So far, we've discovered this compositeness hierarchy: Atoms - bound states of electrons, nuclei, photons Nuclei - bound states of nucleons and other hadrons Hadrons - bound states of quarks and gluons So are any Standard-Model particles bound states of any other particles? The compositeness searches described in the Particle Data Group's pages yield lower limits of a few TeV for 4-elementary-fermion interactions and lower limits from 100 GeV to a few TeV for excited leptons. More generally, work like [hep-ex/0001023] Standard Model Physics at LEP describes very good agreement up to about 100 GeV per particle for electron-positron collisions. This is about 200,000 times an electron's rest mass, and if electrons are bound states, one must somehow get cancellation at least as good as that. That is rather difficult to picture. For atoms, one gets relative binding energies around 10^(-8) - 10^(-5), for nuclei, around 0.01, and for hadrons, around 1. So for an electron to be a bound state, its relative binding energy must be at least 200,000. Has any model builder come up with a way for that to happen?