The leptons are a group of six elementary particles and their six antiparticle counterparts that do not interact with the strong force. They do, however, feel the weak force and the gravitational force, and electrically charged leptons interact by the electromagnetic force. The most familiar lepton is the electron. The electron is the most stable particle in the universe and it does not decay into other particles.

Leptons can either carry one unit of electric charge or be neutral. The charged leptons are the electrons, the muons, and the taus. Each of these types has a negative charge and a distinct mass. Electrons are the lightest charged leptons with a mass less than 1/2,000th that of the proton.

Muons are more than 200 times as massive as electrons and taus are about 3,700 times as massive as electrons. Each charged lepton has an associated neutral partner, called a neutrino--the electron-neutrino...