As the lightest hadron, and consisting of only one up and one down quark, the pion is in a sense the "Hydrogen atom" of light-quark QCD. That is to say, it is the strongly interaction particle which is most amenable to calculations of its structure. We study its structure by measuring the cross section and then relating this to the form factor. The electromagnetic form factor of a particle describes the probability of finding the charged constituents (ie, the quarks but not the neutral gluons or photons) all within a small space-time interval. The size of the space-time interval is inversely related to Q^2, the four-momentum transfer. At Q^2 = 0, the electromagnetic charge form factor of the proton, for example, is simply 1. At larger four-momentum transfer it drops to a small fraction of 1 because the probability of finding all the quarks within such a small interval is nearly vanishing. Nevertheless, these difficult measurements are interesting because they tell us about the pion when it is close to its relatively simple configuration of two quarks, a regime which is only beginning to be explored.
The fine (ie, small distance scale) structure of the pion can be described by perturbative Quantum Chromo Dynamics (pQCD) calculations plus higher order corrections. In contrast to the electromagnetic form factors of other hadrons such as the proton, for example, the normalization of the pQCD calculation for the pion form factor is well-defined. Since form factors often vary smoothly with momentum transfer, and may have the pQCD-predicted dependence on momentum transfer due to very non-pQCD mechanisms, being able to compare the absolute normalization of data and theory is very important! The coarse structure of the pion can be described by Lattice QCD calculations, but an important part of strong phenomenology is the search for viable models which (though inexact) will elucidate the simpler degrees of freedom needed for a qualitative understanding of strongly bound light-quark systems.
Improvements in both experiment and theory are responsible for the current excitement in this field. Our high quality data now probe the pion to about 1/4 of its radius (0.15 fm), and improved measurements at higher resolution are in progress. Lattice QCD calculations by several groups around the world, with better control of systematic errors than only 10 years ago, are now describing the structure of the pion at a scale significantly smaller than its radius. Improvements in higher order corrections are permitting perturbative QCD calculations to describe, not just the fine structure, but larger-scale structure as well.
Our first results up to Q^2 = 1.6 were published in J. Volmer et al., PRL 86 (2001) 1713-1716 . This is a highly cited work ( TopCite 100+ and counting).
Our second phase of measurements is now published in T. Horn et al., PRL 97 (2006) 192001-1 .
In order to continue the program to higher momentum transfer we require beam energies higher than 6 GeV plus the construction of a new, small angle, high momentum spectrometer called the Super High Momentum Spectrometer (SHMS). The SHMS is the core of the Hall C 12 GeV upgrade.