Evidence suggests that's not the case, however. Consider the neutrinos, which also are observed to come in three generations. Cosmological evidence suggests tha

Evidence suggests that's not the case, however. Consider the neutrinos, which also are observed to come in three generations. Cosmological evidence suggests that the sum of the three masses is ~1 eV. Detector experiments suggest that the lightest neutrino must be at least 0.05 eV. Now suppose there's a fourth generation of neutrinos. Per the weak interaction, the Z boson can decay into a neutrino/antineutrino pair through the following diagram: Z -> 2 v (one of these neutrinos v would be an antiparticle). Therefore so long as the decay is kinematically permitted, ie m_Z > 2 m_v, we will eventually see the decay. But just what is the mass of the Z? Well, experiment has its value pinned ~45 GeV -- at least 10^10 times larger than the heaviest of the three lightest neutrinos. If there is a fourth generation of quarks, then the mass the mass hierarchy must be something like* v_1 : 0.00000000005 GeV v_2 : 0.00000000010 GeV v_3 : 0.00000000100 GeV v_4 : 22.50000000000 GeV And of course, v_4 could be larger! There's nothing physically preventing a fourth generation of quarks from being so massive (at least, that we know of), but to a particle physicist that would be a highly unintuitive discovery. -- * The mass splittings for the quarks is known, but not their actual values. Don't quote me on these values -- they're just for illustration!