Leptons are a family of elemental particles that includes the electron, the muon, the tau, and their associated neutrinos. Like quarks, which make up the protons and neutrons in the atomic nucleus, the lepton is a fermion, meaning it has a quantum spin of ½. Other fundamental particles with different spins are called bosons and mediate force interactions between the fermions. Fermions, made up of the lepton and quark, are the “meat” of matter, while bosons are the “skeleton”.
In comparison to quarks, a lepton is light. An electron’s mass is 1⁄1836 of an atomic mass unit (amu), the approximate mass of a hydrogen atom. The non-lepton mass in the hydrogen atom comes from the nucleus. The other two lepton variants – the muon and the tau – are significantly heavier. The tau lepton weighs almost twice as much as a proton.
There are three types of lepton, as mentioned before: the electron, the muon, and the tau. Each of these has an associated neutrino – the electron neutrino, the muon neutrino, and the tau neutrino. Neutrinos have almost zero mass and no charge, and travel close to the speed of light.
The electron and the other base leptons have a negative charge, are more stable than neutrinos, and can orbit atomic nuclei. The electron is the most stable of the three and can be found throughout conventional matter. The muon and tau are created during particle accelerator experiments or through cosmic ray impacts. Each of these six particles has an associated antiparticle of opposite charge, bringing the total number of leptons to 12. These antiparticles are called antielectrons, antimuons, and antitaus.
When systems containing leptons interact with each other, the total number of leptons tends to be conserved. The rare circumstance in which the number isn’t conserved is called the chiral anomaly. Sometimes, leptons change their type, but this usually requires high energies and the heavier leptons aren’t very stable.