What are Leptons?

Like quarks, there are six types of leptons, and again, in three pairs. Electron - neutrino, muon - neutrino, and tau - neutrino (these three neutrino's are different from each other). The electron, muon, and tau each carry a negative charge, whereas the three neutrinos carry no charge. Leptons, unlike quarks, exist by themselves, and, like all particles, have a corresponding antiparticle.

	Mass (GeV/c²)	Electric Charge (e)
electron neutrino	<7 x 10^-9	0
electron	0.000511	-1
muon neutrino	<0.0003	0
muon (mu-minus)	0.106	-1
tau neutrino	<0.03	0
tau (tau-minus)	1.7771	-1

As the chart indicates, the tau and muon are much heavier than the electron. Furthermore, they are not found in everyday matter. This is because they decay very quickly, usually into lighter leptons. There are a couple of rules that govern the decay of leptons.

Rule One: One decay product of a heavy lepton will always be its corresponding neutrino. The other products could be a quark and its antiquark, or a lighter lepton and its antineutrino.

Rule Two: The total number of family members before and after must be conserved (realizing that an antiparticle is considered a negative family member). For example, if a tau particle decays into a lighter lepton, the tau's corresponding neutrino will be a product of the decay, keeping a total of 1 family member before and after the decay. The other products could be a lighter lepton and its corresponding ANTIneutrino, keeping a total of zero members before and after for that family of leptons. This is because:

Let's now go on to the next topic, the Forces Of Nature.