lepton
/ LEP-ton /
A lepton is one of the basic building blocks of matter that, unlike a quark, never feels the strong force. The most familiar lepton is the electron, the lightweight particle that swarms around the nucleus of every atom and carries electric current through wires. But the electron has heavier cousins and a set of nearly invisible partners called neutrinos, and together this little family is the leptons.
There are six leptons, grouped in three pairs called generations. Each pair is a charged lepton plus its matching neutrino: the electron and the electron neutrino; the muon and the muon neutrino; the tau and the tau neutrino. The three charged leptons (electron, muon, tau) all carry the same electric charge as the electron and differ mainly in mass — the muon is about 200 times heavier than the electron, and the tau heavier still. The three neutrinos carry no electric charge and are extraordinarily light and weakly interacting. Every lepton is, as far as we can tell, a true point — it has no internal parts.
Leptons matter because they are half of the matter content of the Standard Model (the other half being quarks). They feel the weak force, and the charged ones also feel electromagnetism, but none of them feel the strong force, so they never get locked up inside protons and neutrons the way quarks do. That is exactly why a free electron is perfectly ordinary while a free quark is never seen.
Switch on a torch and the current flowing through the bulb is a stream of electrons — leptons doing everyday work. Meanwhile, trillions of neutrinos from the Sun, also leptons, are passing harmlessly through your body every second.
Electrons in a wire and solar neutrinos are both leptons — same family, very different roles.
Lepton means 'light one' in Greek, a name from the days when only the electron was known; the tau is actually heavier than the proton, so the name is now a historical accident, not a description.