Universal Extra Dimensions: New DZERO Results
    By Tommaso Dorigo | February 13th 2012 07:03 AM | 11 comments | Print | E-mail | Track Comments
    About Tommaso

    I am an experimental particle physicist working with the CMS experiment at CERN. In my spare time I play chess, abuse the piano, and aim my dobson...

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    Of the dozens of new physics models which are currently on the market of Standard Model extensions and plug-ins, the ones hypothesizing the existence of additional dimensions of space-time beyond the 3+1 we know about are definitely among the most fascinating.

    Imagine what a discovery would be, if we found out that our senses do not really represent any faithfully the reality in which we are immersed. I could hear the tumult from astrologists, occultists, ufologists, sellers of pseudoscience: "Aha! See, there's more than meets the eye!". We would instantly get flooded by ex-post justifications of why Jupiter does influence our lives (of course, through the extra dimension!), as well as how the pyramids got erected by aliens going through some extra-dimensional tunnel. It would be horrible, but it would also be fun! And after a few months spent handling the hit to science caused by the discovery, we could indeed start thinking at interesting ways to exploit it.

    All the above does sound like science fiction. And the general idea does come from a period of time when science fiction was in its golden age: the thirties of the past century. In those years the theorists Kaluza and Klein devised a model of extra dimensions which were too small to be detected by our senses, and "folded" onto themselves. The folding would generate a periodic boundary condition on the wavefunction of the particles, which would result in an apparent "tower" of states. To us they would look like copies of our SM particles, but with higher masses. I think your best bet to know more about this topic is to visit Sabine's Backreaction post on the matter.

    It is a very interesting idea, and we can actually test it at particle colliders. So far, most searches for large extra dimensions have concentrated in a modified version of the KK model, whereby the generic signature involves the disappearance of a particle in the extra dimension: one would then observe one particle recoiling against "nothing at all" -the disappearing partner. DZERO, however, tried to go directly after the Kaluza-Klein particles that are predicted to be associated to each Standard Model particle in the so-called "minimal Unified Extra-Dimensions" model, mUED. If you know supersymmetry, this will not sound too strange: each known particle gets its counterpart. In particular, one also gets a "lightest mUED particle" which escapes undetected. Is large missing energy a signature, then ?

    Not in the DZERO search: the masses of the KK particles might be such that the produced cascade decays do not yield a significant missing energy. The considered model sees the creation of KK quark-antiquark pairs, which decay yielding charged leptons and quarks. The most striking signature arising from such a process might be the presence of pairs of same-sign lepton pairs (from one of the decay chains depicted in the diagram above, or from two separate decays of KK quarks).

    DZERO sought for same-sign lepton pairs of small transverse momentum, which could be sizably contributed by the KK decays. If you look for same-sign electrons or muons (or electron-muon final states) at a hadron collider, you are going to have to fight backgrounds which come mainly from electroweak processes such as W+jets, Z+jets, and diboson processes; plus of course QCD processes yielding fake leptons. This is what appears from the graph below, showing the transverse momentum of the leading muon in the event (I should have said earlier that DZERO base their search on a sample of events collected by a inclusive muon trigger - so a muon is always present there).

    A boosted-decision-tree discriminant constructed with the most discriminating kinematic variables is put together to put in evidence a possible discrepancy of the data with Standard Model processes. The discriminant output is shown below. The black histogram representing a possible signal is well separated from the main background sources. The data points (black circles with error bars) are in good agreement with the sum of backgrounds (dominated by the multijet component, in cyan, and W+jet production, in blue).

    Fitting the output of the BDT allows DZERO to place upper limits on the possible signal contamination. When compared with the expected signal cross section, which is a function of the compactification parameter R, one gets a lower limit on R.  This is shown in the figure below, where the green-and-yellow band shows the predicted range of upper limits that the analysis could set, given the amount of analyzed data and the search technique. Since the masses of KK particles depend on R, the R limit allows to exclude masses of the new KK quarks below 317 GeV, which is the best way of summarizing the search result.

    I congratulate the DZERO folks for this interesting new result. You can read more information on the search in the DZERO paper.


    Does this mean that mUED will join SUSY in her perennial position of being just around the next corner?

    Hmmm, I suppose that such a theory is also not falsifiable, in fact. But I might be overlooking something (e.g. other constraints). I think there was a motivation, some time ago, for expecting LEDs to appear at the sub-millimeter scale (corresponding to testable TeV energies); I do not remember the details, will try to find out (or even better, hope some other reader comments here).
    Depending on how relaxed you want to be about falsifiability, I think UED can be ruled out in a much more definite way than SUSY (if it turns out not to be the physical reality). The mUED introduces only two new parameters: R and a cutoff scale at which a new theory must take over. And the dependence of results on the cutoff scale is much slower than on R. The coupling strengths (and therefore cross sections) are not arbitrary, so as collider experiments gather more data, one can increase the lower limit on the allowable compactification scale. As for an upper limit... if you believe the lightest Kaluza-Klein particle is all of the universe's dark matter, than you can already place an upper limit.

    This is of course all for the mUED. If you allow for funny things to happen with higgs couplings or at the fixed points of the orbifolded extra dimension, or if you allow for more than one universal extra dimension --- all the above is out the window.

    (It's wonderful to see this interesting theory getting attention!)

    A general rule of thumb is that if a new physics model is supposed to solve the naturalness problem of the Standard Model, some of its effect must be visible at the few-hundred GeV scale.
    If you push your limits for a model beyond these scales, the model might still be true, but it will not be "natural" (if it solves the SM problems, it can only solve them by means of some fine tuning or by complicating the model further - like epicycles did) and therefore it becomes less appealing.

    The mUED doesn't address the naturalness problem, since it allows all fields to propagate in the bulk.

    "It would be horrible, but it would also be fun!"

    "..same-sign lepton pairs (from one of the decay chains depicted in the diagram above, or from two separate decays of KK quarks)."
    >How can you get same-sign leptons from a single decay chain, as illustrated? It seems like you need two separate decay chains.

    " gets a lower limit on R."
    >Perhaps you meant R^{-1}? (and perhaps D0 meant to label the plots differently?) Shouldn't R^{-1} have units GeV?

    Sorry that was a blunder. Will correct.
    Hi Tommaso,

    Some of us lay people actually do care what is going at the LHC research department and how this is being done.

    We might have our proclivities to the mysteries of the world and our previous incarnations:) but we all still care about the science and how this is being accomplished. Of course matter expressions objectified are important too. Being particularized, what does "free will" have to do with it?:)

    Kaluza-Klein Partners — Why? Step 1-


    Imagine what a discovery would be, if we found out that our senses do not really represent any faithfully the reality
    Gee - this just proves that you do not even proofread your writing anymore; surely I am not entirely wrong when claiming you have given up thinking stuff through sufficiently. Extra dimensions would prove we do not see reality as it is? And that would be news then, would it?
    Dude, I'm thinking that was irony.

    No -- he's serious. He's deeply into pseudoscience. Doesn't need data to spin tall tales.