ABSTRACT
The grouping into the three families reflects the behaviour of the elementary fermions in the four known interactions, namely the strong, the electromagnetic, the weak and the gravitational interactions. Phenomenologically, four elementary interactions are known. The four interactions are, in decreasing order of strength, the colour interaction between quarks, the electromagnetic and the weak interaction, and gravitation. Perhaps the most important and most generally valid theorem known in quantum field theory or elementary particle physics is the CPT theorem. For all particles, CPT invariance guarantees the existence of an antiparticle with the same half-life, mass and spin but with inverse additive quantum numbers. The parity transformation changes the handedness of a particle, and thus produces a right-handed neutrino from a left-handed one. The charge conjugation operation effects the transition to an antiparticle state; however care is needed if the state of motion of the particle has a role to play, as with the neutrino.
