According to this blog, new standard model calculations have changed the 3 sigma B meson forward and backward production asymmetry observed at LEP into two anomolies:

  • A 2.5 sigma B meson production cross section anomaly and
  • A 2.5 sigma B meson forward and backward production asymmetry

And that both of these anomalies can be explained by Randall-Sundrum-type models with the right-handed b-quark localized near the IR brane (which implies extra dimensions). I have been told that these models are equivalent to saying that the b quark is composite.

Obviously, these 20 year old 2.5 sigma signals are not definitive and could have statistical or systematic error explanations, but if they hold up and are confirmed by LHC B meson measurements, I have the following questions:

  • Would this constitute somewhat convincing evidence for extra dimensions?
  • Would this be the first convincing evidence for extra dimensions or are there other standard model anomalies that imply extra dimensions?
  • How are Randall-Sundrum-type models equivalent to saying that the b quark is composite?

No, this on its own wouldn't be significant evidence of much of anything, especially extra dimensions. These statements about RS are really just saying that the right-handed bottom would be mixing with some heavier quarks that have "vectorlike" masses, i.e. masses that don't come from the Higgs. This could happen in various composite models, but doesn't require compositeness. Since we have no other evidence of anything being composite so far, it would be much more plausible that there are just some new elementary quarks mixing with the b-quark than that something like RS is behind the data. And there may be other ways to explain the data -- most plausibly, that it's a statistical fluke. (People look in hundreds of places for new physics, so it's not unexpected that such flukes happen.)

The equivalence of RS and strong dynamics refers to the fact that Randall-Sundrum geometries are a slice of anti de Sitter (AdS) space, and the AdS/CFT correspondence tells us that physics in anti de Sitter space is equivalent to strongly-interacting conformal field theory. The modified version of AdS that appears in these models corresponds to thinking about confining, rather than conformal, field theory. So RS is a sort of toy model for various kinds of strongly interacting, confining field theories. If any such theory is discovered, it's probably more sensible to think of it as strongly interacting field theory rather than an extra dimension, because the extra-dimensional effective field theory is not likely to be a good approximation to strong dynamics in the real world.


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