Talk:Large extra dimensions


 * This page was briefly wrongly merged with universal extra dimension/Talk:universal extra dimension

Arkani Hamed Schmaltz reference
Thank you for the source, this is a very beautiful paper. It really gives a great argument for proton stability, and lack of direct four-fermi lepton-quark mixing. That really does address criticism based on mixed lepton-quark operators, in particular neutrino masses from pion condensate. That should be explained better here. But the main point is that naturalness assumptions can be naturally violated if there are extra dimensions, which is surprising and important regardless of the validity of the model.

The paper also begins to address neutrino masses, although it doesn't give a detailed mechanism. It might be OK, I don't know.

There is a zoo of other dimension six objects, which need some discussion. Here's the first few I can think of:
 * 1) nonminimal chiral QCD interactions: terms like $$\epsilon_{\mu\nu\lambda\sigma} G^{\mu\nu}Q\sigma^{\lambda\sigma}Q$$, with G the gluon field, Q some quark spinor, and with some appropriate contraction of the color indices. I haven't thought about exactly which ones occur, but there's probably a ton of them. These would lead to chiral quark gluon interactions. What does that look like? These are big for the top quark, but I don't know if there are any experimental bounds. It might be OK.
 * 2) six gauge-boson interactions: terms like FFF and epsilonFFF, or derivative modifications to four-gauge boson terms, whichever ones survive, with F one of the three gauge field strengths. These give new photon,W,Z,gluon interactions. Maybe these ruin predictions based on minimal coupling of gauge bosons to themselves, like corrections to the W and Z mass. Again, there might be no tight bound from them. I don't know, I'm just asking. Is there anyone who parametrizes allowed violations by mass scale anymore?
 * 3) Pauli-like terms for both quarks and gluons, for anomalous magnetic moments of leptons. There are terrific measurements here, but again I don't know if these are evadable. What's needed is a parametrization of the effects of any extra terms in terms of the mass scale.

The Arkani Hamed Schmaltz non-naturalness argument is really about the bigness of the extra dimensions, and the approximate locality of gravitational interactions. It's really really clever, but it needs stabilization. All the nonnaturalness in the low-energy theory might get shunted right into non-naturalness in the stabilization mechanism. But Arkani Hamed has a paper about that, which I haven't read.

Maybe this whole scenario is really nonfiction. That would be mind-blowing. This article needs to read based on current consensus, but I think that the field is really split on this.Likebox (talk) 18:37, 2 April 2009 (UTC)

Naturalness
I was rattled by the example of Arkani Hamed and Schmaltz (AH/S). It so easily blows away a naturalness argument, that you start to suspect that all naturalness arguments are crappy. That's not true.

I think people tended to underestimate the naturalness arguments, because they were brought up at a time when the suppression factor was always considered huge--- each extra dimension is 1/Mpl. If the power suppression is small, the dimension 5,6,7,8 terms should be noticible as strong CP violation. The QCD scale means that sea quarks are always bouncing around off each other at around 100-200 MeV, which means the suppression from each dimension operator at 1-10TeV is only about 10^{-5} at best. That's not so much, especially considering that strong neutron electric dipole moment is excluded to something like 1 part in 10^-18 or something like that. This requires a detailed calculation to make sure there is a bound. I am pretty sure that people writing about this subject didn't consider strong CP to be a major problem.

The reason that AH/S were able to evade naturalness for proton decay is because proton decay mixes quark and lepton operators, and you can make these products disappear by moving the quarks and leptons far apart (there only constraint is that they need to couple to a "bulk" SU(2)) Their mechanism is not going to work for neutrino masses (as they admit) or strong CP. I don't think anything works great for those. But it needs a calculation.Likebox (talk) 15:00, 3 April 2009 (UTC)

AH/S Again
There is another problem with AH/S as physics: since it separates the quark and lepton brane, it requires at least the SU(2) and U(1) gauge fields, which couple to both of them, to be delocalized. This will ruin SU(2) and U(1) RG running above the scale where you notice that the leptons and quarks are separated. Electromagnetic interactions have not suddenly become extraordinarily weak at the last electron-proton collider, so the separation between the quarks and the leptons can't be too big in terms of Planck masses. Biggest EM measurements were at 100GeV, that's (generously) .01 Planck masses, and no hint of a deviation from inverse-square law.

That means the quarks and the leptons aren't separated by more than 100 Planck units. This does not give a direct quantitative bound, because you don't know how "squished" the wave-function of a zero mode can be in quantum gravity, but dollars to donuts it's not going to be smaller than the Planck length. So to get large proton decay suppression, keeping the thing natural, and keeping EM normal at 100GeV, you need a higher Planck scale, all else being equal.

So to test "large extra dimensions" at LHC, you don't need to look for exotica, like black holes. Just check to see if EM events suddenly drop out of all counts. If they don't, large extra dimensions violates proton decay bounds. We at Wikipedia need to point these things out, when the referees for physics journals stop doing their job.Likebox (talk) 17:31, 4 April 2009 (UTC)
 * No that is not wikipedia's job. If you want to point something out that journal referees missed than write a paper and submit it to a journal. (it is as simple as that). Wikipedia is not the place to make points about physics. Anything posted that is not backed by verifiable sources (however true it may be) should be removed as OR. Not adherring to this policy would make it impossible to control crackpot contributions to wikipedia. (TimothyRias (talk) 13:10, 8 July 2009 (UTC))


 * The question here is: who's pot is cracked? The subject of this article is the ADD model, and it was very popular ten years ago, but it was never very plausible. It was the basis of many papers, but it was never accepted as likely by even a large minority of physicists. I can't support this assertion, but I believe that ADD style model building, which is fanciful to say the least, was the main reason that string theory was heckled in this decade.


 * Even proponents don't necessarily believe that ADD is true as physics, they just use it as a starting point for model building, as make-work for theorists. Models inspired by ADD, like Randall Sundrum and Brane compactifications, are OK, so long as they don't pretend that the Planck scale is right around the corner.


 * The referees who aren't doing their job are those that do not make sure to say that ADD style model building is very speculative, and that mainstream thinking still supports the desert. The ADD model predicts that black holes will be formed by accelerators, and it is Wikipedia's job to say whether there is consensus that this is possible. The literature on this subject is vast, but the underlying implausibility of the model is not usually made clear. This leads to undue weight speculations about mini-black holes at accelerators, which, to put it mildly, is very unlikely.


 * I have to admit that I personally dislike this paper very much, but some of the authors have gone on to do good stuff.Likebox (talk) 00:53, 13 October 2009 (UTC)

Article organization
I'm concerned by the current organization of the article, which splits it into two sections titled "Proponents' views" and "Opponents' views". It is especially troubling that the latter section only has a single reference supporting something like a dozen paragraphs of text.

I think the article should be rewritten to provide a topical overview of the history and features of large extra dimensional models. Sections discussing, say, collider phenomenology, neutrino masses and mixings, CP violation, proton decay, and so on could present both strengths of and challenges to these models, without the misleading he-said-she-said framework. Unfortunately, I don't think I have time to do this myself. -David Schaich Talk/Cont 00:31, 22 July 2009 (UTC)