Question: Could there theoretically be more than three generations of normal matter (ignoring sparticles)? Why, or why not?

James M Monk answered on 14 Jun 2011:

This question was tested experimentally by the LEP experiments in the 1990s. There is a particle called the Z boson, which is a bit like a photon with mass, that can decay to all the matter particles in the Standard Model. If there were a fourth (or higher) generation of matter in the Standard Model then the Z would also have to be able to decay to those particles unless the new generation had some very different properties to the generations we already know about.

The rate of decay of a Z is determined by how many things it can decay into (and what their masses are). So by measuring the decay rate of the Z, the LEP experiments were able to determine that there were no additional generations because the Z would decay more quickly if there were.

The caveat to this is if there were a fourth generation in which the neutrinos were more than half of the mass of the Z (which is very heavy at 91 proton masses), then the decay could not happen, so you could not test for such a set of particles in that way. Neutrinos with a mass of over 45 GeV would be pretty weird, since the masses of the neutrinos we know about are tiny! Additionally, it cannot rule out the possibility that the neutrinos in the fourth generation do not interact with the Z, but such a new generation would not be a “Standard Model” generation.