In this article, Lumo introduces and explains the idea (presented by the original authors in this paper) that the LHC signal at about 125 GeV could alternatively be interpreted as a higher dimensional radion. Such a higher dimensional radion would better fit to the branching ratios observed at the LHC so far (at the present state of data accumulation) than a SM Higgs.

While reading Lumo's article, I got very curious about the following:

a) Where would the Higgs hide, if this model is true and the 125 GeV signal is rather a radion than a Higgs?

b) Would a Higgs still be needed in this case for Electroweak Symmetry Breaking (EWSB)?


c) Could the radion itself play the role of the Higgs?

"Acknowledgment": Lumo has started to think about these questions at the end of the article too, so we are both very curious about the answers to these issues.

  • $\begingroup$ Since this question could still be open (again ...) and people who work on this are maybe found at theoretical physics SE, I thought about asking there too. But I'm not 100 % sure if they would like it ... $\endgroup$
    – Dilaton
    Apr 21 '12 at 14:07
  • $\begingroup$ The radion is a massless field, isn't it? So (c) can't be . $\endgroup$ Aug 14 '13 at 12:42
  • 1
    $\begingroup$ @Dimension10 Lumo explained that it should some kind of describe the "thickness" of the Randall-Sundrum world, so is it not some kind of muduli field? Dont know how massive it is expected to be (if it should have any mass at all) out of my head ... $\endgroup$
    – Dilaton
    Aug 14 '13 at 13:30

This is not exactly my area of expertise, so you can probably get a better answer from someone else (perhaps Lubos). But based on a quick overview of the relevant papers, the presence of a Randall-Sundrum radion as originally proposed would not eliminate the need for a Higgs boson. In order to allow gauge invariance, the Higgs mechanism basically requires a ring-shaped potential minimum, which means you need a field with at least two degrees of freedom. The first RS paper mostly only works with the case of one extra dimension, so you would need to generalize the model to additional dimensions, and given that these extra dimensions are supposed to be individually periodic, I'm not seeing how you could get the sort of structure required to produce a Higgs mechanism out of it. Of course, a lot of people have done work that builds on Randall's and Sundrum's papers, so perhaps someone has determined some way to do it, but it seems unlikely to me. (In fact, in the original paper, around equations 17 and 18 they talk about a fundamental Higgs field which is separate from the radion field, so evidently the authors themselves did not consider the radion as a stand-in for the Higgs.)

The LHC experiments have searched the entire allowed mass range for the standard model Higgs, from the lower limit set by LEP to the upper limit set by unitarity bounds, and everything except this region around $125\text{ GeV}$ is excluded at 95% confidence level. So if this bump turns out not to be the Higgs boson, the standard model Higgs is ruled out and we would have to start looking at rather more exotic model which predict Higgs masses in excess of $600\text{ GeV}$. I don't know of any particular model of this sort which has generated much interest among particle physicists.

And another thought: even just based on the data presented in the paper by Cheung and Yuan, the branching ratio excesses measured by CMS have huge uncertainties. It seems pretty premature to me to dismiss the identification of the observed excesses with the Higgs boson, since with more data, the numbers could easily converge to the SM Higgs expectations.

  • $\begingroup$ Lubos has just given a link that examines the coexistence in the same mass region of both a Higgs and a Radion in the LHC data : arxiv.org/abs/1202.5017 . Can be done. $\endgroup$
    – anna v
    Apr 22 '12 at 4:03
  • $\begingroup$ Sure, they can coexist, but that paper doesn't say anything about the radion and the Higgs being the same particle. (Though it does discuss mixing, which is kind of in that direction.) $\endgroup$
    – David Z
    Apr 22 '12 at 4:11
  • $\begingroup$ Hi David, thanks for this nice answer; what you say seems very reasonable to me. Even though other people may have additional things to say, I like it +1. $\endgroup$
    – Dilaton
    Apr 22 '12 at 9:04
  • $\begingroup$ Of course I did not mean to dismiss the identification of the 125 GeV excess as ha higgs... When I asked Prof. Strassler what he thinks about the idea of this alternative interpretation, I probably made him a little bit angry ... :-/ He clearly stated that one should not yet think or ask about alternative interpretations at the present state of things and that such a radion is only one in a zillion of other possibilities and all such papers should be ignored as nois ... $\endgroup$
    – Dilaton
    Apr 22 '12 at 9:11

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