Why Would a Fourth Type of Neutrino Increase Annihilation Rates?

[JZR]

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?

 

[betel]

Basically you would have more possible channels in which your electron and positron can decay to. Providing more options for a decay will increase the rate.

 

[phyzguy]

Here's a nice explanation, with a graph:

http://backreaction.blogspot.com/2007/12/three-neutrino-families.html