Particle Accelerators

Soon after the discovery of muons and pions in cosmic rays, a whole plethora of unstable particles was uncovered. Central to these discoveries was the particle accelerator. In these devices, charged particles, typically electrons or protons, are accelerated to high energy and then smashed into a target. Detectors of various sorts are used to examine the particles created by the collisions of the accelerated particles and the atomic nuclei with which they collide. Sometimes an elastic collision occurs, in which the accelerated particle simply ``bounces off'' of the target particle, transferring a good bit of its momentum to this particle. However, under many circumstances the collision results in the production of new particles which didn't exist before the collision. This is referred to as an inelastic collision.

The simplest type of target is liquid hydrogen since the nucleus consists of a single proton. The orbital electrons of the target atoms are so light that they are generally just ``brushed aside'' without greatly affecting the trajectories of the accelerated particles. However, a variety of targets are used under different circumstances.