graduate research profiles

Graduate Student Research

Research opportunities for graduate students abound. In addition to working directly with a Physics Department faculty member, graduate students also conduct research with faculty in Astronomy & Astrophysics, Chemistry, Geophysical Sciences, Molecular Engineering, as well as at Fermilab, Argonne, CERN, and many other facilities.

In many cases, graduate students can get involved in research immediately upon beginning their first quarter in the program. A great way to explore an experimental research group early-on is to satisfy the experimental physics requirement by doing a year-long project with that group.

Below, you will find a sampling of some of the fore-front-level research being done by our graduate students. 


Alex EdelmanALEX EDELMAN
Research: Condensed Matter Theory
Advisor:  Peter Littlewood

Degrees:
- B.S., Carnegie Mellon University, 2012
- M.S., University of Chicago, 2013
Graduate Student (2012-pres) Dept. of Physics, James Franck Institute
Awards:
Sachs Felowship, 2013

Condensed matter physics is concerned with the emergent behavior of systems with many strongly interacting constituents. Explicitly tracking every particle - every molecule in a glass of water, say - is computationally infeasible, and in any case frankly uninteresting, whereas the dominant low-energy physics that emerges - for instance the wave that forms when the glass is swirled around, characterized by a few simple parameters like an amplitude and a speed - is both tractable and ultimately the behavior that dominates our observations of the system.

I study strongly correlated electronic systems, so called because the collective behavior of the constituent electrons (among others) is mostly determined by their interactions, departing radically from what could be anticipated from the properties of a single particle. The spectacular phenomena that result, such as superconductivity, defy any description in terms of small departures from a non-interacting gas (as would be appropriate for simple metals, for example). I am particularly interested in certain underrated materials whose peculiarities make it possible to tease out physics that remains elusive elsewhere.

The first is actually a class of synthetic "materials" that can be realized in a variety of systems, from quantum well excitons in dielectric mirrors to cold atoms in cavities, and the effective quasi-particles that emerge when the system is optically pumped are known as polaritons. Most generally, polaritons are hybrid particles of light and matter, and through tuning of the light-matter interaction can inherit properties of each that are desirable for pushing the system into a wide variety of behaviors that are otherwise hard to access. From the light, which is of course massless and non-interacting in free space, polaritons inherit effective masses orders of magnitude smaller than an electron mass and a characteristic dynamical time scale that is much faster than for competing effects from electronic or atomic motions. Meanwhile, polariton-polariton interactions are largely inherited from the interactions of the matter component, which can of course be tuned separately.

For instance, in polaritons based on quantum well excitons, where the polariton-polariton interaction is mostly a weak repulsion, polaritons form a liquid which at low enough temperature undergoes a Bose-Einstein condensation transition and forms a superfluid. The transition temperature is controlled by the ratio of kinetic energy to thermal energy, and for extremely light polaritons can end up as high as room temperature - a feat which has been experimentally achieved and remains far out of reach in other systems. Polaritons are therefore an enticing platform to explore phenomena related to condensation and superfluidity, and have for many years been a rich playground in this field.

My own research focuses on the phases of polaritons that can be realized when interactions become strong and long-ranged, such as the van der Waals repulsion between highly-excited Rydberg states of excitons or atoms. Although this system still supports condensation, the interactions no longer serve to merely thermalize it, and all else being equal would favor an ordered, crystalline state. It turns out that the competition between condensation and crystallization supports a plethora of phases with varying spatial order, including so-called super-solids in which a patterned density coexists with superfluidity. These intriguing states of matter have long been sought, for instance in Helium-4, but with ambiguous results. Polaritons may provide a path to explore these and other heretofore inaccessible states of matter.

A second material I study is the superconductor strontium titanate (STO). STO is a semiconductor but upon light doping becomes superconducting, with a transition temperature which forms a "dome" as a function of carrier density, much like the high-temperature superconducting cuprates. Unlike the cuprates, the superconducting phase appears entirely conventional and well-described by the conventional Bardeen-Cooper-Schrieffer (BCS) theory developed for metals. This is curious because the pairing of electrons in a thin shell around the Fermi energy mediated by acoustic phonons, which is the BCS mechanism of superconductivity, cannot possibly explain the behavior of STO. There is already of course a large industry of proposing unconventional superconductivity mechanisms, but these hypotheses are difficult to unambiguously test in the cuprates where a variety of other electronic phases compete and intertwine with superconductivity. In my work to generate a theory of the comparatively simpler STO, the hope is to find an account that is clean enough to inform new directions in the search for better superconductors.

Back to Top


grad_research_micahel_fedderke_thumb.jpgMICHAEL FEDDERKE
Research: Theoretical Particle and Dark Matter Physics
Advisors:  Lian-Tao Wang and Edward W. Kolb

Degrees:
- B.S., University of Cape Town, 2009 (Physics and Applied Mathematics)
- B.S., University of Cape Town, 2010 (Mathematical and Theoretical Physics)
- M.S., University of Chicago, 2012 (Physics)
Graduate Student (2011-pres) Dept. of Physics, Enrico Fermi Institute
Awards: Gregor Wentzel Research Prize, Robert G. Sachs Summer Fellowship

Dark matter is a type of matter with well-measured gravitational interactions, but which appears to otherwise interact only extremely weakly with the kind of ordinary matter of our everyday experience. There is about five times more of it in the universe than there is ordinary matter, and yet we know remarkably little about its identity and basic properties. A broad and intense experimental program is currently underway aimed at detecting the extremely weak non-gravitational interactions of dark matter with ordinary matter (should they exist) in an effort to elucidate its nature.

A major theme of my research is the utilisation of the tools of theoretical particle physics to better understand what exactly it is that this wide variety of experiments can tell us about the nature and identity of dark matter.  My work in this area has involved theoretical aspects of a number of the canonical avenues for dark-matter detection: indirect detection (astrophysical searches for signals of dark-matter decay or annihilation, from regions where it has clumped gravitationally), direct detection (ultra-low-background terrestrial experiments looking directly for interactions of dark matter with ordinary matter in a detector), and to some extent, collider-based detection.

As the approach is particularly well-suited for the low energies involved in direct and indirect detection, my work has made extensive use of the techniques of effective field theory (EFT), which has the advantage of allowing a variety of experimental results to be interpreted in such a way that the conclusions drawn are applicable to broad classes of theoretical models which share common dominant low-energy behaviour, independent of differences in high-energy behaviour among the models in each class.

Together with E.W. Kolb, T. Lin and L.-T. Wang, I have examined how Fermi-LAT and H.E.S.S. observational limits on the emission of high-energy photons from dark-matter-rich regions such as the Milky Way galactic centre can constrain EFT operators describing the pair-annihilation of cold thermal-relic dark-matter particles (a.k.a. Weakly Interacting Massive Particles, or WIMPs) into Standard Model gauge and/or Higgs bosons, assuming that the same interactions are also responsible for fixing the WIMP relic abundance during thermal freeze-out in the early universe. With J.-Y. Chen, E.W. Kolb, and L.-T. Wang, I made use of the limits from the CMS and ATLAS experiments on invisible decays of the Higgs boson, together with direct detection limits from the LUX experiment, to constrain an EFT description of a fermionic WIMP coupled to the Higgs boson through a particularly interesting avenue known as the "Higgs Portal". I am currently working with E.W. Kolb on a project making use of current (and projected future) direct detection limits to constrain effective operators describing the coupling of fermionic WIMPs to Standard Model quarks, with a focus on non-standard types of interactions.

My research interests also extend beyond the physics of dark-matter detection. I have worked on a couple of projects aimed at exploring possible new physics using the Higgs boson as a probe. Although results from ATLAS and CMS indicate that the Higgs has many of the properties we expected it to have, as more, and more precise, experimental data is obtained, there will be much more to learn. Together with T. Lin and L.-T. Wang, I have considered how a simple singlet-doublet Higgs Portal model could be probed by a variety of collider experiments, with a particular focus on how future proposed electron-positron colliders (FCC-ee, ILC, and CEPC) would provide powerful constraints on such models as a result of improved electroweak precision measurements, as well through their precise determination of the production cross-section for a Z boson in association with a Higgs. In an ongoing project with B. Batell, A. Tesi and L.-T. Wang, I am involved in looking at how an idea from early-universe cosmology --- the relaxion mechanism --- can be tied together with so-called Composite Higgs models, in which the Higgs boson is not fundamental, but  is rather a bound-state object analogous to a QCD meson. The aim of this project is to use the relaxion mechanism to naturally explain a scale separation (the Little Hierarchy Problem) which is required to make Composite Higgs models phenomenologically viable.

Finally, I have undertaken some research at the abundantly fruitful interface of particle physics and early-universe cosmology. With E.W. Kolb and M. Wyman, I performed a detailed numerical and analytical study of gravitational particle production effects which can occur when an additional scalar field is present during an epoch of inflation being driven by a slow-rolling inflaton field. We considered a couple of scenarios with a direct scalar-field--inflaton coupling engineered such that the effective time-dependent mass of the scalar field became momentarily light or tachyonic during inflation, finding that this efficiently drives an irruptive burst of scalar-field particle production.

Publications

  • Probing the fermionic Higgs portal at lepton colliders. With Tongyan Lin and Lian-Tao Wang. JHEP 04 (2016) 160. [arXiv:1506.05465].
  • Irruption of massive particle species during inflation. With Edward W. Kolb and Mark Wyman. Phys. Rev. D 91, 063505 (2015). [arXiv:1409.1584]. 
  • The Fermionic Dark Matter Higgs Portal: an effective field theory approach. With Jing-Yuan Chen, Edward W. Kolb, and Lian-Tao Wang. JHEP 08 (2014) 122. [arXiv:1404.2283]. 
  • Gamma-ray constraints on dark-matter annihilation to electroweak gauge and Higgs bosons. With Edward W. Kolb, Tongyan Lin and Lian-Tao Wang. JCAP 01 (2014) 001. [arXiv:1310.6047]. 

Back to Top


Geracie_thumb.JPGMICHAEL GERACIE
Research: Condensed Matter and High-energy Physics
Advisor: Dam Thanh Son

Degrees:
- B.S., UW-Milwaukee, 2010 (Physics and Mathematics)
- M.S., University of Chicago, 2011 (Physics)
- Ph.D., University of Chicago, 2016 (Physics)
Awards: Robert A. Millikan Fellowship, Gregor Wentzel Teaching Prize, Gregor Wentzel Research Prize (Dept. of Physics)

My interests lie on the interface between condensed matter and high energy physics. Though often treated separately, these subjects have a rich shared history. This is possible because of the power of geometry and quantum field theory, which can describe diverse phenomena within a unified way of thinking. I hope to further this program by applying techniques familiar to high energy physicists in new condensed matter settings.

My initial research focused on novel applications of geometry to the quantum Hall effect. Since its discovery, the quantum Hall effect has proven a fruitful playground for both theorists and experimentalists. The first known example of a topological phase, its discovery precipitated a revolution in our understanding of quantum matter that continues to this day. My work in this field involved studying these systems in a new regime, the lowest Landau level (LLL) fluid, in which the system is cold enough to be effectively constrained to the LLL, but is otherwise hydrodynamic. We demonstrated that such flows are highly constrained, characterized by only the equation of state and a thermal Hall conductivity. In addition, we were able to extract a number of new Streda-type formulas for both charge and energy transport.

To be as constraining as possible required the full implementation of spacetime symmetries. Since these samples are typically non-relativistic in nature, this required a number of formal developments in non-relativistic, or Newton-Cartan (NC) geometry. In several papers, myself and collaborators made the necessary advances, writing the first fully diffeomorphism covariant Ward identities for an NC geometry and constructing the first Galilean covariant description of torsionful NC backgrounds, which is necessary to study non-relativistic energy transport.

My current work is concerned with certain strongly coupled field theories at finite density. Large N Chern-Simons theories have attracted much attention among high energy circles in recent years. These systems are unique in that they exhibit a great deal of interesting physics, including anyonic excitations, as well as conjectured holographic and Bosonization dualities, yet also permit exact computations of many relevant quantities at strong coupling. Myself and collaborators use this setting as a playground to investigate the properties of condensed Fermion systems at strong coupling and low temperatures. We have also found a number of novel phenomena, including an enhancement of quantum degeneracy that leads to a breakdown of the classical regime at strong coupling. By examining linear response in this system, we have also learned that Fermi liquid theory, long a pillar of condensed matter phenomenology, needs to be augmented to completely characterize zero temperature transport. Going forward, we seek to continue this program in the hopes of gaining new insights into strongly coupled condensed phases of matter.

Publications

  • Spacetime Symmetries of the Quantum Hall effect, M. Geracie, C. Wu, S. Wu and D. T. Son, Phys. Rev. D91 (2015) 045030; arXiv:1407.1252 [cond-mat.mes-hall].
  • Hydrodynamics on the lowest Landau Level, M. Geracie and D. T. Son, JHEP 1506 (2015) 044, arXiv:1408.643 [cond-mat.mes-hall].
  • Fields and fluids on curved non-relativistic spacetimes, M. Geracie, K. Prabhu and M. Roberts, JHEP 1508 (2015) 042, arXiv:1503.02680 [hep-th].
  • Cold, dense Chern-Simons with Fermions at Large N, M. Geracie, M. Goykhman and D. T. Son.

Back to Top


chen_he_heinrich_thumb.jpgCHEN HE HEINRICH
Research: Cosmology
Advisor: Wayne Hu

Degrees:
- B.S., McGill University, 2012 (Joint Honors Mathematics and Physics)
Graduate Student (2012-pres) Dept. of Physics, Kavli Institute for Cosmological Physics
Awards: Winstein Travel Award (Dept. of Physics), Best Poster Prize (Conference Essential Cosmology for Next Generations)

The cosmic microwave background (CMB) contains a wealth of information about our universe. Measurements of the CMB have been used to extract key information such as the amount of baryons and dark matter, the existence of dark energy, and key signatures of inflation. Nevertheless, the nature of dark matter, dark energy and the details of the inflationary physics remain unanswered mysteries.

With current and upcoming CMB experiments such as Planck, SPT, BICEP/Keck and CMB-Stage 4, we will see a dramatic increase in cosmological data in the near future. However, converting this data into useful physical information remains a challenging task, and novel techniques for connecting theories to observables must be devised.

I work at the intersection of theory and data, constraining physical models using cosmological observations. Together with my advisor Prof. Wayne Hu, I developed and applied theoretical and data analysis techniques to constrain a wide class of inflationary models using the CMB. We approach problems from three directions: data, phenomenology and theory.

In our latest work, using data from the Planck satellite we derived model-independent constraints of reionization -- the epoch when the universe goes from neutral to ionized. Using a principal components (PC) analysis, we were able to extract all the information avilable in the CMB data. This analysis revealed high redshift ionization and corrected bias in certain cosmological parameters compared to the standard analysis, for example, that of the optical depth which is crucial for measuring neutrino mass hierarchy with the CMB. Our public code ReLike provided novel tools for studying the formation of first stars and opened a new route for probing dark matter annihilation. In the future, the PC method will be used jointly with other reionization probes such as 21 cm measurements.

Beside data analysis, a major part of my research has focused on doing simulations and forecasting works to design the best data analysis techniques for probing the curvaton model, a type of inflation model with an extra scalar field. In particular, with Prof. Daniel Grin and Wayne Hu, I developed and tested new techniques to improve the signal-to-noise of the curvaton signatures in the CMB and to remove bias effects from gravitational lensing. Measuring those signatures with CMB-Stage 4 will confirm or eliminate certain scenarios of the curvaton model in which baryons or dark matter originates from the curvaton.

Working from the theory side, Prof. Wayne Hu, Dr. Miranda, Dr. Motohashi and I have developed theoretical and numerical techniques for improving the accuracy of power spectrum predictions for the class of inflationary models with features in the inflaton potential. We found a universal template leading to much more accurate theory predictions and capturing all non-linear excitations in the inflaton field with a simple form. This non-linear template corrects bias of lower-order templates used in past searches of axion monodromy and will enable searches of potential features with higher amplitude and frequency.

The upcoming CMB experiments will tell us even more about the beginning and the evolution of the universe. Planck’s final data next year will deliver definitive constraints from CMB temperature on a wide class of inflation models. Ground-based experiments such as SPT, BICEP and CMB-S4 will provide precision measurements of CMB polarization, useful for determining the neutrino mass hierarchy, mapping the mass in the universe, and measuring primordial gravitational waves – another key prediction of inflation. 

Publications

  • Signatures of metal-free star formation in Planck 2015 Polarization Data. Vinicius Miranda, Adam Lidz, Chen He Heinrich, and Wayne Hu. arXiv:1610.00691 (2016).
  • Complete Reionization Constraints from Planck 2015 Polarization. Chen He Heinrich, Vinicius Miranda, and Wayne Hu. arXiv:1609.04788 (2016).
  • Lensing Bias to CMB Measurements of Compensated Isocurvature Perturbations. Chen He Heinrich, Daniel Grin, and Wayne Hu. Phys. Rev. D94(4) 043534 (2016). (Selected as part of the PRD kaleidscope.)
  • Nonlinear Excitations in the Inflationary Power Spectra. Vinicius Miranda, Wayne Hu, Chen He, and Hayato Motohashi. Phys. Rev. D93(2) 023504 (2016).
  • Compensated isocurvature perturbations in the curvaton model. Chen He, Daniel Grin, and Wayne Hu. Phys. Rev. D92(6) 063018 (2015).
  • Prospects for detecting gamma rays from annihilating dark matter in dwarf galaxies in the era of Dark Energy Survey and Large Synoptic Survey Telescope. Chen He, Keith Bechtol, Andrew Hearin, and Dan Hooper. Phys. Rev. D91(6) 063515 (2015).
  • Hunting for Orphaned Central Compact Objects among Radio Pulsars. Jie Luo, Chi-Yung Ng, Wynn C. G. Ho, Slavko Bogdanov, Victoria M. Kaspi, and Chen He.  Astrophys. J. 808(2) 130 (2015).
  • The Correlation between Dispersion Measure and X-ray Column Density from Radio Pulsars. Chen He, Chi-Yung Ng, and Victoria M. Kaspi. Astrophy. J. 768, 64 (2013).

Back to Top


Li-Chung_thumb.jpgLI-CHUNG HA
ResearchAtomic Physics
Advisor: Cheng Chin

Degrees:
- B.S., National Taiwan University, 2006 (Chemistry)
- M.S., National Taiwan University, 2009 (Physics)
- Ph.D., University of Chicago, 2016 (Physics)
Awards: Grainger Graduate Fellowship (Dept. of Physics), Government Scholarships for Study Abroad (Taiwan)

Modern computers, based on silicon transistors, have experienced exponential year-on-year growth in computation power. Soon, this technology will reach the limitation defined by the tunneling effect of electrons in the semiconductor material. To respond to the increasing demand for faster data processing, a new type of computation technology is required. For data processing involving physical systems, the concept of quantum simulation was proposed by Richard Feynman. It involves the study of a complex system with a different system governed by the same equations, but which is relatively clean and controllable.

Ultracold atomic systems have been demonstrated as excellent candidates for doing quantum simulation. With the advantage of its cleanness and tunability, researchers are able to probe physical phenomena which are difficult to study in a complicated condensed matter system. Over the past decade, there have been numerous scientific results verifying the models shown in the textbook, e.g. realization of the Bose-Einstein condensate (BEC) and degenerate Fermi gas, demonstration of the Bose-Hubbard model, quantum magnetism, and quantum criticality.

The primary research direction of my thesis resides on engineering the band structure of a BEC in an optical lattice which is an optical potential created by interfering two laser beams. We load the condensate in a one dimensional optical lattice and phase modulate the lattice potential. The ground and the first excited bands from the lattice are coupled through the modulation, and the dispersion of the new admixed ground band evolves from having one single minimum to having two minima. This system exhibits effective ferromagnetism, and we directly observe the formation of domain and extract the correlation function.

Based on this new type of quantum phase, we are also able to simulate the roton which is a quasi-particle previously only observed in superfluid helium. The existence of roton in superfluid helium was speculated by Lev Landau to explain many experiments on the suppression of the superfluid critical velocity. In our system, the roton originates from the double-well dispersion in the shaken lattice, and can be controlled by both the atomic interaction and the amplitude of the modulation. We determine the excitation spectrum using Bragg spectroscopy and measure the critical velocity by dragging a weak speckle potential through the condensate. We observe good agreement between our measurement and the modified Bogoliubov model.

A future research direction is to study the scaling of the domain size versus the speed used to cross the phase transition in order to understand and simulate the early universe. As was predicted by Tom Kibble, the density of frozen-out cosmological defects, such as cosmic domains and strings, is set by the cooling rate of the universe after the Big Bang. Wojciech Zurek later extended the idea to condensed matter systems, which is the formation of vortices (an excitation with quantized angular momentum) in liquid helium after being quenched to the superfluid phase. The Kibble-Zurek mechanism is of particular interest in grand unified theories of symmetry-breaking phase transitions in the early universe, 10^{-35} s after the Big Bang. While it is infeasible to perform an experiment or classical computer simulation on a cosmological scale, a quantum simulator would be readily applicable to the question.

Publications

  • Roton-maxon excitation spectrum of Bose condensates in a shaken optical lattice, arXiv:1407.7157 (2014). L.-C. Ha, L. W. Clark, C. V. Parker, B. M. Anderson, C. Chin.
  • Anomalous thermoelectric transport in two-dimensional Bose gas, arXiv:1306.4018 (2013). E. L. Hazlett, L.-C. Ha, and C. Chin.
  • Direct observation of effective ferromagnetic domains of cold atoms in a shaken lattice, Nature Phys. 9, 769 (2013). C. V. Parker, L.-C. Ha, and C. Chin
  • Strongly interacting two-dimensional Bose gases, Phys. Rev. Lett. 110, 145302 (2013). L.-C. Ha, C.-L. Hung, X. Zhang, U. Eismann, S.-K. Tung, and C. Chin
  • Extracting density-density correlations from in situ images of atomic quantum gases, New Journal of Physics 13, 075019 (2011). C.-L. Hung, X. Zhang, L.-C. Ha, S.-K. Tung, N. Gemelke, and C. Chin.

Back to Top


Prabhu_thumb.jpgKARTIK PRABHU
Research: General Relativity and black holes
Advisor: Robert M. Wald

Degrees:
- M.Sc. and B.Sc., Indian Inst. of Tech., Kharagpur, 2010 (Physics)
- Ph.D., University of Chicago, 2016
Awards: Jerry Rao (Dept. of Physics), Wentzel Research Prize (Dept. of Physics)

General Relativity is the best description of gravity that we have presently. Some of the most exotic predictions of this theory include spacetime singularities and black holes. Even though the theory is a 100 years old, there are many aspects of black holes at remain unresolved. One such problem is whether black holes (specially in higher dimensions) are stable. The problem essentially is: If we perturb a black hole by a small amount, does the perturbation "settle down" to some other configuration (stability), or does it become larger and larger without any bound (instability)?

With Prof. Robert M. Wald, I investigated the instabilities of stationary and axisymmetric black holes in General Relativity in vacuum. The key idea is to investigate an energy-like quantity for small axisymmetric perturbations of the black hole. In an earlier work Hollands and Wald had already shown that if this energy is negative then the black hole is unstable in the sense that it does not settle down to any other stationary black hole. Expanding on this work, using a reflection symmetry of such black holes, we proved that the kinetic energy part of the total energy is always positive. Further, if the remaining potential energy part is negative then the small perturbation will grow exponentially in time i.e. we have an unstable perturbation. We also found a variational formula that give the rate of this exponential growth. At present, along with former UChicago student Josh Schiffrin, we are extending this result to black holes with matter fields and also to perfect fluid stars in General Relativity.

Along with this, I am working on generalising the first law of black holes mechanics and the formula for black hole entropy to situations where matter fields like magnetic monopoles or fermions are present around the black hole.

Another topic that I have worked on along with Michael Geracie and Matthew M. Roberts, is to formulate non-relativistic spacetimes and matter fields in a geometric way in a manner that is very close to General Relativity. Such a geometric viewpoint on non-relativistic systems was already shown to be very fruitful in condensed matter systems like the Quantum Hall Effect by Prof. Dam T. Son. We are continuing to work on further applications of this non-relativistic geometry to look at other applications that might give some insight into other condensed matter systems.

Selected Publications:

Back to Top


Scheeler_thumb.jpgMARTIN SCHEELER
Research: Experimental Soft Matter - Topological Vortex Dynamics
Advisor: William Irvine

Degrees:
- A.B., Princeton University, 2011 (Physics)
Graduate Student (2011-pres) Dept. of Physics
Awards: McCormick Fellow, Sachs Fellow, Wentzel Teaching Prize, Yodh Research Prize (Dept. of Physics), GAANN Fellowship (Dept. of Education)

Vortices lie at the heart of fluid flow, driving all fluid motion in the absence of boundaries. Naturally this vorticity can organize into vortex loops—thin, closed loops which house compact arrangements of vortex lines—resulting structures akin to a smoke ring. Since these compact, geometric objects provide enough information to reconstruct the flow everywhere in space, they can be thought of as building blocks of a flow, with more complex flows corresponding to more complicated tangles of these loops. In the absence of viscosity, the dynamics of these elemental structures are bound to a simple rule: they are not allowed to cross. What would happen then if you took one of these vortex loops, like a smoke ring, and tied it into a knot?

Once a vortex loop is knotted, the only way to remove the knot is to pass one segment of the loop through another, but in an ideal flow, it is exactly this sort of filament crossing that is forbidden, and thus, vortex knots should stay knotted for all time. This topological stability manifests mathematically as the conservation of a quantity known as the fluid helicity, which measures the degree of vortex line linking and knotting in a flow. Like energy or momentum conservation, this topological conservation provides a strong constraint on the evolution of the fluid throughout all of space, and in doing so, provides a powerful framework for understanding the messy vortex tangles that produce complicated flows.

But in a real flow, one which has even the slightest amount of viscosity, vortex tube dynamics acquire a feature which seems to jeopardize this conservation law: the ability to reconnect. Rather than crossing, two small sections of vortex tube can align anti-parallel to each other, becoming so close that their separation approaches the viscous length scale, allowing the small regions to diffuse into each other and annihilate. The removal of these sections and the subsequent reconnecting of the tubes swaps the connectivity of the branches of the loop, providing knots a chance to reconfigure their topology and eventually unknot.

Working with Prof. William Irvine, I study the dynamics of these topological vortices experimentally. To generate these vortex knots and links in water, we take advantage of a phenomenon known as a "starting vortex", in which an airplane wing, when suddenly accelerated from rest, sheds a vortex along its trailing edge. Leveraging advances in 3D printing technology, we design and fabricate hydrofoils – airplane wings extruded along closed, curved paths in space – which when impulsively accelerated, produce vortex loops that match their geometry. This method allows us to produce isolated vortex links and knots for the first time in experiment.

How does the behavior of these experimental knots compare to our theoretical expectations? Recall that in an ideal flow, once we generate a knotted vortex, we expect it to remain knotted. Surprisingly, what we observe in a viscous fluid is instead the rapid unknotting of these vortices via topology-removing reconnections. At first glance, this result could indicate that helicity is a very poorly conserved quantity, discontinuously dropping to zero as the divergent reconnection process undoes the knotting. But what if instead of being lost, this knotting was flowing into a different form?

This potential discrepancy can be resolved by thinking a little more carefully about what helicity is measuring: it is not a measure of the knotting of the vortex tube containing the field lines, but rather of the knotting of the field lines themselves. Clearly, by knotting the tube containing the field lines, we've also knotted the lines themselves, but we could also wind the vortex lines around the center-line of a tube which is unknotted in a way that links or knots them (visually similar to the fibers wound together into rope). This linking inside a tube is generated in two independent ways: by coiling the center-line of the tube into a helix, and by twisting the field lines about the center-line.

With this knowledge, we can return to the experimental evolutions, where we observe that whenever there is a reconnection that changes the tube topology, it also introduces a helical coil in the post-reconnection geometry. Measurements of the helicity before and after the reconnection show that the formation of this coil is enough to exactly compensate for the tube unknotting, conserving helicity throughout the process. These results suggest that vortex field line topology might provide a useful interpretive key in understanding real fluid flows, and inspire future work intended on understanding how helicity can be cascaded towards dissipation and its role in vortex stability.

Selected Publications:

  • M.W. Scheeler, D. Kleckner, D. Proment, G.L. Kindlmann, W. T. M. Irvine, "Helicity conservation by flow across scales in reconnecting vortex links and knots" PNAS 111 (43) 15350 (2014).

Back to Top


grad_research_bjorn_scholz_thumb.jpgBJORN SCHOLZ
ResearchDark Matter and Astroparticle Physics
Advisor: Juan I. Collar

Degrees:
- B.S., Julius Maximilian University of Würzburg, 2010
- Dipl., Technische Universität Dresden, 2012 (Physics; M.S. equivalent)
Graduate Student (2012-pres) Dept. of Physics, Kavli Institute for Cosmological Physics

In 1970 Vera Rubin measured the rotation curve of the Andromeda galaxy and found that the rotational velocity of its stars flattened out for large distances to the center. This was in strong contrast to the theoretical predictions of a decaying rotational velocity if only luminous matter were present. One way to account for this behavior was to postulate the existence of so called 'dark matter', i.e matter that does not interact or emit electro-magnetic radiation. Since then a multitude of different experimental observations have found independent evidence for 'missing' mass. The leading hypothesis is that dark matter is composed of weakly interacting massive particles (WIMPs), which only interact via the weak force and gravity. However, until now no hard evidence has been found to substantiate this assumption.

A vast number of experiments are currently searching for WIMP induced signals using a wide variety of target detector materials, such as xenon, NaI[Tl] or germanium. The challenge for all of these experiments is to minimize all potential backgrounds. These backgrounds could obscure a hypothetical exponential excess at low energies caused by WIMP-induced recoils. This means that all experiments are found deep underground to minimize cosmic-ray associated background such as muon-induced neutrons. In addition all experiments are encapsulated in a sophisticated onion-layered array of active and passive radiation shields, preventing environmental radiation to reach the detector. My advisor and I are part of the CoGeNT-4 (C-4) collaboration. The C-4 dark matter experiment is located at the Soudan Underground Laboratory in Soudan, MN. It uses a 1.2 kg low-background p-type point contact (PPC) germanium detector cooled to liquid nitrogen temperatures to look for WIMP induced signals. An ultra-low energy threshold allows us to look for nuclear recoil events due to dark matter particles with a relatively low mass. Besides exploring new phase-space, C-4 will also test an annual modulation in the event rate seen by its predecessor CoGeNT, which so far only had minor statistical significance. Due to the almost three times larger mass and the reduced energy threshold, a new measurement of this modulation will help us to properly assess its origin. During the last couple of years I have been involved in almost all aspects of this experiment, including programming the FPGA based data acquisition system, designing the external neutron shield, assembly and calibration of the muon veto, characterizing the PPC detector as well as the final C-4 deployment at SUL in 2015. C-4 is currently taking data and I am eager to analyze the data to look for any potential WIMP-induced signal and its annual modulation.

I am also part of the COHERENT collaboration. We are aiming to be the first to observe coherent elastic neutrino-nucleus scattering (CEvNS). Even though CEvNS has been predicted more than four decades ago as a weak interaction standard-model process, it still remains unobserved to this day. Due to the coherence condition, which requires the wavelength of the momentum transfer to be larger than the target nucleus the experimental signature of CEvNS consists of difficult-to-detect nuclear recoils of a few keVnr, putting them out of reach of conventional neutrino detectors. Advances in ultra-sensitive detector technologies, mainly used in rare event searches such as dark matter or neutrinoless double beta decay experiments, now make it feasible to search for such tiny recoil signals. Validating our standard model predictions of CEvNS is crucial as it depicts the same interaction channel thought to be used by WIMPs. As such any deviation would directly influence our ability to interpret the results of dark matter search experiments.

We are currently taking physics data with a ~14 kg low-background CsI[Na] detector at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory, TN. The SNS provides an ideal environment for CEvNS detection. A narrow proton beam strikes a liquid Hg target at a repetition rate of 60 Hz producing pions. Throughout the π+ chain a total of three neutrinos are being emitted within a couple of microseconds. This compact emission profile combined with a sharply pulsed proton beam is highly beneficial for background rejection. The emitted neutrino energies perfectly match the criterion for a fully coherent interaction with medium-sized nuclei. Combined with the good light-yield and moderate quenching factor of CsI[Na], this should result in an unambiguous, statistical significant CEvNS observation over the next couple of years. For this experiment, I have been involved in the programming of the data acquisition system, its deployment in June 2015 and the data analysis that is currently underway.

Publications

  • Measurement of the low-energy quenching factor in germanium using an Y-88/Be photoneutron source, B. J. Scholz, A. E. Chavarria, J. I. Collar, P. Privitera, A. E. Robinson (under review), arXiv:1608.03588.
  • Measurement of the ionization produced by sub-keV silicon nuclear recoils in a CCD dark matter detector, A. E. Chavarria, J. I. Collar, J. Pena, P. Privitera, A. E. Robinson, B. Scholz, C. Sengul, J. Zhou, J. Estrada, F. Izraelevitch, J. Tiffenberg, J. R. T. de Mello Neto, D. Torres Machado, Physicsal Review D, arXiv:1608.00957.
  • A Low-noise Germanium Ionization Spectrometer for Low-background Science, C. E. Aalseth, J. I. Collar, J. Colaresi, J. E. Fast, T. W. Hossbach, J. L. Orrell, C. T. Overman, B. Scholz, B. A. VanDevender, K. M. Yocum (2016), IEEEE TNS, DOI: 10.1109/TNS.2016.2614431, arXiv:1603.01584.
  • Cycling State that Can Lead to Glassy Dynamics in Interacellular Transport, M. Scholz, S. Burov, K. L. Weirich, B. J. Scholz, S. A. Tabei, M. L. Gardel, A. R. Dinner (2016), Physical Review X 6, 011037, arXiv:1602.04269.

Back to Top


Wu_thumb.jpgMILES WU
ResearchExperimental High-energy Physics
Research Advisor: David Miller

Degrees:
- A.B., Princeton University, 2012 (Physics)
Graduate Student (2012-pres), Dept. of Physics, Enrico Fermi Institute
Awards: McCormick Fellowship, Sachs Fellowship (Dept. of Physics)

As a member of the ATLAS Collaboration, my research involves studying the collisions of highly energetic protons at the Large Hadron Collider (LHC) at CERN in Switzerland. The sheer amount of data collected and the high energies of the LHC enable us to probe the interactions between the fundamental particles of the universe with great sensitivity and look for possible new physics.

At the LHC the production of energetic jets (showers of hadrons and other particles formed from a quark or gluon) is extremely large, and a small percentage of these jets during the shower can emit a W boson via electroweak radiation. Many searches for new physics involve Lorentz-boosted top quarks, and these collinear W's can imitate the final state of these top quarks' decays and become a significant background for these searches. Therefore it is important to know how often these collinear W's are produced and what their properties are, so that these searches can account for them. As a detailed study on the emission of collinear W's from jets has never been performed before, I have been working on performing a measurement of their cross-section and looking at their kinematic properties. This process is also of significant theoretical interest in the context of Monte Carlo event generators as there are different approaches on how to model it, so I have been in contact with phenomenologists and have been comparing my measurements with different theoretical predictions. While this process is somewhat rare at the current energies of the LHC, it will become very common at the the much higher energies of currently proposed future colliders (such as the 100 TeV Future Circular Collider at CERN). As such, it will be increasingly crucial for the future that we understand this process well.

I am also involved with developing the software that the ATLAS experiment uses to probe the internal structure of jets. These so-called jet substructure observables can be very useful in identifying the hadronic decays of heavy resonances and distinguishing them from the often large unwanted background. These techniques and the software I help create enable a large variety of analyses to look for new physics with greatly increased sensitivity.

Previously, I was part of the analysis that searched for new physics that utilised a jet substructure approach. As many beyond the standard model theories predict heavy resonances that decay into two vector bosons, we looked for events where one of the bosons decays leptonically (into either electrons or muons) and where the other boson decays hadronically (into quarks, and ultimately jets). This semi-leptonic final state happens more often than the fully-leptonic final state, but the identification of the hadronic boson is not as easy due to the large backgrounds and the poorer detector resolutions. Here the jet substructure techniques proved to be very useful in enabling us to discriminate the signal from the backgrounds and, compared to the previous ATLAS search that did not use these techniques, we were able to provide much stronger limits.

Publications:

  • ATLAS Collaboration, "Search for resonant diboson production in the llqq final state in pp collisions at sqrt(s) = 8 TeV with the ATLAS detector," Eur. Phys. J. C 75, 69 (2015) [arXiv:1409.6190 [hep-ex]].

Back to Top