In the standard model, there are three families of matter particles, headed up by the electron, tau and muon, and their associated neutrinos, and quarks. Is there any reason to suspect the existence of a 4th or even 5th+ series of matter particles? (which, if they existed, would probably be nearly impossible to produce.) In a similar vein, does anyone know if the tau/tau neutrino/top/bottom quark family predicted to exist, or was one of them found by "accident" first? Finally, I don't quite understand the technical details, but it appears that neutrinos may actually be an oscillating superposition of electron, tau and muon neutrinos. Is it possible that something similar could happen with the electron, tau, muon, or is the neutrino oscillation specific to how it interacts only via the weak force?