collision-energy increase is a factor of four if two balls of the same speed collide head-on as opposed to one of them being at rest. However, because of relativity and the conservation of momentum, the effective energy of the collision of particles aimed at each other is far greater. At the Fermilab Tevatron, collisions have been made between 900-GeV protons and 900-GeV antiprotons. Achieving the same collision energy with a stationary target would require an accelerator with a circumference about 2,000 times as large as Fermilab's, or about 8,000 miles!
There are different types of colliders operating today that are important for particle physics. The relatively large mass of protons and antiprotons makes it more efficient to accelerate them to high energies than to accelerate electrons or positrons. However, in the collisions, it is their constituents—quarks and gluons—that interact. Since many subatomic constituents make up the proton and antiproton, no single one carries the full energy of the accelerated particle. For example, in proton-proton collisions, the effective collision energy is about a factor of 10 lower than the full energy of the beam. By increasing the intensity of the beams, it is sometimes possible to study higher-energy processes.
In collisions between electrons and positrons, the energy of collision is the full energy to which the particles are raised (since electrons and positrons appear to have no substructure). Such collisions cleanly probe the electromagnetic and weak interactions: They do not create the extraneous debris characteristic of proton collisions and are easier to interpret.
Finally, because an electron lacks substructure and behaves in a point-like way, it is a useful probe for exploring the structure of the proton. An electron-proton collider provides information about the structure of a proton that is not available from a proton collider and provides an opportunity to search for hypothesized objects that combine both quark and lepton characteristics.
The variety of phenomena that particle physicists have uncovered and are studying has been surveyed in this chapter. Although these phenomena occur at the smallest distance scales, what is observed has relevance in understanding the physics of forces that govern the atom, the energetic processes in cores of stars, and even the structure of the universe.
The collisions of high-energy particles have been shown to reveal new and important structure. These collisions re-create the conditions of the universe just after its birth. The laws that are discovered have existed for all time and everywhere in the universe. A small number of forces have been discovered, all of which could arise from a single one. The particles on which these forces act have a mysterious structure; the lighter ones make up our everyday world, whereas the role of the next two generations still represents a major puzzle.
The following chapter presents the theoretical framework in which these phenomena are currently understood.