Here is an excerpt from my textbook: "The mass of a Z boson is about 90 Gev. By selecting the energy of the electron and positron beams in the LEP collider to be 45 Gev each, a high rate was achieved for the production of neutrino-antineutrino pairs in the process. The experiment shows that there are no more types of neutrino than the three already discovered. Had there been a fourth type of neutrino, the rate of electron-positron annihilation would have been higher than observed." My question is, why would the rate of annihilation be higher if there were four types of neutrino?