Why people are still discussing the 4th generation models?

In summary, there are three generations required for anomaly cancellation in the Standard Model of particle physics. However, there are still papers discussing a potential fourth generation. The fourth generation is theoretically allowed, but it is almost certainly ruled out experimentally due to the heavy mass of the particles and their instability. However, in non-standard scenarios, such as those involving a non-standard Higgs boson, a fourth generation may still be possible.
  • #1
Accidently
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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
 
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  • #2
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).
 
  • #3
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!
 
  • #4
yes, I was referring to standard scenarios only; I can't say much about (e.g.) non-standard Higgs ...
 

FAQ: Why people are still discussing the 4th generation models?

1. Why haven't newer generations replaced the 4th generation models?

The 4th generation models were released in 2010 and were considered groundbreaking at the time. They have since become iconic and still have a large user base, making it difficult for newer generations to completely replace them.

2. Are there any significant differences between the 4th generation models and newer generations?

Yes, there have been significant improvements and updates in newer generations such as faster processors, better camera quality, and enhanced features. However, the 4th generation models still have a loyal following and are considered reliable and user-friendly.

3. Why are some people still using the 4th generation models?

Some people may prefer the design or functionality of the 4th generation models. Others may not see a need to upgrade if their current device is still functioning well. Additionally, some users may not be able to afford or justify the cost of upgrading to a newer generation.

4. Will the 4th generation models eventually become obsolete?

As with any technology, the 4th generation models will eventually become obsolete. However, Apple continues to support and provide updates for these models, so they can still be used for several more years. Eventually, newer software and apps may not be compatible with these models, making it necessary to upgrade.

5. Are there any potential health risks associated with using older models?

No, there are no specific health risks associated with using older models. However, as with any electronic device, it is important to follow proper safety guidelines and to take breaks from prolonged use to avoid eye strain and other potential issues.

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