What is a lepton?

Answer 1

Leptons are (as far as we know) fundamental particles, meaning that as far as we know, there are no particles that make up leptons themselves. (Leptons are not the only fundamental particles; they are just a class of fundamental particles.) They have no strong interactions, meaning that they cannot experience the strong force (the strong force is the force that holds the nucleus of an atom together, and the force that holds the quarks inside of protons and neutrons).

There are six known leptons: electrons, electron neutrinos, muons, muon neutrinos, taus, and tau neutrinos.

Electrons are probably the most familiar leptons. You may have learned about electrons "orbiting" the nucleus of an atom. ("Orbiting" is in quotes because that is only a rough approximation of what electrons do. In realty, electrons act more like waves oscillating, or vibrating, in an atom than like particles orbiting the nucleus of an atom.)

Muons and taus are heavier than electrons and are also unstable. Muons and taus decay within a tiny fraction of a second, usually into a combination of electrons and neutrinos.

Neutrinos have zero electric charge and very little mass. Since neutrinos have no electric charge, they are only affected by the weak force and the force of gravity. This means that neutrinos don't interact with other particles very much. In fact, there are thousands of neutrinos passing through you (without interacting with the particles in you) right now!

As for now, scientists have determined that leptons have no internal structure or even size. (That doesn't make much sense; how can something not have size? But remember leptons aren't made of anything as far as we know. It wound't make sense for something not made of anything to have size... Quantum mechanics and particle physics can get confusing.) It is possible that future research may prove this to be false, showing that there is substructure within the particles, but for now leptons are considered to be fundamental particles.

A current theory that would explain what leptons (and everything else) is made of is string theory. This theory basically says that everything is made up of "strings" of energy. (This still doesn't really help in determining the size of a lepton. If a leptons is made up of energy, how much space does that energy take up? But does energy even take up space...?)