the six leptons
/ LEP-tonz /
Leptons are the matter particles that, unlike quarks, can roam freely on their own. The most familiar by far is the electron — the lightweight particle whose dance around atoms makes all of chemistry, electricity, and light from a bulb. Leptons feel electromagnetism and the weak force, but, crucially, they are deaf to the strong force, which is why they are not trapped inside protons the way quarks are.
There are six, again in three generations. Three are charged: the electron, the muon, and the tau, each carrying charge -1 and each heavier than the last (the tau is roughly 3,500 times heavier than the electron). The other three are neutrinos — the electron neutrino, muon neutrino, and tau neutrino — which carry no electric charge, almost no mass, and barely interact with anything. Trillions of neutrinos from the Sun pass through your thumbnail every second, and essentially all of them sail straight through the entire Earth. The muon and tau are like heavier electrons that decay in fractions of a second; only the electron is stable, which is why ordinary atoms are built from electrons and not their heavier cousins.
Leptons matter because the electron underpins all of chemistry and electronics, while neutrinos are messengers from places light cannot escape — the Sun's core, supernovae, the early universe. The discovery that neutrinos have a tiny mass and can change flavor as they travel was the first solid crack in the original Standard Model, which had assumed they were exactly massless.
About 100 trillion neutrinos from the Sun pass through your body every second, yet over your whole lifetime perhaps only a handful will ever bump into one of your atoms.
Neutrinos are matter particles that almost never interact.
The original Standard Model treated neutrinos as massless; experiments later showed they have tiny masses, and where those masses come from is still unknown — a clear sign the model is not the full story.