Though they are typically not directly involved in the planning, fabrication, or operations of experiments, theoretical physicists play key roles in determining what kinds of experiments would likely be the most interesting. They also explain experimental results in terms of a fundamental underlying theory that describes all of the components and interactions of matter, energy and space-time. Our understanding of the universe relies on the active, integrated participation of theorists in interpreting the results of particle physics experiments.
Theorists employ a range of methods and tools—including new mathematical techniques as well as the construction and use of powerful computational facilities—to extract precise predictions about the Standard Model and beyond. They push the boundaries of knowledge by constructing models to predict new physical phenomena and developing the means to experimentally search for them. In doing so, they identify when new physical principles are needed and what other consequences may be. New principles and ideas, such as supersymmetry and the possibility of extra dimensions, have helped guide the search for new physics at such experiments as the Large Hadron Collider at CERN in Geneva, Switzerland.
Theorists have been busy identifying the most promising and sensitive methods for finding signs of new phenomena in the mountains of data that the LHC has and will produce. Many attractive ideas have been proposed for the solution of fundamental problems, such as the origin of mass and the mechanisms for breaking symmetries in nature. Theorists anxiously anticipate analyzing data from the LHC to identify which of these many ideas are true.
Lattice Quantum Chromodynamics (QCD) is one very successful theory. It describes strong interactions between the quarks and gluons that make up the protons and neutrons in the nuclei of atoms. Although the equations that define the theory are in principle exact, none of the methods that have been successful in other theoretical areas are adequate. Those techniques rely on the assumption of relatively weak interactions among quarks and gluons, which fail at distance scales larger than a fraction of the width of a human hair.
At very powerful proton colliders like the LHC, scientists understand how strong interactions will affect the physics of the quark and gluon. They cannot, however, easily predict what will happen as the byproducts of the collision move away from each other and lose energy. A better understanding of the strong interactions would go a long way towards helping physicists hunt for hints of new physics, leading to lattice QCD research.
Recent advancements in numerical algorithms coupled with increased computer performance have made it possible for theorists to make very precise QCD calculations. The Scientific Discovery through Advanced Computation (SciDAC) program provided some of the computational tools for this effort, in partnership with the Office of Advanced Scientific Computation Research and the Office of Nuclear Physics.
Theoretical physicists continue to develop ideas that push the exploration of the quantum universe in new directions. Motivated by the effort to unify Einstein’s theory of gravity with quantum mechanics, the LHC will make it possible to test some of these ideas. Perhaps they can shed light on dark matter or dark energy, or even suggest new cosmologies to explain the history and evolution of the universe.