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Positron - electron annihilation.

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Sep29-10, 03:48 PM
P: 4
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?
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Sep30-10, 05:12 AM
P: 319
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.
Sep30-10, 07:14 AM
P: 2,195
Here's a nice explanation, with a graph:

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