Theoretical Physics

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The PIs on this proposal will perform research in a variety of exciting areas in theoretical physics, including extensions of the Standard Model, electroweak symmetry breaking, quantum chromodynamics, neutrino physics, supersymmetry, anomalies and boundary conditions in quantum field theory, conformal field theory, the quantization of the superstring, and the geometry of string compactifications. Quantum field theory is the language of elementary particles and forces. Among quantum field theories, the so-called Standard Model describes the known forces in Nature aside from gravity, namely the electromagnetic, weak and strong forces. Among the mysteries not addressed by the Standard Model is the problem of "generations", a three-fold repetition of particle types. Another is the origin of electroweak symmetry breaking, or precisely how the weak interactions become weak. It is widely hoped that the Large Hadron Collider at CERN will begin providing qualitatively new insights into these issues within the next few years. Indeed, it is expected that many proposals for extensions of the Standard Model, especially those with supersymmetry, will be tested. In supersymmetric models, known particles all have as-yet undetected "partners", whose presence improves the quantum behavior of the theory. Experiments over recent years have also confirmed that neutrinos, which only interact via the weak forces, have small but nonzero masses, not envisioned in the construction of the Standard Model. Neutrinos masses also have profound consequences for astrophysics and cosmology. The strong force in the Standard Model is described by the field theory quantum chromodynamics (QCD), which is central to experimental programs for observing new signals. It also exhibits crucial aspects of quantum field theory that are not yet understood, and presents both "weak coupling" and "strong coupling" behaviors. Work in weak-coupling, or perturbative, QCD serves both to make predictions for signals of new physics and their backgrounds. The strong-coupling behavior of the theory is studied to understand the structure of the nucleons, and for its relation to other theories, including string theory. String theory is the only known finite quantum theory of gravity, and has inspired many new insights into quantum field theories, especially QCD and its supersymmetric extensions. It is the only candidate for a theory that unifies all the known forces. It has given insights into the nature of black holes and revealed profound links to many aspects of modern mathematics. It suggests many proposals for new physics to be seen at the LHC. The proposed research will have a broad impact by advancing our knowledge of the laws on Nature and contributing toward a better understanding of the physical universe. Indeed, many of the topics under study by the senior participants are widely discussed in the media, such as the role of "ghostly" neutrinos in the Universe, and hopes of physicists for the soon-to-be-commissioned Large Hadron Collider. The research outlined in this proposal will serve in the training of graduate students and postdoctoral fellows. The faculty on this proposal are often involved in outreach efforts to reach a broader public. These include public lectures through the Run Run Shaw series and summer workshops in physics and mathematics supported by the Simons foundation. They are also involved in writing textbooks for graduate students.