I.

Introduction

Elementary Particles, originally units of matter believed or provisionally assumed to be fundamental; now, subatomic particles in general. Elementary-particle physics—the study of elementary particles and their interactions—is also called high-energy physics, because the energy involved in probing extremely small distances is very high, as the uncertainty principle dictates. The term “elementary particle” was originally ascribed to these constituents of matter because they were thought to be indivisible. Most of them are now known to be highly complex, but the name “elementary particle” is still applied to them.