Nonperturbative Quantum Field Theory
Understanding the strong interactions, the interactions of quarks and gluons. In ordinary conditions, quarks and gluons can never exist as free particles, but are instead confined within composite particles such as protons and neutrons. At extremely high temperatures or densities, which existed in the early universe and may exist in the cores of neutron stars, it is predicted that quarks and gluons can be liberated, forming new states of matter called the quark-gluon plasma or quark matter.
It is only at extremely high energies that standard perturbative methods can be applied to the strong interactions. At low and intermediate energies, the theory can be defined by discretising space and time to form a lattice, and studied by computer simulations.
Confinement of quarks and gluons is still far from understood. One way of approaching this issue is by studying the properties of the quarks and gluons themselves and how they couple to each other at different energy and momentum scales. Signatures of confinement will show up in the infrared régime or quark and gluon correlators. The same forces that produce confinement also give rise to about 98% of the mass of everything we see around us.