Faculty

Theoretical cosmology, quantum field theory, models for quantum gravity effects My current research involves several related topics in quantum fluctuation phenomena, with applications to gravitation and cosmology. One topic is the study of energy density fluctuations for quantum fields such as the electromagnetic field. My collaborators and I have shown that large vacuum energy density fluctuations are more probable than previously expected. These large fluctuations can drive quantum fluctuations of gravity and provide insight into effects in quantum gravity, an area which is not well understood. Energy density fluctuations may also produce observable effects in atomic or condensed matter systems, and may play a role in the evolution of the early universe. I am also working on analog models for quantum gravity, in which quantum fluctuations in a nonlinear optical material might produce fluctuations in the speed of light, analogous to an effect expected in quantum gravity.

Condensed Matter Physics, Soft materials, Colloids, Liquid Crystals, Computational Physics, Physics Education Soft matter physics is the study of matter that is all around us in everyday life: soaps, oil, foods, sand, foams, and biological matter. All of these are readily deformable at room temperature and combine properties of both fluids and solids. Despite their ubiquity, these materials are extremely complicated. Unlike simple fluids like water, they have rich internal structure; unlike crystalline solids they are typically not periodically ordered. Moreover, they exist in long-lived metastable states far from equilibrium and respond to stimuli such as applied electric and magnetic fields, temperature and pressure. My work seeks to understand how these materials respond to shape: how they self-organize on curved surfaces or in complex geometries and how this knowledge can be used both to sculpt desirable shapes at the microscopic scale and create shape changing systems like soft robots. We use high performance computing to simulate and predict these behaviors and work closely with experimentalists at Tufts and beyond.

Experimental High Energy Physics My research focuses on the discovery of new fundamental particles of nature, as well as on the understanding of the behavior of the known particles. To do this, I participate in the ATLAS experiment, one of the two general-purpose detectors at the Large Hadron Collider at CERN. My work currently consists in analyzing data in order to: Perform precision measurements leading to a better understanding of the strong interaction within the QCD theoretical framework; Search for new physics in events involving large amount of missing energy, typical signature of new particles that interact very weakly with normal matter such as dark matter candidate; Develop and estimate the performance of the ATLAS trigger system. This last aspect of my work also involves software development and a participation in the detector operation. I'm focusing my efforts on the Missing Energy trigger. The Standard Model of particle physics, despite being very successful, cannot be the end of the story. It contains a certain number of theoretical dissatisfactions. Of all the possibilities, I believe that dark matter is one of our best guess. Its existence is based on experimental facts, and the mass scale of dark matter particles, in the case where it is the right explanation, should be accessible at the LHC. Its existence would be inferred by the observation of missing energy in subset of all collected events. Looking for excesses of events involving large amount of missing energy over expectations is a promising way to look for dark matter at the LHC. My approach is to carry such search by performing precision measurements of Standard Model quantities, to optimize the sensitivity of the analysis to such new particles. Predictions using quantum chromodynamics (QCD) implies many approximations, assumptions or simplifications at various levels. These could lead to large systematic uncertainties on various Standard Model predictions, possibly leading to significant limits in our sensitivity to new phenomena. My research try to determine which of the simplifications and approximations are acceptable at the level of precision needed for a new physics discovery. To this end, I investigate events that contain a vector boson and jets, as they are sensitive to such physics and yet provide a clean enough environment to allow for high precision measurements. These are also the most important background to a wide range of new physics signature. As a side, I am also interested in the philosophy of physics, focusing on epistemological aspects of experiments and simulations as used in High Energy Physics.

Condensed Matter Physics

Physics Education Research: Scientists are professional learners who employ a range of skills and qualities to learn new things. This view has significant bearing on how students advance in their understanding of scientific concepts. For this reason, my current research focus is to investigate how learners come to engage in the practices of science. To make progress on the question, I have studied how learners' views of knowledge (personal epistemologies) impact their scientific engagement in the contexts of introductory physics, quantum mechanics, and science teacher education. I have also studied the interaction of personal epistemology with emotions that come up in the doing of science (epistemic affect). Most recently, I have looked at how personal epistemology interconnects with social caring and epistemic empathy. These studies help outline some paths to progress in equity and inclusion in STEM fields, and inform my approaches to teaching.

Experimental particle physics, neutrino oscillations, neutrino interaction physics, neutrino astrophysics, computer simulations of neutrino-nucleus interactions. The main thrust of my research is the study of the neutrino. Through neutrino oscillation experiments, we are gaining insights into neutrino masses and mixing parameters. Precise measurements of these quantities may allow us to uncover the reason behind the matter-antimatter asymmetry in the universe, or point the way to a theory beyond the standard model. Precise measurements of oscillation parameters require good models of neutrino-nucleus interactions. I work on experiments that are studying neutrino oscillations (NOvA and DUNE), on experiments that are providing new data on neutrino-nucleus interactions (MINERvA), and on a widely-used software package (GENIE) that is used to simulate neutrino-nucleus interactions.

Theoretical high energy and nuclear physics, Science and society, Science education Theories of fundamental constituents of matter, Quantum Chromodynamics, tests of the Standard Model and beyond, the role of spin and angular momentum in particle interactions at medium and high energies. The role of science in public policy; non-proliferation of nuclear arms; education for peace.

Theoretical Physics: Cosmology, Particle Physics, Astrophysics My primary research is in physics at the interface between theoretical cosmology and particle physics, including astrophysics and aspects of quantum field theory. By studying the extreme conditions of the very early universe, as well as the properties of the late universe's dark constituents, and analyzing the results of various ground based experiments, we can gain insights into the fundamental laws of nature. This acts as the driving force behind much of my research, although I sometimes investigate other interesting subjects. A central focus has been on trying to understand the nature of dark matter, which forms the majority of matter in the universe. There are various interesting candidates for the dark matter, including so-called axions, which may organize into new interesting types of structures. Furthermore, I have worked on the understanding the large scale structure of the universe, which gives insights into the initial conditions of the early universe. Another focus has been on understanding cosmological inflation, which is the leading idea for the earliest moments of our universe, involving an early phase of rapid expansion. I have worked on connecting inflation to the matter anti-matter asymmetry of the universe and worked on the post-inflationary era where the universe needs to transition to a hot soup of particles. A recent interest is in pursuing a fundamental understanding of gravitation. I am interested in understanding the full set of theoretical and observational constraints that determine the structure of gravitation, including constraints from quantum mechanics. Furthermore, I sometimes investigate interesting quantum phenomena, including entanglement entropy and the Casimir effect.

Quantum Information, Quantum Simulation, Adiabatic Quantum Computation, Computational Physics Quantum information faces three basic questions. Firstly, what are quantum computers good for? Secondly, how do we build one? Thirdly, what will quantum information contribute if technological obstacles to constructing a large scale quantum computer prove insuperable? The first question is the search for problems which quantum computers can solve more easily than classical computers. The second is an investigation of which physical systems one could use to build a quantum computer. The third leads to the search for spinoffs in classical computation, and the question of where the classical/quantum boundary lies. I am interested in all three questions.

Experimental high energy physics, elementary particle interactions, neutrino oscillations, neutrino-nucleus interactions, baryon instability searches. Design and execution of experimental measurements that reveal or constrain the existence of new elementary particles, that delineate the properties of known elementary particles, and that quantify the interactions and symmetries that govern fundamental energy systems of the subatomic realm.

Astronomy; galaxy formation and evolution; extra-galactic surveys; active galactic nuclei; near-infrared astronomy Understanding how galaxies form and evolve means understanding how the tiny differences in the distribution of matter inferred from the cosmic microwave background radiation grew and evolved into the galaxies we see today. The working hypothesis is that galaxies form under the influence of gravity, and galaxy formation can be seen as a two-step process. First, the gravity of dark matter causes the tiny seeds in the matter distribution to grow bigger with time. As they grow more massive, the gravitational attraction becomes stronger, making it easier for these structures to attract additional matter. As the dark matter structures grow, they pull in also the gas, made of hydrogen and helium, which is the primary ingredient for the formation of stars, and hence for the formation of the stellar content of galaxies. The formation of the stellar content inside these dark matter structures involves many physical processes that are much more complicated and quite poorly understood from a theoretical perspective. These physical processes include, for example, how gas cools and collapses to form stars, the process of star formation itself, merging of galaxies, feedback from star formation and from active super-massive black holes. My research activity in the past decade has focused on understanding how galaxies formed after the Big Bang, and how their properties (e.g., the stellar mass, the level of star formation activity, the morphology and structural parameters, the level of activity of the hosted super-massive black hole, etc.) have changed as a function of cosmic time. Since we cannot follow the same galaxy evolving in time, we need to connect the galaxies we observe at a certain redshift (i.e. a certain snapshot in time) to those we observe at a smaller redshift (i.e., at a later time in cosmic history) in order to infer how the properties of galaxies have actually changed and what physical mechanisms are responsible for these changes. The better we understand the galaxy properties at a certain time and the more finely in time we can probe the cosmic history, the easier it becomes to connect galaxies' populations seen at different snapshots in time, linking progenitors and descendants across cosmic time. Ultimately, my research aims at understanding what galaxy population seen at one epoch will evolve into at a later epoch, and what physical processes are responsible for the inferred changes in the galaxies' properties. In order to do this, I have adopted two different but complementary approaches. The first approach consists of statistical studies of the galaxy populations at different cosmic times; the second approach consists of detailed studies of individual galaxies to robustly derive their properties.

- Extra-solar planets "exoplanets" - Planets in multiple-star systems, including circumbinary planets - Stellar populations and fundamental parameters - White dwarfs - Black holes - M-dwarfs - Stellar activity (spots and flares) - Celestial mechanics, including the Kozai-Lidov effect - Planet formation - Observational astrophysics

Experimental Particle Physics, Electromagnetic Theory, Computational Physics. High Energy Physics: studies of heavy quarks, new particle searches, tests of the Standard Model. Computational Physics: data analysis, simulation, electromagnetism.

Cosmic strings, gravitational waves, anthropic reasoning in cosmology, energy conditions in general relativity. Research: My main area of research is the study of cosmic strings, in particular their production of gravitational waves, which might enable us to detect the cosmic string network, if it exists. Cosmic strings are one of the potential sources of a stochastic background of gravitational waves that (as of September 2020) may have been seen by pulsar timing arrays. With my collaborators, I have developed a large-scale simulation of cosmic strings, and we are the process of to going from simulation results to predictions for the gravitational wave background, with all effects included. I also study gravitational waves and the possibilities of observing them more generally. I am an associate member of NANOGrav (the North American Nanohertz Observatory for Gravitational Waves), which observes gravitational waves using precision timing of pulsars. I work on issues of anthropic reasoning in cosmology and how to make sense of the infinite multiverse produced by eternal inflation. I study the structure of eternally inflating universes and also the philosophy of how to get from a model of what actually exists everywhere in the universe to an understanding of what we ourselves should expect to observe. I have worked for many years on the question of exotic phenomena such as superluminal travel and time travel in general relativity. My collaborators and I showed that, subject to certain conditions, a minimally-coupled quantum scalar field obeys the "achronal averaged null energy condition", which prohibits exotic spacetimes. I am currently working on the interaction between eternal inflation and energy conditions.

Extragalactic astrophysics How did galaxies and their central black holes co-evolve from the Big Bang to the present? Despite much progress through large scale galaxy surveys as well as ever more sophisticated numerical simulations, we are still hampered by the fact that much of the star-formation activity and black hole growth are buried in thick cocoons of dust and gas. Observations suggest that much of this activity took place in the past, before the Universe was half its present age, and likely involved mergers of nearly equal sized galaxies. As the merger progresses, gas and dust are more and more concentrated, triggering prodigious star-formation and gradually increasing accretion onto the central black hole (Active Galactic Nuclei or AGN). The process is short lived as supernovae- or AGN-driven winds lead to a 'blow-out' event which disperses the intervening gas and dust halting further star-formation and black hole growth. Indications that starbursts and AGN may regulate each other as above can be seen in the local correlation between the mass of a central black hole and the stellar mass of its host galaxy. The same galaxy observed at different stages of this process can appear very different. Therefore observations of different types of galaxies at different epochs and in different wavelength regimes are crucial to build a more complete understanding of the whole process.

Physics of elementary particles The Standard Model, gauge theories; also topology, differential geometry and other branches of modern mathematics to better understand quantum gauge theories, the origin of mass and the structure of space-time, matter and all interactions, including gravity. I am a member of the ATLAS collaboration at the LHC. Studies of Higgs boson and top quarks. The main objective is to find out whether the new particle discovered in 2012 is a minimal Standard Model Higgs, or some other kind. Studies of top quarks are very interesting on their own. Because of very large mass of the top quark, its lifetime is very short, ~ 5x10^{-25} seconds, much shorter that the characteristic time of the strong interactions. As a consequence, top quark decays before any strong interaction effects may take place. This allows a direct access to the information about the quark spin, which is very difficult, if not impossible, for any other quark. Studies of top quarks are very important for other searches, as top quarks will constitute the most important background for almost any final states due to "new physics" and have to be understood very well. We are using very advanced multidimensional analysis techniques, developed by our group (Ben Whitehouse and I). Topology and geometry of the Universe In the Standard Cosmological Model (SCM), the starting point is an interpretation of the observed redshift of spectral lines from distant galaxies as a Doppler shift in the frequency of light waves as they travel through an expanding Universe. Acceptance of this hypothesis led to the ideas of the Big Bang and the LambdaCDM, the Standard Model of cosmology. Remarkably, there exist another explanation of the cosmological redshift. As shown by Irving Ezra Segal, a mathematician and a mathematical physicist, the same axioms of global isotropy and homogeneity of space and time, and its causality properties, are satisfied not only by the Minkowski spacetime R x R^3, but also by a Universe whose geometry is R X S^3. In Segal's model, the geometry of the spatial part of the Universe is that of a three-dimensional hypersurface of a four-dimensional sphere. Locally, it is indistinguishable from the flat Minkowski spacetime. It is the geometry of the Einstein static Universe, which he abandoned when the interpretation of the increase of redshift with distance was universally accepted as evidence for expanding Universe. The redshift in Segal's model arises in a geometric way analogously to distortions which appear when making maps using stereographic projection from S^2, a two-dimensional curved surface of a sphere in three dimensions, onto a flat surface of a map, R^2. Segal's theory makes a verifiable prediction for the redshift as a function of distance. The comparison, although in principle very simple, is non-trivial. For more distant objects, one can only estimate the distance using various proxies, for example the magnitude, if one assumes that the chosen sources have the same absolute luminosity. Surprisingly, Segal's model cannot be falsified with the currently available data. The magnitude-redshift data for supernovae agree very well with SCM, but it also agrees with Segal's model. There exist another independent observable, the number of observed galaxies as a function of redshift z, N(< z). Assuming that galaxies are uniformly distributed in the Universe, their number is proportional to the volume enclosed in a given fixed angular field of view, and the dependence of this volume on the manifold distance is sensitive to the geometry of the Universe. Two Tufts undergraduate students, Maxwell Kaye and Nathan Burwig, joined me in this analysis. We examined the data from several Hubble Deep Fields, and found that the number of observed galaxies as a function of redshift is also in very good agreement with Segal's model. We are continuing with a study of these fundamental questions about the topology and geometry of our Universe. Interestingly, I have also shown recently that one can explain the observed value of the CMB temperature, following Segal's original idea that the CMB appears unavoidably as a result of light traveling many times around a closed spatial part of the R X S^3 Universe. Magnetic monopoles I am also a member of MoEDAL, a small collaboration looking for magnetic monopoles at the LHC.

Biological Physics, Condensed Matter Physics, Quantum Mechanics My research interests cover a broad array of topics in biological physics and condensed matter physics. In Biological Physics our group is performing both experimental and theoretical work to uncover fundamental physical principles that underlie the formation of functional neuronal networks among neurons in the brain. One of the primary challenges in science today is to figure out how as many as 100 billion neurons are produced, grow, and organize themselves into the truly wonderful information-processing machine which is the brain. We combine high-resolution imaging techniques such as atomic force microscopy, traction force and fluorescence microscopy to measure mechanical properties of neurons and to correlate these properties with internal components of the cell. Our group is also using mathematical modeling based on stochastic differential equations to predict axonal dynamics and network formation. The aim of this work is twofold. On the one hand we are using tools and concepts from experimental and theoretical physics to understand biological processes. On the other hand, active biological processes in neuronal cells exhibit a wealth of fascinating phenomena such as pattern formation, collective behavior, and non equilibrium dynamics, and thus the insights learned from studying these biological systems broaden the intellectual range of physics. To capture the complexity of neuronal interactions we use theoretical methods from statistical mechanics and many-body physics such as: stochastic differential equations, Fokker-Planck formalism, Markov processes, effective energy landscape etc. I am also interested in the foundations of quantum mechanics, particularly in applying the theory of stochastic processes to open quantum systems. My interests in condensed matter physics includes experimental work on quantum transport in nanoscale systems (carbon nanotubes, graphene, polymer composites, hybrid nanostructures), as well as on scanning probe microscopy and the study of novel biomaterials.

Experimental condensed matter physics; physics education My primary physics research is in experimental surface science. In my lab at 574 Boston Ave., my students and I study what happens when foreign atoms and molecules form chemical bonds with metal surfaces. We examine how the interaction between the foreign molecule and the metal modify properties of both of them. In recent years a particular focus has been on how the attachment of the foreign molecule changes the electrical resistivity of the metal substrate. This area of research has relevance to a range of potential applications including catalysis, chemical sensing, and the growth of thin films and nanoparticles on surfaces. A second area of activity is physics education, particularly at the elementary school level. Together with collaborators at a local nonprofit organization and at other universities, I am working to develop and study curriculum materials and professional development strategies for teachers to improve instruction in science in grades 3-5.

Theoretical cosmology I do research on cosmic inflation, dark energy, cosmic strings and monopoles, quantum cosmology, and the multiverse.

My current focus is on measuring the properties of the neutrino, one of the fundamental particles of the Standard Model. We know a few things about the neutrino: it has a very small mass, has no electric charge, comes in three types — or flavors — and interacts only via the weak force and gravity. However, there are many things we do not know. What is the exact mass of the neutrino? And how does it get its mass? Are the three we know about the only kinds that exist? Answers to these questions impact not only our understanding of the fundamental laws of matter but also have consequences for our understanding of how the universe evolved. These and many other questions make the neutrino a fascinating particle. However, as mentioned above, neutrinos interact only via the weak force. They interact so rarely that, at the energies, we typically work with, neutrinos can pass through light-years long block of lead without striking it. This makes neutrino experiments challenging as we need to build massive, building-sized detectors which are instrumented with relatively, low-cost sensors. However, the challenge is often fun, as we are often forced to apply the newest technologies in both hardware and software to design and complete our experiments.