TABLE OF CONTENTS After the discovery of the neutron in 1932, it was thought that the electron, the proton, and the neutron were the fundamental constituents of all matter. Subsequent experiments, involving cosmic rays as well as accelerator beams, revealed that there was a host of other particles that could be regarded as equally fundamental. We already mentioned the families of leptons and some of their properties in connection with nuclear ?-decay. In addition, we also know of the existence of hadrons such as ?-mesons, K mesons, ? mesons, hyperons, and their many excited states. All these can be referred to collectively as elementary particles. Usually, an elementary particle is thought to be an object without any substructure, namely a point particle. However structure can be probed only up to any given scale that is limited by the available energy. Consequently, our definition of what is elementary or fundamental is always tentative, and must rely on experimental verification at ever higher energies. For example, to examine the structure of matter at length scales of ? r 
0.1 fm, requires transverse-momentum transfers ( ? p T) at least of the order
| (9.1) |  |
In other words, to be sensitive to small length scales, the energy of the particles used as probes must be very high. Because of this need, the study of elementary particles has also come to be known as high-energy physics.
Whenever a higher-energy accelerator starts operating, we can probe deeper into the structure of...
