With the discovery or particle accelerators it was found that the higher the energy of the colliding particles:

* The greater the variety of new particles produced

* The greater the mass of the particles

Until the 1960's there was no apparent relationship found between these new particles or any way of categorising them, so they became known as the 'particle zoo'

Classification of the Particles - Leptons and Hadrons

Eventually physicists began to see some order in the particles. They classified them according to whether they felt the strong force or not.

So two families of particles emerged: