# High Energy Physics -- Theory

## Particles and Fields: Theoretical High Energy Physics

University of Chicago Theory Group Home Page

The aim of theorists working in particle physics is to develop a coherent picture of the structure and properties of matter at its deepest level: to interpret experimental data, build models of fundamental particles and their interactions, predict or speculate on new phenomena at increasingly high energies or more minute levels, and search for a mathematical framework in which to express the basic laws.

According to the highly successful "standard model", the fundamental particles are leptons (the electron and neutrino, and their relatives) and quarks (the constituents of neutrons and protons), which are fermions. The fundamental interactions--strong, weak, electromagnetic, and gravitational--are mediated by gauge fields (generalizations of the vector potential of electromagnetism), which are bosons. All of these forces follow from the same general principle, local gauge symmetry, which is geometric in nature. One speculation is that all gauge fields are different manifestations of an all-embracing "grand unified" gauge theory, as suggested by the success of the Glashow-Weinberg-Salam theory that joins electromagnetic and weak forces. Another idea is that it might even be possible to unify bosons and fermions under a principle known as supersymmetry. The most ambitious such proposal is string theory, wherein all known particles and forces are realized as different manifestations of one-dimensional extended loops of string. The characteristic size of strings is extremely small, so the macroscopic observer would see them as point-like objects. The various internal vibrational states realize the different quantum numbers of a low-energy particle spectrum--both matter and force mediating quanta. The subject is still in its developmental stages; Chicago is one of the major centers of string research.

Many other questions intrigue the theorists. For example, how does quantum chromodynamics (QCD, the theory of strong interaction) lead to the binding of quarks into nucleons? How do the properties of nucleons, such as magnetic moments, follow from quark dynamics? How many families of quarks and leptons exist in nature, and why? (Six leptons and six quarks are known to exist in nature.) What fixes the values of physical parameters--the masses, charges, and interaction strengths of elementary particles? What mechanism gives weak gauge bosons their large masses? Do neutrinos have mass? What is the fundamental origin of CP or T violation? What do we expect from the next generation of particle accelerators? Can particle physics shed light on the physics of the early universe? Some of these problems are currently being pursued with different emphasis by individual members of the theory group.

In addition, several members of the condensed matter theory group work on problems having substantial overlap with particle physics, including the study of lattice gauge theories and patterns of dynamical symmetry breaking. There is also substantial interchange of ideas with the theoretical astrophysics group.