the lepton
/ LEP-ton /
Not everything that makes up matter feels the crushing grip of the strong force. The electrons that carry electricity, the ghostly neutrinos that stream through your body by the trillions each second, and their heavier cousins the muon and the tau are all leptons, fundamental fermions that stand entirely apart from the quark's colored world. If quarks are the bricks locked forever inside atomic nuclei, leptons are the free agents: an electron can be knocked clean out of an atom, a neutrino can cross a light-year of lead.
A lepton is a spin-1/2 fermion that carries no color charge, so it does not participate in the strong interaction at all. There are six leptons in three generations: the charged leptons, electron (e), muon (mu) and tau, each with charge -1 e and progressively larger mass, and their three neutral partners the electron, muon and tau neutrinos. Charged leptons feel both electromagnetism and the weak force; neutrinos feel only the weak force and gravity. Each generation carries a lepton-flavor number that is conserved in nearly every process, which is why a muon decays to an electron plus a muon-neutrino and an electron-antineutrino, balancing the books.
The muon was such a surprise when found in cosmic rays in 1936 that the physicist I. I. Rabi is said to have asked 'who ordered that?', a heavy copy of the electron with no obvious purpose. Leptons are where the Standard Model is cleanest: unlike quarks they appear as free particles, so the electron's magnetic moment can be measured and predicted to twelve decimal places, the most precise agreement between theory and experiment in all of science. A caveat worth stating: neutrino oscillation shows that lepton flavor is not perfectly conserved, the first laboratory crack in the original Standard Model.
A cosmic-ray muon reaching the ground is a lepton created miles up in the atmosphere; it survives the trip only because relativistic time dilation stretches its 2.2-microsecond lifetime enough for it to travel the distance.
Muons are heavy, unstable leptons whose survival to sea level is a textbook demonstration of special relativity.
Do not picture the neutrino as merely a 'small' particle; what makes it elusive is not its size but that it interacts only weakly, so it passes through matter that would instantly stop an electron.