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As far as we know, all matter in the universe is composed of two kinds of particles, the hadrons (from which the LHC – Large Hadron Collider – gets its name) and leptons. Hadrons are not fundamental particles, in the sense that they are known to be composed other particles (quarks, and anti-quarks); leptons, on the other hand, are fundamental.
We are very familiar with one (of six) leptons, the electron, whose existence as an independent entity was discovered by J. J. Thompson, in 1897. The next lepton to be discovered – the positron – is actually the electron’s antimatter counterpart (it was discovered by Carl Anderson, in 1932 … but predicted by the brilliant Paul Dirac four years earlier). Not long afterwards (1937) a member of next lepton flavor was discovered, the muon. Back then, cosmic rays were the particle colliders used to do high energy particle physics (both the positron and muon were discovered in cloud chambers; today these experiments are repeated in many a high school science class). The third, and last, lepton flavor, the tau, was discovered in 1975.
Muons (and taus) behave just like electrons, only more massive. Except they decay; the muon rather slowly (2.2 µsecs!), into an electron (and …), the tau much more quickly (290 femtoseconds!!), and in many different ways (because it’s more massive than many hadrons, several hadron/anti-hadron decay modes are possible).
Actually, the electron anti-neutrino was discovered, sorta, before the positron … conservation of energy applied to beta decays (radioactive decays in which beta particles – electrons or positrons – are emitted) lead Wolfgang Pauli (in 1930) to postulate the existence of a neutral particle … but it took over a quarter of a century for the neutrino’s existence to be directly observed (in 1957, by Cowan and Reines; the word ‘neutrino’ is often used to mean both the matter particle and its antimatter counterpart).
Completing the discovery histories: 1962 (the muon neutrino), 2000 (the tau neutrino), … 1998 (neutrino flavor oscillation).
Leptons ‘feel’ the weak (nuclear) force (they ‘participate in weak interactions’), and the charged leptons (electron, muon, tau, and their antimatter counterparts) the electromagnetic force; none participate in strong interactions.
So what’s this neutrino flavor oscillation? While each of the three flavors of neutrino are different, and distinct (per their interactions with other particles), they are not immutable … a neutrino can change its flavor (from electron to muon, say, or tau to muon; of course a tau can, and does, decay to a muon … but it can’t oscillate back to being a muon!). This discovery both closed a chapter on solar physics (the ‘solar neutrino problem’ was solved, no new astrophysics required) and opened a new one in particle physics (the Standard Model is incomplete; neutrinos have mass).
Looking for more? SLAC’s Virtual Visitor Center has a good, if brief, introduction to Leptons; at the opposite end of the scale is the PDG (Particle Data Group) Review of Particle Physics (Leptons in the 2006 Review).
Leptons get a mention in several Universe Today articles, including Book Review: The Mystery of the Missing Antimatter (great book by the way, I highly recommend it), Astronomers on Supernova High Alert, and First Collisions of the LHC.
Leptons make an appearance – as indeed they must – in the Astronomy Cast episode The Search for the Theory of Everything.