As you would expect, fixed target accelerators shoot fast moving particles at stationary ones. This is useful as you can select whatever target you like - only a few different types of particles can practically be accelerated. Typically, the target constitutes a piece of metal or a gas-filled tank. However, far higher energies can be generated in colliding beam accelerators than in fixed target ones. If the collisions are repeated many enough times, any particle will be produced (if only for a very short time) that we could wish to study. The only fundamentally limiting factor is that the mass of the new particle must equate to an energy lower than the combined energy of the colliding particles (remember Einstein’s E = mc2). Hence, colliding beam accelerators are the best means we have to produce exotic, massive particles such as the top quark.
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