Theoretical Nuclear and Particle Physics: 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 is working on applications of effective field theories, and recently has developed a framework that extends this formalism to processes with energetic hadrons. For weak B meson decays this provides a new approach to understanding QCD effects which were previously thought to be intractable or model-dependent. Jesse Thaler is using insights from effective theories to improve our understanding of QCD jets produced in high energy collisions. This is particularly relevant for the Large Hadron Collider (LHC), where jet production is expected to be the dominant background to 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 Center for Theoretical Physics (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.