Theoretical high energy physics

This is a summary of the topics of recent research. "Theoretical high energy physics" here means

the attempt to understand the fundamental constituents of matter and energy

We will not discuss here somewhat related areas of theoretical physics, such as

• astrophysics/cosmology
• statistical mechanics/condensed matter theory
• nuclear physics

Of course, the borders are not sharp, but here is a rough classification:

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Phenomenology

This is what used to be called "theory" yesterday (so some will resent it being called phenomenology today), and will be called "pedagogy" tomorrow. It now changes little in its fundamentals. Phenomenology in the strictest sense is often little more than extending old calculations or evaluating data or comparing experiment to theory. More generally it involves finding new applications of old theories, new methods to analyze them, or modifications that do not radically alter their basic premises. Sometimes it is also called "particle theory" to distinguish it from string theory.

Color physics or QCD (Quantum ChromoDynamics)

studies the strong interactions responsible for holding together the protons and neutrons that make up the atomic nucleus, and also the quarks (and gluons) that make up the protons and neutrons.

• Perturbative QCD studies situations, namely high energies, where the strong coupling is relatively weak, so it can be approximated as a small modification to the free theory.
• Lattice QCD studies situations where the strong coupling is relatively strong. The method of calculation is to approximate space and time as a finite number of discrete points, so numerical approximations can be made on computers.

Flavor physics

studies the electroweak interactions responsible for other particle properties, including the electromagnetic force. (Sometimes strong interactions are included, but in a minimal way.)

• QED (Quantum ElectroDynamics) focuses on electromagnetism. This theory makes the most accurate predictions (better than a part per million) in any scientific theory, period. At this point it is little more than extending approximations to the next order.
• Extensions to the Standard Model are sometimes classified as model building, intermediate between hard-core phenomenology and hard-core theory. The Standard Model is particle physics as we know it today, almost all its basic assumptions confirmed experimentally. But it could stand some improvement. Some current topics are unification (supersymmetry or Grand Unified Theories) and the possibility of masses for the neutrinos.
• Branes are (the latest version of) the application of string theory to model building, and so sometimes considered part of string theory. This method applies properties of higher dimensions; it uses a minimal set of results of string theory, or builds string-inspired models.

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Theory

This is sometimes called "formal theory" by phenomenologists; but some theorists distinguish phenomenology as a separate class between experiment and theory. It is often called "string theory", since almost all of it is, or is closely related, or would like to be. Strings are generalizations of point particles to the simplest extended objects. They have some interesting properties beyond what is normally found in particle theories: The most important one is that they provide a consistent theory for the interactions of particles of all possible types. Nowadays "string theory" refers to theories that include gravity, in addition to the strong and electroweak forces of the Standard Model.

Compactification

is an approach to strings that begins by choosing a "vacuum", or "ground state" (state of lowest energy) different from the obvious one: String theory is naively a theory in higher dimensions, so the higher dimensions need to be eliminated. Then low energy modifications to that vacuum are studied.

• The AdS/CFT correspondence (Anti de Sitter/Conformal Field Theory) tries to relate string theory near a particular vacuum to a particular particle theory with a large symmetry, in an effort to gain insight into both. It involves the concept of holography, which tries to describe physical spacetime as the boundary of a larger space, and is related to (or a special case of) branes.
• M theory generalizes strings to membranes, and also adds one dimension to the 10 dimensions usually associated with (super)stings; F theory is a vague proposal to add yet another dimension. No formulations of these theories have been found that are powerful enough for general calculations of the kind possible in string theory, but general arguments indicate they are just different formulations of string theory that expose some of its otherwise hidden features. (All formulations of string theory are thought to be related by different forms of duality.) Presently the most practical application of these theories is to imply new compactifications, much more general than those apparent in the usual string theory formulations.

Non-standard approaches to string theory

• The Green-Schwarz approach to superstring theory is a modification of the usual approach that is less understood but potentially more powerful. It tries to incorporate more of the symmetry (specifically, the supersymmetry) into the formalism in a more direct way.
• String field theory is an approach to describing both strong and weak coupling within the same formalism. It is basically the same approach used in particle physics. Most recent research in this area focuses on the nature of the uncompactified vacuum in string theory.
• Noncommutative field theory is a particle theory inspired by string theory. It uses a string vacuum different from the usual one (but not compactified), one which violates rotational invariance. An effect is that low and high energies become related in a way reminiscent of string theory. This theory is not realistic, but provides a simpler model for studying some features of string theory.
• In the random lattice approach, a worldsheet that describes the string is approximated by an irregular 2-dimensional lattice, whose vertices and links are treated as those of the Feynman diagrams of particles that make up the string. In particular, a QCD string would be one that describes hadrons, made up of the quarks and gluons of QCD.