Quarks are difficult to study, because the force that binds quarks together (through the exchange of gluons) is so strong that we observe them only when they are combined into larger, composite particles, such as protons and neutrons. So if we wanted to measure the size of these smallest building blocks of matter, how would we do it?
Ernest Rutherford first discovered a potential probe in 1909 when he "scattered" particles from gold. Rutherford and his students, Hans Geiger and Ernest Marsden, sent a beam of alpha particles into a thin sheet of gold foil. They knew the projectile, an alpha particle (now known to be a helium nucleus with two protons and two neutrons), was passing close to an atom in the gold when its path was altered. This experiment demonstrated that the mass of the atom was concentrated in a space much smaller than the atom itself. In this way, it revealed that the atom had structure: it was built of even smaller particles.
Physicists use this same technique, called "scattering," to learn about protons, neutrons and the quarks and gluons inside. Here at Jefferson Lab, the Continuous Electron Beam Accelerator Facility (CEBAF) propels electrons into targets in each of the three experimental halls. Some of the electrons may be deflected as they pass close to the nuclei of the atoms in the target. The electrons may even knock some particles out of nuclei or create new particles.
The resulting interactions are observed and recorded by huge detectors — house-sized arrays of electronics that measure the speed, direction and energy of these particles. By observing the scattered particles, physicists are learning about how the nucleus is put together.
Physicists are also exploring matter by improving theoretical models. Whether it's using new or existing theories to predict what an experiment is likely to find or a computer-aided theoretical calculation of the interactions of quarks and gluons inside matter, theories are integral in helping scientists paint better pictures of the building blocks of matter.