LEP facility offers the first opportunity for the direct observation of hadronic decays of W bosons, and the decay rate to charmed quarks provides a measurement of the Cabibbo-Kobayashi-Maskawa (CKM) parameter Vcs. Data collection with the OPAL experiment was completed in 2000, and there is the opportunity for thesis work using the full data sample of up to 1000 pb?1. We are also involved in the preparation of the ATLAS experiment for the Large Hadron Collider (LHC) being built at CERN. This facility will allow most of the possible mass range for the Higgs boson to be directly studied and will also allow important searches for supersymmetric states. When it begins operation in 2006, it will be the highest energy facility in the world, surpassing the Fermilab energy by over a factor of 7. At Chicago, we are preparing the detector's hadron calorimeter, which measures the energies and trajectories of quarks and gluons produced in the high energy collisons. In particular we are building very high speed readout electronics for the calorimeter signals. There is the exciting opportunity for new graduate students to gain experience in state-of-the-art electronics. We are also preparing physics studies for searches for extra dimensions and missing energy channels which exploit the calorimeter capabililties. Thomas F. RosenbaumAt temperatures near absolute zero, new collective phenomena become possible. The quantum mechanical nature of materials is highlighted at the low temperatures, leading to a different class of phase transitions and to states with unusual excitation spectra. I use dilution refrigerator techniques to explore quantum magnets and glasses with connections both to quantum phase transitions and to the encoding of information, metal-insulator transitions with choreographed charge and spin degrees of freedom, new magnetoresistive compounds, vortex creep and tunneling, and exotic superconductivity. MilliKelvin temperatures often are combined with symmetry-breaking fields, such as uniaxial stress, to help constrain theory on fundamental grounds. These disparate topics are united by the theme of the interplay of correlation effects and disorder and by the issue of how macroscopic order can emerge from microscopic disorder. Jonathan L. RosnerRecent experiments at Fermilab and CERN and some non-accelerator experiments (including measurements of parity violation in atoms) have permitted tests of the theory of electroweak interactions with unprecedented accuracy. Studies are in progress to determine how these experiments shed light on new physics in the mass range of 100 GeV to several TeV. A parallel line of investigation deals with the weak couplings of quarks to one another, as parametrized by the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Various ways of learning these couplings more precisely are being studied. A key role is played by experiments on decays of mesons containing the "charm" and "bottom" (or "beauty") quarks. Proposals are made, based in part on present data, for ways to detect the violation of CP symmetry (the combination of charge and space inversion) in decays of these mesons. Central to these studies is the "top" quark, discovered in experiments at Fermilab. Knowledge of the top quark mass sheds valuable light on other parameters of the electroweak theory, including the mass of the Higgs boson and the nature of the quark couplings governing CP violation. Other topics being investigated include properties of systems involving one or more heavy quarks (charm, bottom, and top), the possibility that quarks and leptons have a composite structure, the role of neutrino masses in a more general understanding of fermion masses and couplings, the possibility of detecting new quarks and leptons with unusual quantum numbers as predicted in certain unified theories of the electroweak and strong interactions, and the experimental signatures of gauge theories beyond the standard SU(3) X SU(2) X U(1) electroweak-strong theory. Rosner also has completed an experiment to search for and study radio-frequency (RF) pulses accompanying cosmic ray air showers, and is currently analyzing the data. This work is intended as a prototype for a system at the proposed Auger Giant Air Shower Detector. Other possibilities for RF detection of cosmic ray showers are being investigated as well. Robert Rosner (See Department of Astronomy and Astrophysics)Guy SavardMy current research is centered around low-energy tests of the Standard Model of electroweak interaction. These studies are performed on samples of radioactive ions captured in ion traps where they are available for high-precision experiments. New techniques developed by our group to efficiently capture short-lived isotopes of essentially any species in ion trap allow us to select isotopes with decay properties which enhance and isolate specific effects and hence increase our sensitivity to the physics of interest. The present experimental program is looking at a more precise determination of the weak vector coupling constant and a more precise test of CVC and the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. A related experiment currently in preparation will search for scalar currents outside the standard V?A form for the charged electroweak interaction using samples of trapped superallowed emitters. The type of experiment I am performing would greatly benefit from a more intense and versatile source of radioactive ions, and such a source has been assessed a high priority for new construction in nuclear physics in the US. I am therefore also heavily involved in the R&D for such a facility, which will use a novel technical approach for the fast extraction of the radioactive species based on some of the technologies we developed for ion trapping. (The leading contender for this facility is Argonne National Laboratory, located a short distance from Chicago.) John P. SchifferSearch for simple symmetries in nuclear structure using heavy-ion reactions; understanding dynamics of interactions between nuclear systems. Study of crystalline order, phase transitions, and degrees of freedom in confined ionic systems in ion beams and ion traps. Search for exotic objects in nature; e.g., particles of integral charge but anomalous mass such as strangelets, electron-positron peaks observed in conjunction with very high electromagnetic fields, "17-keV" neutrinos, etc. Measurement of nuclear properties that are important in the processes of nucleosynthesis, with particular attention to the breakout point from the hot CNO cycle that is crucial in producing elements heavier than oxygen, and to the properties of some of the nuclei that are the principal visible remnants of supernova explosions. Savdeep S. SethiIn order to answer basic questions about the nature of spacetime and gravity, we need a quantum theory of gravity. String theory remains the most promising candidate for such a theory. In addition, string theory naturally melds the strong and electroweak forces with gravity in a unified framework. For many years, it appeared that there were a number of distinct string theories. However, it has become clear in recent times that each of these distinct theories is a limit of a unique eleven-dimensional theory known as M theory. Uncovering the structure of M theory is likely to radically alter our notions of spacetime and gravity. My research centers on understanding various aspects of M theory, string theory, and Yang-Mills theories. Among the topics that I am currently studying are the constraints imposed by supersymmetry on string theories and Yang-Mills theories, D?brane dynamics, and compactifications of M theory to three and four dimensions. Melvyn J. ShochetMy research is carried out with the Collider Detector at Fermilab (CDF), which we built to study collisions between 1000 GeV protons and 1000 GeV antiprotons. With the large data samples we have accumulated, at a center-of-mass energy three times larger than any previously produced, we have been able to study the strong and electroweak interactions and search for new phenomena. These studies include measuring the production cross sections for energetic quark and gluon jets, photons, and W and Z bosons; making a precision measurement of the mass of the W boson; measuring the production and decay properties of mesons containing b quarks; and searching for objects that are not included in the Standard Model of the elementary particles and their interactions, such as supersymmetric particles, heavy gauge bosons, and leptoquarks. Our most important result is the discovery of the top quark and the measurement of its mass. The initial evidence for the top quark was presented in April 1994, with the confirmation of the discovery announced in March 1995. At a mass of 174±5 GeV, the top quark is by far the most massive of the elementary constituents of the universe. We have just built a new CDF detector that will take data for the next six years using the upgraded Fermilab Tevatron accelerator. We will collect two orders of magnitude more data than before, which will greatly improve our measurement of the properties of the top quark and our sensitivity to new phenomena. In Chicago we designed and built electronic systems for selecting in real time the most important 300 collisions out of the 10,000,000 that will occur each second. This includes the Silicon Vertex Trigger (SVT), an important new tool for hadron colliders that selects collisions based on the presence of b quarks in the event. The SVT is needed for the study of the top quark, the search for new phenomena, and the study of the asymmetry between matter and antimatter. Simon P. SwordyMy research is directed to observations and analysis of high-energy radiation from space mainly through the following experimental efforts. I am involved with several projects to make measurements of high energy gamma rays and cosmic ray particles: The VERITAS experiment (Very Energetic Radiation Imaging Telescope Array System) is a collaboration formed to build the next generation of atmospheric Cherenkov telescopes for gamma ray measurements. This will consist of an array of telescopes located near Tucson, Arizona, which can be used for gamma ray detection on dark, moonless nights. VERITAS can be used to directly search the putative supernova remnant sources of cosmic rays for the gamma rays expected to be produced at these sites. The level of sensitivity of VERITAS is sufficient to determine whether or not hadrons are accelerated to energies near 1000 TeV in these sources as expected in the "standard model" of Galactic cosmic rays. The CREAM experiment (Cosmic Ray Energetics And Mass) is a new experiment which combines transition radiation and thin calorimetry to measure cosmic rays near the "knee" of the spectrum at 1000 TeV. This is planned for future 100?day ultra-long-duration balloon flights. This experiment is presently under development in collaboration with Pennsylvania State and the University of Maryland. ACCESS is a cosmic ray experiment planned for the International Space Station. This will be the largest experiment on the Space Station at about 11,000 lbs. launch weight. The science goals include the measurement of all cosmic ray nuclei into the "knee" of the overall spectrum. At present this instrument is under study at NASA for launch in 2007. DICE, the Dual Imaging Cherenkov Experiment, operated for nearly three years (1994?97) at the Dugway, Utah, site of the Chicago Air Shower Array (CASA). This experiment was specifically designed to detect changes in cosmic ray composition across the knee of the spectrum. The results indicate that contrary to expectation the mean mass of cosmic rays tends to decrease slightly across this spectral feature. Recent efforts at correlations of the DICE measurements of atmospheric Cherenkov with particles at ground level have supported these composition measurements. Future developments of Cherenkov imaging of air showers are being explored. Michael S. Turner (See Department of Astronomy and Astrophysics)Carlos E.M. WagnerTwo of the most relevant open questions in particle physics are the ones related to the origin of mass and to the existence of a unified theory of all known elementary particle interactions. The answer to the first question relies on the mechanism of electroweak symmetry breaking, which can be tested through the corresponding Higgs physics at high energy collider facilities. A first step towards the answer of the second question is the search for unification of couplings at high energies, in theories which go beyond the standard model description of particle interactions. I am mostly interested in issues connected to these two fundamental questions. I am currently working on Higgs physics in low energy supersymmetric theories; on the question of unification of couplings and radiative electroweak symmetry breaking in these theories; and on cosmological issues, mostly related to the possibility of generating the observed baryon number at the electroweak phase transition. I am also interested in looking for ways of testing these theories through the data coming from high energy collider facilities like the Tevatron collider, which started operation in April 2001 at the Fermi National Accelerator Laboratory in the Chicago area. Yau W. WahMy research centers around neutral kaon rare decays with emphasis on the question of charge-conjugate-parity (CP) violation. The decay modes of kaon into a neutral pion and a pair of leptons are particularly interesting in that they are CP violating, but have never been seen. A series of experiments, Kaons at the TeVatron (KTeV), has been conducted at the nearby Fermi National Accelerator Laboratory. This experiment had the best sensitivities for a large variety of kaon and pion decay modes and provided unprecedented high sensitivity studies of particle properties and searches for violations of basic symmetries. We are currently analyzing the full data sample (collected in 1996, 1997, and 1999) to make a precise measurement of direct CP violation. Robert M. WaldMy research mainly has focused upon the theory of quantum phenomena in strong gravitational fields, particularly quantum effects involving black holes and black hole thermodynamics. My interests also span attempts to formulate a quantum theory of gravitation (where no background classical metrical or causal structure of spacetime is present), mathematical investigations of classical general relativity, and applications of general relativity to cosmology and astrophysics (such as gravitational lensing phenomena and gravitational radiation reaction effects). Paul WiegmannMy research focuses on the non-perturbative aspects of quantum field theory, primarily electronic systems, where interaction controls the properties of the system. This includes electronic physics in low dimensions, quantum Hall effect, and strongly correlated electronic systems. In recent works on condensed matter physics, I have concentrated on topological aspects of quantum interference. In the area of mathematical physics, my interests include completely integrable models of quantum field theory and statistical mechanics, quantum groups, and anomalies in quantum field theory. In recent years I have also become interested in singular behavior developing in dynamical systems. Bruce WinsteinI have been engaged for the past several years in precise studies of very small symmetry violations in kaon decays. During my recent sabbatical in Princeton, I worked on the detection of cosmic microwave background radiation. I will describe each of these in turn. Kaon Physics. CP symmetry, which reverses left and right and changes particles into antiparticles, was shown to be violated in the decay of the neutral kaon, this in 1964 by James Cronin and colleagues. The only established manifestation of this violation to date is in the transition from the neutral kaon to its own antiparticle: that transition amplitude differs from the "time reversed" one by 0.23%. CP violation is important since it likely accounts for the lack of antimatter in the universe: there is a force which distinguishes matter from antimatter. Along with Professors Blucher, Wah, and Winston and investigators at ten other institutions, we constructed a new detector (KTeV) which was designed to detect another manifestation of CP violation, "direct CP violation," which should be present if our understanding of the mechanism (Standard Model) is correct. The core of the new experiment is an array of 3100 CsI crystals to precisely measure the energies of high energy gammas from kaon decays. The work on the crystals and on the electronics for triggering purposes was carried out on the Chicago campus. The Fermilab experiment, which also studies a host of very rare kaon decays, ran in 1996 and in 1997 and was upgraded for a very successful run in 1999. The search for direct CP violation requires many millions of CP violating decays and an unprecedented level of understanding of any possible biases or small asymmetries in their detection. We have so far reported on only 15% of the data collected. But a very large and significant result-0.00280 with an error of ±0.00041-was obtained which serves to establish the new manifestation at nearly 7 standard deviations. This value is larger than the majority of calculations available prior to our result, and it is not clear if it can be accommodated within the standard model. Hence there is great interest in the results we will obtain with the rest of the data, as well as in the results that a CERN group will report. KTeV has so far published about 12 papers. One particularly pretty result concerns the observation for the first time of what is called a T?odd asymmetry in a particle decay. This comes about because of the known CP violation in the kaon quantum state, but it had been difficult to see before KTeV. The analysis of the rest of the KTeV data will continue through 2001, and many more results are expected. KTeV is not expected to run again, but our group is playing a key role in determining the viability of a new initiative (KAMI) which would build on the core of the KTeV detector to study other CP violating and rare kaon decays at significantly higher sensitivity. Cosmic Microwave Background Radiation. During my sabbatical at Princeton, I worked on an experiment which aims to detect the polarization in the microwave radiation. The anisotropies in the polarization are expected to be at the level of 10% of the temperature anisotropies and should be evident at small (1 degree) angular scales. Gravity waves at the time of inflation, depending on the details of the "inflation potential" can also be detected in the polarization, but this will be far fainter and at larger angular scales. Polarization has not been detected but is being sought by many groups using a variety of technologies. It will help confirm our picture of the physics of the early universe as well as contribute to the determination of the cosmological parameters. I am starting a small group to continue in this area. Roland WinstonSolar energy. The subdiscipline of nonimaging optics makes it possible to concentrate sunlight in excess of solar surface intensities. Our group is using ultra high solar flux to pump lasers and generate very high temperatures. Nontracking collectors are being developed for solar cooling. Experimental particle physics. The weak interactions of hyperons provide an important testing ground for theories of the symmetry and structure of elementary particles. Our group has been participating in precision studies of neutral kaon and hyperon decays, Kaons at the TeVatron (KTeV) at Fermi National Accelerator Laboratory. The KTeV Collaboration is currently analyzing the full data sample (collected in 1996, 1997, and 1999) to make a precise measurement of direct CP violation. Thomas A. WittenMy research concerns the statistics of "structured" fluids, containing polyatomic species such as polymers, surfactant micelles, or colloidal particles and aggregates. One current interest is heterogeneous polymers, in which competing interactions give novel structures and metastability to a polymer fluid. We are studying new structures that such polymers take on when they are near a solid substrate. We are studying the consequences of topological and twisting constraints in polymer fluids. We are also studying the mechanics of crumpled elastic membranes, surfactant monolayers, and granular materials. We wish to understand the distinctive ways in which stress propagates and relaxes in these materials to give them their distinctive strength characteristics.
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