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

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


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.

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.)


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.


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.

Non-standard approaches to string theory