The concern of the HEP group is the origin and structure of all of the matter and forces we see around us.

The bedrock of the field is called the ``Standard Model of Fundamental Particles and Interactions'' (SM). It is the established framework into which essentially all of the data about the quantum structure of matter and force learned since around the turn of the Twentieth Century has a place.

The Standard Model (and most slight generalizations of it) is the concern of Particle Physicists.

The main conceptual and calculational tool used by particle physicists in constructing the SM and studying its implications is called ``Quantum Field Theory''  (QFT).  In particular, the class of QFT's called ``Gauge Theories'' turn out to describe Electromagnetism, the Weak Nuclear Force, and the Strong Nuclear Force.

They are very powerful and rich theories, which help us study many aspects of the physics of the SM particles.
 

A common theme is to understand the nature of the strong force by studying the consequences of the theory of  ``Quantum Chromodynamics'' (QCD).  This is problematic.  Perturbation theory is sufficient to study ``Quantum Electrodynamics'' (QED) since the coupling between photons and electrically-charged particles (e.g. electrons)  is small.

But the strong force is inherently non-perturbative as the coupling between gluons and color-charged particles (e.g. quarks, gluons)  is strong enough to confine quarks and gluons into color-neutral particles (hadrons)
which makes the connection between theory and experiment more indirect.

Non-perturbative techniques are necessary.
 

• what is the size of the nucleon as determined by various probes (e.g. electromagnetic)?
• where in the proton does its spin reside (spin of quarks, gluons, orbital angular momentum)?
• how are the light quarks and gluons distributed about the heavy quark in a B-meson (the hydrogen atom of QCD)?
• and how do they respond when the heavy quark suddenly changes upon electroweak transitions?

One of the best-motivated extensions is known as supersymmetry, a hypothetical symmetry that exchanges fermions and bosons. Supersymmetry thus posits the existence of an as-yet-unobserved ``superpartner'' boson for every fermion, and vice versa. There is some hope -- at least among proponents of supersymmetry -- that such superpartners will be observed in the next generation of particle accelerations.

Supersymmetric theories also exhibit many nice mathematical properties, and to some extent can be solved exactly.  Their exact solvability can be used to shed light on such difficult problems as quark confinement and phase transitions in gauge theories.  Group member Alfred Shapere has been engaged in finding new solutions to supersymmetric theories, and in using these solutions to understand the phase structure of QCD and its extensions.

After many years of research, our best theory of quantum gravity is described by a class of tools known as ``String Theories''. There are only five such theories, and it is believed that the list is complete. Actually, we now believe that there is a single underlying theory, called ``M-Theory'', which gives rise to each of the five known string theories in particular limits.

After further work, it is now understood that string theory (or M-theory) not only describes gravity very well, it also at the same time describes gauge theories of the type used in the SM.  It is possible to construct string theories that look very much like the Standard Model at ``low'' energies.  This observation has led to new methods for performing calculations in gauge theories and new insights into their non-perturbative structure.

We have also learned much from String Theory about the physics of Black Holes, the strange nature of space-time at ultra high energies, and (possibly) the theoretical foundations for why our world is quantum mechanical.

Two members of the group (Clifford V. Johnson and Alfred Shapere) are concerned with aspects of this program. (See links from Johnson's personal web page for more discussion of these issues.)

Link To: HEP Theory Seminars; Physics Department Colloquium