When nucleons come closer than about 1 fm (femtometer), the force becomes strongly repulsive. This ``repulsive core'' is partially responsible for the saturation of nuclear forces (in other words, the incompressibility of nuclear matter.) It is also of fundamental interest since no good theoretical description of this force exists. The force between two nucleons separated by more than 1 fm can be well understood as the exchange of a pi meson (a bound quark-antiquark system.) When the inter-nucleon separation becomes very small, it is expected that the underlying structure of the nucleons (quarks and gluons) will begin to play a role. Thus it may be necessary to use a description based on Quantum ChromoDynamics (QCD).
Most of the experiments I do use high energy (0.5-4.0 GeV) electron beams to induce (e,e'p) nuclear reactions. Electrons make a good ``probe'' of the nucleus since the interaction is fairly weak, and they can therefore easily penetrate deep into the nucleus. There the nuclear density is large and the short-range effects are expected to be greatest. The research is carried out at several electron-beam facilities. Most of the work is done at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. At Jefferson Lab, I am also participating in an experiment on Compton scattering from the proton, and I am a member of the core working group on charm production at the proposed upgrade of Jefferson Lab.There is a long-term project at the Mainzer Mikroton at the University of Mainz in Germany in which I am quite involved. I also collaborate with researchers at NIKHEF in Amsterdam and at the Bates Laboratory of M.I.T., and short projects may be carried out at these laboratories if the opportunity arises.
Students working on these experiments are exposed to an impressive variety of scale, which is one of the reasons that Ph.D. nuclear physicists tend to do so well in the industrial sector. The physics being studied is of extremely small scale, using beams of the lightest known particle (the electron) and studying processes that involve distances smaller than ten femtometers. On the other hand, at Jefferson Lab the electron beams are produced by an accelerator which is 1400 m in circumference. This electron beam is the most powerful in the world; at 4 GeV energy per particle and 100 microamp beam current, the beam has a power of 400 kilowatt. The beams and reaction products have such high momenta that superconducting magnets (operating at liquid helium temperature, 4 K) are needed to control their trajectories. Liquid deuterium is often used as a reaction target, and a cooling power of several hundred watts (at cryogenic temperatures) is used to keep the liquid from boiling due to the intense beam.
The spectrometers in Hall A at TJNAF are about 20 m tall and momentum-analyze the reaction products using superconducting magnets providing precisely-shaped fields of up to 1.6 Tesla over an area of more than 6 square meters. The superconducting coils carry currents of up to 1800 amps. The particles produced in the nuclear reactions arrive at the detectors at rates approaching one million per second, and electronics operating at typical time scales of 10 nanoseconds analyze the detector signals for ``good'' events, which are then collected and analyzed by a network of high-speed computers at a rate of about 2000 per second.
Students working with my group should expect to spend several months at one (or more) of the laboratories mentioned above. Some time would be spent collaborating on experiments with other groups, learning about the apparatus at the lab; then of course substantial time will need to be spent on-site during the execution of a thesis experiment.
Work on the UGA campus focuses on software development, detector development, and data analysis. Results of a recent simulation project have just been published. The group has three high-performance workstations running Linux, on which this and other projects are carried out. Detector development takes place in a lab in the physics building, which is equipped with a data-acquisition workstation (the same system used at Jefferson Lab) and a one-ton crane (for future large-scale projects.)