They are tiny bits of matter assumed to be the most basic constituents of the universe. Certain elementary particles combine to form an atom, which is the basic unit of any chemical element and from which all forms of matter are built.

The first elementary particle to be discovered was the Electron, identified in 1897 by Joseph John Thomson. The nucleus of ordinary hydrogen was subsequently recognized as a single particle and was named the Proton. The third basic particle in an atom, the Neutron, was discovered in 1932.

Although the models of the atom consisting of just these three particles are sufficient to account for all forms of chemical behaviour of matter, Quantum theory predicted the existence of additional elementary particles.

A search for the positron, or antiparticle (Antimatter) of the electron, led to its detection in 1932, but a search for a particle predicted by Yukawa Hideki in 1935 led to the unexpected discovery of the mu meson or muon, the following year.