# Theoretical Nuclear and Particle Physics

## Areas of Research

### COSMOLOGY AND THE EARLY UNIVERSE

The cosmology and particle astrophysics program in the CTP focuses on the implications for fundamental physics and applications of field-theoretic techniques, complementing the work of the Astrophysics group. Astrophysical and cosmological datasets provide an observational window on the very early universe and afford some of the strongest evidence for physics beyond the Standard Model.

The nature of dark matter, constituting ~ 80% of the matter in the universe, is one of the outstanding questions of particle physics. Annihilations or decays of dark matter could modify the thermal and ionization history of the universe, with possible observational consequences for nucleosynthesis, the cosmic microwave background and the redshifted 21cm line. In the present era, the same phenomena could provide striking signals in regions of high dark matter density. Tracy Slatyer has worked extensively on the interpretation of such signals, developing new constraints and identifying possible signatures of dark matter annihilation, from radio to gamma-ray wavelengths, with a particular focus on data from the *Fermi* Gamma-Ray Space Telescope. The AMS-02 experiment, led by Sam Ting and including MIT faculty Paolo Zuccon and Ulrich Becker, is now providing copious and detailed measurements of cosmic rays, allowing both novel dark matter searches and a better understanding of the astrophysical backgrounds. In the next 5-10 years several new gamma-ray telescopes are expected to begin operation, covering energy scales from GeV up to tens of TeV and improving sensitivity to heavy dark matter beyond the reach of collider experiments. Observations of the 21cm line – a focus of research by <="" a="" style="font-family: Lucida, Verdana, Helvetica, Arial, sans-serif; font-size: 12px; color: rgb(152, 14, 3); margin: 0px; padding: 0px; text-decoration: none;">and Max Tegmark at the MIT Kavli Institute – promise to probe the reionization epoch and early structure formation.

In the area of model-building, Jesse Thaler and Tracy Slatyer have developed innovative models for dark matter in the context of expanded "dark sectors", motivating new experimental searches. Jesse Thaler is closely involved with one such search – the DarkLight experiment – together with MIT faculty Peter Fisher and Richard Milner. The DarkLight experiment will search for the hypothesized *A*' particle, a massive (0.01 – 10 GeV) particle which resembles a photon, but interacts at least 10,000 times more weakly. While great effort has been invested into the search for certain dark matter candidates, such as Weakly Interacting Massive Particles (WIMPs) and similar particles, the range of possibilities is actually much larger than that, so new theoretical insights can lead to rapid developments in experimental searches. In addition to the DarkLight experiment, new lines of research include new laboratory searches for axions, and adaptations of existing direct detection experiments to probe very light dark matter. This work builds on MIT's leadership on the experimental side, highlighted by Peter Fisher's leadership in the MIT-BU-Brandeis Dark Matter Time Projection Chamber (DMTPC) project.

Current experimental searches for primordial *B*-modes in the polarization of the cosmic microwave background have the ability to probe the most fundamental questions of physics, opening a window on the earliest history of the cosmos. Any detection of such *B*-modes would be a strong piece of evidence in favor of the inflationary universe model, pioneered by Alan Guth, and would also provide a powerful tool for studying the details of inflation. If the recent claim of a detection by the BICEP2 experiment is confirmed, it would suggest an energy scale for inflation around 1016 GeV, essentially the scale of grand unified theories – an example of cosmological observations probing physics not far below the Planck scale.

Alan Guth's recent research has included the study of eternal inflation, with the issues it raises concerning the definition of probabilities, and also the study of hybrid inflation, a particular model of inflation with an unusual prediction for the spectrum of density fluctuations. In particular, Guth and collaborators are studying the production of primordial black holes from hybrid inflation, with the hope of making contact with the supermassive black holes observed in the centers of galaxies.

Faculty members at MIT working on theoretical problems related to cosmology include Frank Wilczek in the CTP, as well as Ed Bertschinger in astrophysics, and David Kaiser in Science, Technology, and Society, and physics. A number of experimentalists in the astrophysics and particle experiment groups are working on research relevant to cosmology. String theorists Hong Liu and Washington Taylor are also currently working on problems related to cosmology.

### ENERGY RESEARCH

Energy production and use are clearly crucial to the functioning of modern economies and to enhanced quality of life. However, they also drive major environmental problems. The prospect of global warming and climate change driven by greenhouse gas emissions from fossil fuel combustion presents a particularly difficult challenge. Considerable science and technology development are needed, as well as policies that enable widespread and timely global deployment of key energy technologies.

Robert Jaffe in collaboration with faculty colleagues, postdocs, and students in the MIT Energy Initiative and in MIT's Engineering Systems Division is studying the constraints on the possibility of scaling novel, low carbon energy technologies to high levels of deployment presented by potential shortages of scarce materials known as "energy critical elements".

### FIELD THEORY

Quantum Field Theory (QFT) is the theoretical framework underlying the standard model of particle physics, a theory which unifies all forces of nature other than gravity, and which reproduces to astonishing precision all terrestrial experiments to date not including gravity. Many of the basic structural features of quantum field theory were developed by physicists at the CTP; the work of Jeffrey Goldstone on Goldstone bosons, Roman Jackiw on anomalies, and Frank Wilczek on asymptotic freedom, form an integral part of our current understanding of field theory, and are featured prominently in all textbooks on the subject.

Roman Jackiw is studying possible mechanisms and signatures of CPT violation. Edward Farhi and Robert Jaffe are studying the role that quantum fluctuations may play in stabilizing extended field configurations such as solitons in gauge theories like the Standard Model. Jaffe is also applying field theory methods to the study of the Casimir effect in nanoscopic systems. Here his work overlaps the interests of Leonid Levitov and Mehran Kardar in our Condensed Matter Theory Group. Iain Stewart has developed new types of effective field theory suited for studying phenomena in gauge theories like QCD. Finally, Frank Wilczek retains a keen interest in applications of field theory to condensed matter systems, including the description of anyons, a subject he pioneered.

There is a close connection between quantum field theory and statistical physics; in addition to work on field theory in the CTP, related research is done in the Condensed Matter Theory Group.

### PARTICLE THEORY IN AND BEYOND THE STANDARD MODEL

The goal of particle theory research in the CTP is to enable discoveries of physics beyond the standard model (BSM), both through precision tests of the standard model itself and through detailed studies of possible new phenomena. With the momentous discovery of the Higgs boson at the LHC in 2012, the standard model is now complete, yet its shortcomings loom larger than ever. The standard model cannot account for the nature and origin of dark matter, nor does it address the puzzling hierarchy between the electroweak and Planck scales. For this reason, particle theorists in the CTP are developing new theoretical frameworks to address physics in and beyond the standard model, and studying the resulting experimental signatures at dark matter detection experiments, high intensity experiments, and colliders like the LHC.

The CTP has a long history of leadership in particle theory. Emeritus faculty Dan Freedman, Jeffrey Goldstone, and Roman Jackiw are responsible for some of the fundamental theoretical ideas – especially those associated with symmetries and symmetry breaking – which lie at the heart of the standard model and its extensions. Frank Wilczek is one of the authors of the standard model and is regarded as luminary in particle theory, with long-standing interests in axions, unification, and supersymmetry. Eddie Farhi and Robert Jaffe have taken techniques developed in particle theory and applied them to the fields of quantum computation and fluctuation physics, respectively. Tracy Slatyer and Jesse Thaler represent the next generation of particle theorists, whose work draws on experimental and theoretical developments in areas ranging from dark matter detection to formal supergravity.

The current particle theory effort in the CTP includes research that has a direct impact on experiments as well as research that pursues more formal theoretical directions. Successful particle theorists have an appreciation and understanding of experimental methods, and the CTP prides itself on maintaining close connections to experimental research conducted in the LNS. At the same time, research in particle theory offers opportunities to push the boundaries of knowledge in quantum field theory (QFT), and excellence and creativity in QFT has long been a theme that unites the research conducted in the CTP.

Higgs physics will become increasingly important with the upgraded 14 TeV LHC run starting in 2015. Precent-level measurements of the Higgs boson couplings are needed to test the Higgs boson's role in generating fundamental particle masses. Wilczek has long emphasized that BSM scenarios such as supersymmetry predict small deviations in these couplings as well as additional Higgs particles. More recently, Wilczek has shown how the Higgs boson can act as the portal to dark matter, and Thaler has shown how presence of Higgs bosons can be used to tag new physics signals at the LHC. The connection between Higgs physics and BSM physics remains an active area of research. Precision calculations are crucial for studying the detailed characteristics of the Higgs boson, and Iain Stewart has applied effective field theory methods to calculate key Higgs cross sections and thus reduce theory uncertainties in Higgs measurements.

Another area of increasing importance for the 14 TeV LHC is jet physics. Jets are collimated sprays of particles that arise when quarks and gluons are produced at the LHC, and copious jet production is a potential smoking gun for supersymmetric theories. Jesse Thaler has been at the forefront of the emerging field of jet substructure, developing new jet analysis techniques to capitalize on the exceptional ability of the LHC experiments to resolve jet constituents. These jet substructure methods can enhance BSM signals above standard model backgrounds, and they are currently being implemented in new physics searches by the MIT CMS $pp$ group. Stewart and Thaler have also developed new techniques to perform precision jet calculations, capitalizing on recent development on applying resummation techniques to hadronic collisions. Jet substructure may also offer new probes of the phenomena of jet quenching in the quark/gluon plasma, an area of considerable interest to the MIT CMS heavy ion group.

Dark matter is a key research direction in the CTP, bridging particle physics and astroparticle physics (see Cosmology page). The gravitational evidence for dark matter is overwhelming, but the nature and origin of dark matter is still unknown. The two leading paradigms for dark matter are axions and stable relics (possibly of supersymmetric origin), but given the lack of any conclusive dark matter signals to date, CTP researchers are taking imaginative approaches to dark matter and its potential signatures.

Particle theory also connects to more formal developments in QFT (as well as string theory). Almost all collider studies involve the calculation of scattering amplitudes, but independent of collider applications, Freedman has shown that scattering amplitudes themselves have a rich mathematical structure with hidden symmetries. Inspired by potential LHC signatures of supersymmetry, Thaler has shown that the dynamics of supersymmetry breaking can be richer than previously thought, leading to new results in formal supergravity. Strong dynamics is a feature of many extensions of the standard model, and one can gain some analytic handles on these scenarios by treating them as if they were conformal field theories (i.e.~special QFTs with a scaling symmetry). Conformal field theories may also be relevant for understanding jet physics, since the interactions of quarks and gluons can sometimes be approximated as having a scaling symmetry. More generally, techniques developed in particle theory have the potential to offer new insights in other fields, especially condensed matter physics.

### QCD: QUANTUM CHROMODYNAMICS

The challenge of understanding strong interactions is a unifying theme that cuts across many areas of CTP research and also plays a central role in aspects of the physics of condensed matter and ultracold atoms. The interactions among quarks and gluons, described by Quantum Chromodynamics (QCD), are particularly important because they exhibit many characteristic and challenging features of a strongly coupled theory while at the same time they are described at short length scales by a well- understood and well-tested theory, QCD, which is a central part of the Standard Model.

Understanding strong QCD interactions is crucial to interpreting collider searches for new short distance physics, within and beyond the Standard Model, as well as to understanding the properties of the hot matter that filled the microseconds old universe and the dense matter in the centers of neutron stars. They are also the key to understanding how quarks and gluons form protons, neutrons, and other hadrons – which were the earliest complex structures formed in the universe – and subsequently nuclei. QCD provides a defining example of a theory in which the entities and phases that it describes do not resemble the elementary constituents of which they are made. This feature is characteristic of strongly interacting systems in many areas of physics and makes them both interesting and challenging. Effective field theory provides a crucial tool both for probing the fundamental description of nature embodied by the electroweak part of the Standard Model, and for understanding QCD. In recent years the number of physical phenomena successfully described by effective field theory methods has been rapidly expanding, and MIT faculty have made crucial contributions to these developments. This includes Iain Stewart's co-invention of the soft-collinear effective theory, which provides a rigorous description of the energetic jets formed by high energy quark and gluon collisions. This formalism has enabled improvements in precision by a factor of ten for cross section calculations, and has made a broader range of sophisticated reactions theoretically tractable. Other examples include probing fundamental symmetries like the Standard Model description of CP violation and weak decays, and precisely determining essential parameters like the strong coupling constant. Recently there has also been a renaissance in the realm of jet physics, including both our understanding of jet properties and the invention of new jet observables, where both Jesse Thaler and Stewart have played important roles, for example providing theoretical tools and results that are now used in new physics searches and Higgs analyses at the LHC, including those carried out by Markus Klute and Christoph Paus as part of the CMS collaboration. Thaler has also been a key pioneer involved in developing methods that for the first time make it possible to use the substructure of a jet to determine its parentage, for example whether it came from a gluon, a quark, a boosted *W* boson, or even a supersymmetric squark.

The physics of hadrons and nuclei arises from the same Standard Model that is probed at colliders, but it requires different theoretical methods. William Detmold and John Negele study QCD at lower energies from first principles using a lattice field theory approach and thereby understand how QCD, whose fundamental degrees of freedom are quarks and gluons, gives rise to the rich and complex structure of protons, neutrons, and eventually nuclei. By employing innovative analytic and computational methods, they are able to make fundamental progress in solving complex problems in QCD that are not amenable to other techniques. Detmold's research centers on obtaining quantitative understanding of how the complexity of nuclei emerges from their underlying quark and gluon degrees of freedom, and of the dynamics of the rearrangement of the light quarks and gluons that occurs when a heavy quark decays, for example at particle colliders such as the LHC where LNS colleague Mike Williams measures these decays using the LHCb experiment. Detmold's advances in the QCD study of nuclei have the potential for transforming nuclear physics as they provide a path towards ab initio calculations of nuclear processes with fully quantifiable uncertainties. Negele's research focuses on understanding the underlying structure of the proton. His calculations are now elucidating the contributions of quarks and gluons to the spatial, momentum, and spin structure of protons and neutrons. Both Detmold and Negele also perform carefully quantified calculations of unmeasured properties of nucleons and nuclei that are needed in experimental searches for dark matter and other new physics. Detmold and Negele are key members of a national initiative exploiting the country's most powerful computers for lattice QCD and also use large-scale resources at MIT.

At high enough temperature and/or density, QCD describes various phases of matter in which the quarks and gluons do not coalesce into hadrons or nuclei. Understanding these liquids requires linking particle and nuclear physics, cosmology, astrophysics and condensed matter physics. Experimentalists including Yen-Jie Lee, Gunther Roland and Bolek Wyslouch have used heavy ion collision experiments at RHIC and the LHC to show that the hot quark-gluon plasma that filled the microseconds-old universe is a strongly coupled liquid. Understanding the properties of this new phase of matter and how it emerges from QCD is a central challenge for the coming decade. Together with Hong Liu, Krishna Rajagopal is using gauge/string duality to study similar fluids from first principles and to discern the most effective ways to use measurements of jets and heavy mesons in heavy ion collisions at the LHC to probe the liquid quark-gluon plasma. For example, he has shown how a high energy quark plowing through this liquid can lose substantial energy and yet emerge looking similar a jet in vacuum, with small modifications as seen in the data. Rajagopal has also analyzed the critical point in the QCD phase diagram and has proposed signatures for its experimental detection, showing how to use the collision-energy scan now underway at the Relativistic Heavy Ion Collider (RHIC) to search for the critical point in a large region of the QCD phase diagram. Rajagopal and Frank Wilczek have previously analyzed the properties of the superfluid, color superconducting, quark matter that may lie at the centers of neutron stars, providing a clear understanding of the properties of such matter at very high densities.

The longer term challenge to theorists is to use the data to gain an understanding of how a strongly coupled liquid, which shows no signs of the individual particles of which it is made, can emerge from QCD. This quest resonates with challenges that are central to contemporary condensed matter physics, where Hong Liu has used gauge/string duality techniques developed to study quark-gluon plasma to gain insights into superfluids, and some of the most interesting and most puzzling materials, including "strange metals".

### QUANTUM INFORMATION AND QUANTUM COMPUTING

There is a worldwide research effort exploring the consequences of quantum mechanics for information and computation. The field began with Feynman's 1981 proposal to build a computer that takes advantage of quantum mechanics and has grown enormously since Peter Shor's 1994 quantum factoring algorithm. Leaving aside the extensive experimental efforts to build a quantum computer, theory research in QI/QC (quantum information and quantum computing) investigates several themes:

*Quantum algorithms and complexity*. If a perfectly functioning quantum computer were built, which problems could it solve faster than conventional computers, and which problems do not admit any speedup?*Quantum information theory*. How can we communicate and compute using quantum information in the presence of noise? What are the properties of entanglement?*Measurement and control*. How can quantum information be useful in applications apart from computers and communication devices, such as clocks and precision measurements?*Applications and connections*. How can ideas from QI/QC contribute to areas as diverse as convex optimization, black holes and condensed-matter physics?

QI/QC theory research at MIT spans all of these areas. The CTP faculty involved are Eddie Farhi, Jeffrey Goldstone (emeritus) and Aram Harrow, and the larger QI/QC group at MIT includes Isaac Chuang (EECS/physics), Seth Lloyd (Mech. Eng.), Jeff Shapiro (EECS), and Peter Shor (Math). Together this forms a large and vibrant group working in all areas of QI/QC.

Some of the notable contributions involving the CTP include the quantum adiabatic algorithm and quantum walk algorithms (Farhi, Goldstone), the first example of a problem for which quantum computers exhibit no speedup (Farhi, Goldstone), proposals for unforgeable quantum money (Aaronson, Farhi, Shor), a quantum algorithm for linear systems of equations (Harrow, Lloyd), efficient protocols for simulating quantum channels (Harrow, Shor) and both algorithms and hardness results for testing entanglement (Harrow). Ongoing research at MIT in QI/QC includes work on new quantum algorithms, efficient simulations of quantum systems, connections to convex optimization, understanding the role of decoherence in excitonic transport (e.g. in photosynthesis) and many other topics.

The larger QI/QC group at MIT shares a seminar series, a weekly group meeting, regular events for grad students.

Interdepartmental course offerings include an introductory and an advanced class in core QI/QC, a graduate class in quantum complexity theory and a special-topics class. Quantum information has also recently entered the undergraduate physics curriculum with a junior lab experiment on NMR quantum computing and some lectures in the 8.04/8.05/8.06 sequence on quantum computing.

### STRING THEORY/HOLOGRAPHY/GRAVITY

Quantum physics and Einstein's theory of general relativity are the two solid pillars that underlie much of modern physics. Understanding how these two well-established theories are related remains a central open question in theoretical physics. Over the last two decades, efforts in this direction have led to a broad range of new physical ideas and mathematical tools. These have deepened our understanding not only of quantum gravity, cosmology, and particle physics, but also of intermediate scale physics, such as condensed matter systems, the quark-gluon plasma, and disordered systems. Ideas from string theory have also led to new insights and approaches to problems in many areas of mathematics. Indeed, the interface of quantum physics and gravity is a vibrant area of research that is expected to be extremely active in the coming decade. Researchers in the CTP have been at the forefront of many of these developments. CTP faculty members work on string theory foundations, the range of solutions of the theory, quantum cosmology, and the application of string-inspired "holographic'' methods to strongly coupled field theories. The group in the CTP has close connections to condensed matter physicists, astrophysicists, and mathematicians both at MIT and other departments.

Even though we understand string theory better, there is still no clear fundamental description of the theory in a background-independent framework, and the set of solutions, or string vacua, is still poorly understood. The work of Washington Taylor and Barton Zwiebach combines physical insight with mathematical consistency to address these questions, and has led to the development of new mathematical results and ideas.

Taylor's recent work has given a new systematic understanding of certain types of complex manifolds used for string compactifications, and has identified specific constraints and generic properties that large classes of string theory solutions imply for quantum gravity theories in four and higher dimensions. This program has led to evidence that with additional dimensions and supersymmetry, the spectrum of any consistent quantum gravity theory must arise from a solution of string theory.

Zwiebach has been at the forefront of developments in double-field theory. This program has identified structures of generalized geometry that are relevant to the description of gravity in string theory and, despite advances, is still in its infancy.

Outside the direct application to questions of quantum gravity, string theory has in recent years spawned a rapidly-developing new area of application where "holographic" dualities relate theories of gravity in one spacetime to strongly-coupled quantum theories in a spacetime of one less dimension. Originally discovered in the context of string theory, these dualities appear to provide a rigorous mathematical equivalence between the two related theories. Such dualities give both a new perspective into quantum gravitational phenomena as encoded in quantum field theory, and a way to explore aspects of strongly coupled field theories using the gravitational dual. CTP faculty have played a pioneering role in several applications of holographic duality. Hong Liu and Krishna Rajagopal were at the forefront of efforts that used holography to find new insights into the physics of the quark-gluon plasma.

In recent years, a set of interesting new developments has begun to draw unexpected connections between a number of problems relating aspects of gravity, black holes, quantum information, and condensed matter systems. In particular, it is becoming clear that quantum entanglement, through holographic duality, underlies a characterization of spacetime geometry. These developments tie into the research activity of several CTP faculty members, including Aram Harrow and Hong Liu.

The string group in the CTP interacts broadly with the other groups within the CTP, and with the astrophysics group in the physics department. Faculty in other departments working in string-related areas include Isadore Singer (math). In addition to the regular MIT faculty, Ashoke Sen spends two months each year with the group as the Morningstar visiting professor.