Why people are still discussing the 4th generation models?

  • Context: Graduate 
  • Thread starter Thread starter Accidently
  • Start date Start date
  • Tags Tags
    Generation Models
Click For Summary

Discussion Overview

The discussion revolves around the theoretical and experimental considerations regarding the existence of a fourth generation of particles in the context of anomaly cancellation and its implications in particle physics. Participants explore the necessity and validity of multiple generations, particularly focusing on the fourth generation, and its relevance in current research.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Exploratory

Main Points Raised

  • One participant claims that three generations are required for anomaly cancellation, suggesting that the fourth generation appears redundant.
  • Another participant challenges this by stating that the completeness of multiplets, rather than a specific number of generations, is necessary for anomaly cancellation, indicating that multiple generations can exist without being required.
  • Some participants express uncertainty about the experimental status of the fourth generation, suggesting it may be ruled out due to the heavy mass of potential fourth generation particles and their instability, which complicates direct detection.
  • It is noted that while the fourth generation is generally ruled out in standard scenarios, there are conditions under which it could still be relevant, such as the presence of non-standard Higgs bosons affecting the constraints on the fourth generation.
  • One participant acknowledges their limited knowledge regarding non-standard Higgs scenarios, indicating a gap in understanding that could affect the discussion on the fourth generation.

Areas of Agreement / Disagreement

Participants exhibit disagreement regarding the necessity of the fourth generation and its experimental viability. While some argue it is theoretically allowed, others suggest it is almost certainly ruled out experimentally. The discussion remains unresolved with multiple competing views present.

Contextual Notes

Participants express uncertainty about the implications of non-standard Higgs bosons on the fourth generation, highlighting limitations in their understanding of how these factors influence the theoretical framework.

Accidently
Messages
37
Reaction score
0
3 generation is required for anomaly cancellation, then the 4th generation seems redundant. But there are still papers about the 4th generation, what is the reason?

Thanks
 
Physics news on Phys.org
Accidently said:
3 generation is required for anomaly cancellation ...
That's not quite true.

The completeness of the multiplet {electron, electron-neutrino, up-quark, down-quark} and the completeness of corresponding multiplets in all higher generations are required. So two generations with {electron, electron-neutrino, up, down} and {myon, myon-neutrino, strange, charm} are perfectly valid. What is ruled out due to anomaly cancellation is an incomplete multiplet, i.e. the situation we had about 20 years ago, namely {tau, tau-neutrino, *, bottom} where the * indicates the missing top.

There is no requirement for the second, third or forth generation; theoretically they are all unnecessary waste ;-) Already after the discovery of the muon Rabi asked "who ordered that?".

Accidently said:
But there are still papers about the 4th generation, what is the reason?
It's allowed theoretically, just as the 2nd and 3rd one.

I am not absolutely sure but I thought that a 4th generation is almost certainly ruled out experimentally (the 4th generation particles would be too heavy to be produced directly in colliders, they may by too instable to be detected directly, but they do contribute to loops even at lower energies therefore there are indirect indications regarding the number of generations).
 
tom.stoer said:
I am not absolutely sure but I thought that a 4th generation is almost certainly ruled out experimentally (the 4th generation particles would be too heavy to be produced directly in colliders, they may by too instable to be detected directly, but they do contribute to loops even at lower energies therefore there are indirect indications regarding the number of generations).

They are ruled out in most STANDARD scenarios, but dig a little deeper and they can come back! For example, they contribute to "oblique electroweak corrections" (the so called "S parameter" mainly), but if there is a non-standard Higgs boson, then that constraint goes out the window!
 
yes, I was referring to standard scenarios only; I can't say much about (e.g.) non-standard Higgs ...
 

Similar threads

Undergrad The standard model from loop quantum gravity via spinors octonions
  • · Replies 26 ·
Replies
26
Views
6K
Graduate Three generations of Fermions from octonions Clifford alegbras
  • · Replies 61 ·
3
Replies
61
Views
10K
Graduate Is the Rotating Lepton Model a Valid Approach to Hadron Masses?
  • · Replies 15 ·
Replies
15
Views
4K
ABC reference frame synchronous generator modelling in Ltspice
  • · Replies 1 ·
Replies
1
Views
3K
Graduate Quantum theory without classical time: quantum gravity and unification
  • · Replies 1 ·
Replies
1
Views
2K
Graduate The "electroweak eta meson"
  • · Replies 8 ·
Replies
8
Views
2K
What is the difference between smart phones
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad A novel explanation of CKM and PMNS matrix parameters
  • · Replies 3 ·
Replies
3
Views
4K
Graduate Why do the proton and electron have equal and opposite electric charge?
  • · Replies 9 ·
Replies
9
Views
5K
Electric heat generation from parallel rods carrying current in an oil bath
  • · Replies 11 ·
Replies
11
Views
2K