# Theoretical Nuclear and Particle Physics

## Areas of Research

Cosmology | QCD: Quantum Chromodynamics |

Energy Research | Quantum Computing |

Field Theory | String Theory |

The Standard Model and Beyond |

### COSMOLOGY

Cosmology is the study of the large-scale structure and history of the universe. In recent years, cosmology has changed from being a highly speculative area of theoretical physics into a field where precise experimental data and deep theoretical considerations interact to form an increasingly clear picture of the early history and future fate of the universe. Recent high-precision results from the WMAP satellite, from supernova studies, and from other extragalactic observations seem to confirm the inflationary model of the universe developed by Alan Guth. Inflation can explain why the universe is so large, so uniform, and so close to geometric flatness. Inflation also helps to cement the link between particle physics and cosmology by proposing that the ripples that we measure today in the cosmic background radiation originated as quantum fluctuations perhaps only 10-35 seconds after the instant of the big bang. The recent observational results also show that the rate of expansion of the universe is accelerating, due to a nonvanishing cosmological constant or something similar. These new results raise profound theoretical questions which drive one of the most exciting fields of physics in this decade.

Faculty members at MIT working on theoretical problems related to cosmology include Alan Guth and Frank Wilczek in the CTP , as well as Ed Bertschinger and Max Tegmark 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.

Ernest Moniz, in collaboration with faculty colleagues across the Institute, is examining the linked technology and policy pathways to mitigating greenhouse gas emissions on a large scale. Recent and current work focuses on nuclear power, photovoltaics, and coal utilization with carbon dioxide geological sequestration. In addition, evolution of the electricity transmission system is under study.

### 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.

### THE STANDARD MODEL AND BEYOND

Effective field theory provides a crucial tool both for probing the electroweak standard model and for understanding QCD. For example, it allows a clean separation between QCD and electroweak physics in the description of B meson decays. This means that some processes, in which the electroweak physics is understood, can be used to study hadron structure, whereas other processes, in which the QCD physics is understood, can be used to look for deviations from the standard model, such as in its description of CP violation. Iain Stewart works on developing new effective theories and on applications of known effective field theories. He co-invented the soft-collinear effective theory which allows these techniques to be applied to processes with energetic jets or energetic hadrons. In jet physics, these techniques allow one to derive factorization theorems which make it possible to use high energy collisions to investigate short distance physics. Iain Stewart and Jesse Thaler are using effective theories to improve our understanding of jet properties and jet substructure. This is particularly relevant for the Large Hadron Collider (LHC), where jet production is often the dominant background for potential signals of new physics.

For all its intellectual depth and empirical success, the standard model of fundamental interactions (including QCD, electroweak gauge theory, and minimally coupled gravity) has significant conceptual and esthetic shortcomings. There are also several observed phenomena that the standard model does not address, e.g. the nature of cosmological dark matter. An important branch of theoretical physics is concerned with addressing these shortcomings by suggesting ways to augment the standard model. Phenomenological beyond-the-standard-model physics focuses specifically on questions that are sufficiently concrete and well posed that they will receive experimental illumination in the near future. An outstanding example is the possibility of weakly broken supersymmetry at the LHC; this is suggested by quantitative aspects of theories that unify the interactions, and if correct would lead to a rich and informative flow of new discoveries, that will both call for and reward insight. Other examples are axion physics, many aspects of neutrino physics, and attempts to understand the patterns of quark and lepton masses and mixings.

Beyond-the-standard-model physics takes inspiration from cosmology, quantum field theory, symmetry, and string theory as well as from experiment and observation. Many relevant CTP activities are mentioned in the separate descriptions of those areas. Frank Wilczek studies unified supersymmetric models and axion physics. Jesse Thaler studies the theoretical frameworks and LHC signatures for a variety of beyond the standard model scenarios, hoping to gain insight into the origin of mass, the nature of dark matter, the apparent weakness of gravity, and the symmetry structure of our universe.

### QCD: QUANTUM CHROMODYNAMICS

Understanding the structure of hadrons is one of the great unsolved problems in physics, and as such is the subject of both theoretical and experimental effort at MIT. Robert Jaffe and his collaborators developed the MIT bag model of confinement, still one of the favorite models of quark dynamics, and applied it to the spectrum and structure of hadrons. Professor Jaffe is also one of the leaders in the quest to use new experiments to elucidate the spin structure of the nucleon. Iain Stewart and Jesse Thaler study many manifestations of QCD dynamics in high energy proton collisions at the LHC and in other particle colliders.

John Negele uses lattice field theory to solve QCD ab initio and thereby understand from first principles how QCD gives rise to the observed quark and gluon structure of protons, neutrons, and other hadrons. The combination of numerical computation and analytic techniques enables one to make fundamental progress in solving complex problems in QCD that are not amenable to either technique alone. Current lattice studies range from calculating the contributions of quarks and gluons to the spatial, momentum, and spin structure of nucleons measured by MIT experimentalists Stanley Kowalski and Richard Milner to understanding the role of diquarks and instantons in hadron structure. Professor Negele is one of the founders of a national initiative to develop Terascale computers optimized for lattice QCD and is leading a collaboration to exploit them to understand hadron structure. As part of this initiative, a dedicated 5.7 Teraflops Blue Gene supercomputer at MIT provides essential resources for lattice research.

Understanding QCD in extreme conditions requires linking usually disparate strands of theoretical physics, including particle and nuclear physics, cosmology, astrophysics and condensed matter physics. Krishna Rajagopal and Frank Wilczek study the properties of the cold dense quark matter that may lie at the centers of neutron stars. This stuff is the QCD analogue of a superconductor. However, if probed with ordinary light it looks like a transparent insulator and not like an electric conductor, as previously assumed. The properties of sufficiently dense quark matter have now been understood from first principles, but many interesting questions remain to be answered at lower densities. Progress requires coupled advances in theory, astrophysical observation, and experiments on analogue systems made of ultracold fermionic atoms. Robert Jaffe and Edward Farhi did the first work on quark matter in astrophysics. This work makes contact with research in Xray astronomy, condensed matter theory and ultracold cold atoms carried out elsewhere in our department.

Hong Liu and Krishna Rajagopal do research on hot quark matter, of the sort that is created in current experiments at the Relativistic Heavy Ion Collider (RHIC). They are using gauge/gravity duality to understand properties of the strongly coupled, liquid-like, quark-gluon plasma, which these experiments tell us filled the universe for the first microseconds after the big bang. For example, they study how a high energy quark plowing through this liquid loses energy and under what conditions a pair of heavy quarks moving through this fluid can bind into a meson. Bolek Wyslouch and Gunther Roland are leading the related experimental effort at the Large Hadron Collider, where definitive measurements are anticipated. Rajagopal has also analyzed the critical point in the QCD phase diagram and has proposed signatures for its experimental detection, making it possible for experimentalists at RHIC to do a definitive search for the critical point in a large region of the phase diagram.

### QUANTUM COMPUTING

Edward Farhi and Jeffrey Goldstone are working on quantum computing. Their work has focused on designing quantum algorithms, that is, finding ways to use quantum mechanics to achieve algorithmic speedup for certain computationally difficult problems. The quantum computing group in the Center for Theoretical Physics is responsible for the quantum adiabatic evolution algorithm and the idea of quantum walk algorithms both of which are continuous time Hamiltonian based approaches to quantum computing. This group showed that a quantum computer can determine who wins a game faster than any classical computer. The method involving scattering theory and demonstrates how ideas from physics can be used in the design of quantum algorithms.

The quantum computing group in the Center for Theoretical Physics has ties to many other researchers and centers at MIT. For example Professors Farhi and Goldstone are members of the W.M. Keck Foundation Center for Extreme Quantum Information Technology at MIT. The CTP group is the focus of much of the theoretical efforts in quantum computing and quantum information at MIT. The extended group includes collaborator Peter Shor who is in the Math Department at MIT. He showed that a quantum computer can factor faster than a classical computer, which ignited the field. The group also includes Seth Lloyd (Mechanical Engineering and Engineering Systems), Isaac Chuang (Physics, Electrical Engineering, and Computer Science) as well as Scott Aaronson (Electrical Engineering and Computer Science) all of whom are key players in quantum information science.

### STRING THEORY

String theory (and its alter ego M-theory) is currently the most viable candidate for a unified theory of physics which describes all forces of nature, encompassing the physics of gravity as well as quantum field theory. MIT is a main center of research in string theory, with six faculty members and numerous postdocs and graduate students working in this area.

Work on string theory at MIT is currently focused in several different directions. Dan Freedman (math/physics) is currently working on aspects of the AdS/CFT correspondence, which can be used to derive quantitative non-perturbative information about gauge field theories using gravity and string theory.

Hong Liu is working on string theory in time-dependent backgrounds, holography, and the AdS/CFT correspondence. Washington Taylor is working on nonperturbative formulations of string theory and on relating the space of string vacua to observable physics. Barton Zwiebach is working on tachyon condensation and string field theory, and he has recently written an undergraduate textbook on string theory.

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.