Number of Lepton Generations in string theory

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Discussion Overview

The discussion revolves around the number of lepton generations in string theory, exploring whether this number is fixed at three or if it can vary depending on the specific model or landscape of string theory. Participants examine the implications of different string theories and brane configurations on the generation count.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants question whether the number of lepton generations in string theory is restricted to three or if it is "landscape dependent."
  • One participant cites neutrino oscillations as evidence for exactly three generations.
  • Another participant suggests that various brane configurations in string theory could allow for the construction of different numbers of lepton generations, potentially including values other than three.
  • It is noted that the number of generations may be topological and influenced by the geometry of compact dimensions, indicating a landscape dependence.
  • Participants discuss the constraints on gauge groups and matter in different string theory models, mentioning that while some models do not require three generations, they may complicate the realization of larger gauge groups.
  • There is a mention of multiple copies of E8 in some string theories, with a distinction made that these copies do not interact except through gravity, leading to questions about their implications for the number of generations.
  • A participant raises the issue of whether the existence of only three generations is robust enough to constrain the topology of compact dimensions, or if alternative models with heavier generations or sterile neutrinos are still being considered.

Areas of Agreement / Disagreement

Participants express differing views on whether the number of lepton generations is fixed or variable, with some arguing for a landscape-dependent approach while others reference empirical evidence suggesting three generations. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

The discussion highlights the dependence of conclusions on specific string theory models and the complexities involved in relating gauge groups and matter to the number of lepton generations. There are unresolved questions regarding the implications of hidden sectors and the role of topology in determining generation counts.

JustinLevy
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Is the number of lepton generations in string theory restricted to 3? Or is this "landscape dependent"?

I don't really see what could limit the number of eigenmodes on a continuous object like the string.
 
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I've heard that the study of the neutrino oscilations proves that there are exactly 3 generations
 
I thought that by various brane configurations, one could construct almost arbitrary combinations of gauge groups in string theory, including the standard model. This would mean that in String Theory a lepton generation number could be constructed to be 3, but also (almost?) any other value. Am I mistaken here?
 
Hi Justin---

It depends on which string theory you are working in, but in general the number of generations is topological. In other words, the geometry of the compact dimensions regulates the numbers of generations that you observe. In this sense, it is "landscape dependent".

In general, Orbb is correct when dealing with Type II model building, where you can make branes intersect. In other types of models, there are constraints on the types of gauge groups and matter that you can get. None of these constraints require three generations, but they can make it very difficult to see how one gets a gauge group that is larger than E8, for example.
 
BenTheMan said:
In other types of models, there are constraints on the types of gauge groups and matter that you can get. None of these constraints require three generations, but they can make it very difficult to see how one gets a gauge group that is larger than E8, for example.
Don't some string theories have multiple copies of E8?
If so, would that mean those theories predict more than 3 generations?
 
JustinLevy said:
Don't some string theories have multiple copies of E8?
If so, would that mean those theories predict more than 3 generations?

Well, yes to the first question and no to the second.

To the first, the multiple copies of E8 don't really talk to each other, except via gravity. One E8 is typically "hidden" from the other.

To the second, you have to work out the details, as this is a very model dependent question. Everything depends on the compact manifold.
 
BenTheMan said:
To the first, the multiple copies of E8 don't really talk to each other, except via gravity. One E8 is typically "hidden" from the other.
Ah, okay. I didn't realize those were "hidden sector" theories.

BenTheMan said:
To the second, you have to work out the details, as this is a very model dependent question. Everything depends on the compact manifold.
Are people confident enough that there are only three generations to use it as a constraint on the topology of the compact dimensions? Or are people still seriously considering a very heavy fourth generation, or "sterile neutrinos" and the like?
 

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