Why do we need more fermion generations in particle physics?

[zankaon]

Old quantum adage: Anything not forbidden, will occur. So why not more fermion generations - on the cheap, in regards to energy? Look for higher mass charged leptons. Tau has mass of ~1.7 GEV. A number of accelerators could search for such charged leptons.

 

[malawi_glenn]

i) we don't know if it is forbidden or not, we don't know what or even if there is anything that governs the number of fermion generations. We don't even know it is quantum or not.

ii) don't you think that one has searched for those? You really think that particle physicists are that naive? btw one has strong believes that there should not be more than 3, by inspecting the Z-boson decay width (we deduce that there are only 3 neutrino families with mass less than 45GeV)

 

[hamster143]

New charged heavy leptons with masses below 100 GeV have been ruled out experimentally.

As malawi_glenn correctly says, we have good evidence that there are exactly 3 weakly-interacting neutrinos with masses below 45 GeV. Known neutrinos all have masses in sub-eV range. Why would there be a fourth that is ten orders of magnitude heavier than the others?

 

[malawi_glenn]

The question has nothing to do with QM, that is my strongest point.

 

[zankaon]

Old Jul9-08, 08:31 PM #4
Vanadium 50

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Re: Why three generations of particles
Originally Posted by alphanzo View Post

For instance, the width of the Z boson measurement is sensitive to the number of generations, and experiments agree with only three generations (a fourth generation would increase the width more than is observed--see Peskin and Schroeder's "Intro to QFT").

Strictly speaking, the Z boson width is sensitive to the number of light neutrinos. A 4th generation with a neutrino weighing more than 45 GeV is a logical possibility. (There are other difficulties, but this particular one can be evaded)


Old Jul12-08, 10:56 PM #9
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Re: Why three generations of particles
Originally Posted by wlln View Post

There are no reasons that why only three-generation fernions exit. But if more element feimions exist, they will lead to inconsistency with the konwn experimental results and severe ultra-divergence.

Not true. A heavy and almost degenerate 4th generation is compatible with the existing data. Granted, the fit is better with 3 generations than 4, but 4 can be made to work.

 

[humanino]

Strictly speaking, the Z boson width is sensitive to the number of light neutrinos. A 4th generation with a neutrino weighing more than 45 GeV is a logical possibility

Are you suggesting to add only neutrinos ? How do you deal with anomalies ?

Not true. A heavy and almost degenerate 4th generation is compatible with the existing data. Granted, the fit is better with 3 generations than 4, but 4 can be made to work.

It's a also compatible with the green spaghetti monster.

 

[Vanadium 50]

Old quantum adage: Anything not forbidden, will occur. So why not more fermion generations - on the cheap, in regards to energy? Look for higher mass charged leptons. Tau has mass of ~1.7 GEV. A number of accelerators could search for such charged leptons.

First, your "old quantum adage" is nonsense. A particle just like an electron but weighing half as much is possible. But it doesn't exist.

Second, I would like to reamplify malawi_glenn's comments: "You really think that particle physicists are that naive?" although I would have said "You really think that particle physicists are that stupid?" You are implying gross incompetence on their part.

If they are that stupid, how did they discover the tau?

Third, I don't particularly appreciate being quoted in a way that makes it difficult to see what I actually said and what I was replying to, and also in a way that removes it from any context. I'm somewhat puzzled why you chose to do it this way, as you clearly have done it properly in the past.

Now that that's out of the way, a fourth generation is not excluded by the data. It can't be - one can always make a 4 generation model appear arbitrarily close to a 3 generation model. However, it is highly constrained, and the only models that are not excluded must look very, very similar to one where they are not present at all.

Finally, if you just type "fourth generation lepton searches" into Google, you get almost 14000 hits.

 

[kashiark]

we don't know why there are only three generations, but we at least have a little evidence for it: in the solar neutrino experiment, scientists did all the math and predicted a number of solar neutrinos that they should detect but they only found one third of the amount that they expected. their explanation for this is that neutrino flavor periodically propagates which means that neutrinos switch from one flavor to another. the fact that they found a third of the amount that they calculated is evidence that they are exactly three generations

 

[Vanadium 50]

we don't know why there are only three generations, but we at least have a little evidence for it: in the solar neutrino experiment, scientists did all the math and predicted a number of solar neutrinos that they should detect but they only found one third of the amount that they expected. their explanation for this is that neutrino flavor periodically propagates which means that neutrinos switch from one flavor to another. the fact that they found a third of the amount that they calculated is evidence that they are exactly three generations

That has nothing whatever to do with the number of neutrinos. Malawi_Glenn's answer is much better.

 

[DoubleHelix]

Sorry for resurrecting this old thread, but going back to something that was mentioned earlier (much much earlier), why is the resonance width of the neutral Z boson sensitive to the number of light neutrinos? I've been looking for the answer to this for ages!

 

[DoubleHelix]

Wait I think I understand this thing now, the neutral Z can decay into a neutrino anti-neutrino pair so it will contribute to the overall rate of the neutral Z interactions meaning that it must increase the resonance width. So if you measure the total width and then the partial width of each contribution and you can calculate the neutrino resonance width, the resonance width left over once all other contributions have been subtracted from the total should be 3 times this theoretical width?

 

[kloptok]

Yes exactly, the total width is the sum of the partial widths, which com from the decays: charged lepton pairs (e, mu and tau), quark-antiquark pairs (all flavours multiplied by number of colours) and lastly the number of (light) neutrino-antineutrino pairs. When summing the data strongly indicates three light neutrino generation. Strictly, a fourth generation is according to above comments not entirely excluded, but personally I think it sure seems unlikely with a fourth lepton family.