Banner
    Gordon Kane On SUSY At The LHC
    By Tommaso Dorigo | August 28th 2011 05:16 AM | 38 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...

    View Tommaso's Profile
    Despite the hopes of most and the preconceptions of many, news from the Lepton-Photon conference in Mumbay, India, report that the Standard Model is as alive and strong as it has ever been. Indeed, the recent searches for Supersymmetry by ATLAS and CMS, now analyzing datasets that by all standards must be considered "a heck of a lot of data", have returned negative results and have placed lower limits on sparticle masses at values much larger than those previously investigated (by experiments at the Tevatron and LEP II).

    Similar is the tune being sung on the B-physics sector, now being probed with unprecedented accuracy by the dedicated LHCb experiment (along with again precise measurements by ATLAS and CMS, plus of course the Tevatron experiments). I have not reported on those results here yet, but will duly do so in the next weeks. In a nutshell, anyway, deviations from the Standard Model predictions are all well within one sigma or two; the hypothetical contribution of SUSY particles in virtual loops taking part in the decay of B hadrons must be very small in order to fit in this picture.

    A poignant question is in the air and must be asked. Do those results shake the confidence of those particle theorists whose beliefs in the correctness of Supersymmetry are strongest ? What do those physicists think today ?

    I decided to pose this question to Gordy Kane, a leading particle theorist and advocate of Supersymmetry. Prof. Kane is not only the author of countless scientific papers and several books on the topic, but he is one of the true fathers of Supersymmetry, in particular its "minimal" version, the MSSM. I had the pleasure of meeting Gordy at a PASCOS conference in Albuquerque three years ago (the picture on the right portrays us at the conference dinner). He graciously agreed to write a couple of paragraphs for me -i.e., for us. Here you are:

    I thought it was worthwhile to comment a little on recent LHC searches, since they have led to a number of surprising statements. First consider gluino searches. The results of a search for gluinos are very sensitive to squark masses. Theoretically the only well motivated values for squark masses are very large, tens of TeV, because they are generically predicted in compactified string/M-theories when the associated moduli satisfy cosmological restrictions. Then (a) the gluino production rates are considerably reduced, and (b) the decays or gluinos to 3rd family final states dominate. Existing gluino searches cover this region poorly. The current limit on gluino masses is not above 500 GeV. Whether the squarks are indeed so heavy is not the issue, the point is that if they are the limits on gluino masses are smaller than is often stated. I and others expect this decay to tops and bottoms is the signature by which gluinos will be found, with masses well below a TeV.

    Second, when squarks are heavy the two doublet Higgs sector is an effective single doublet since the heavy partners decouple. There is a single light Higgs boson observable. If the gauge group of the theory is the MSSM one then the Higgs mass is between about 115 and 128 GeV (essentially a function of the parameter tan(beta)). It will not be above that range. It has the SM production rate. The LHC searches are not yet sensitive to this region, and should not yet have seen a signal, so not seeing a signal does not allow any meaningful conclusions about Standard Model or MSSM Higgs bosons.

    I am happy to see a definite prediction by Gordy on where the gluino should be. I think we have a chance to see the decay of gluinos to third generation particles at the LHC if gluinos exist, but we'll have to be patient for a while longer...

    As for the Higgs, of course the null results on MSSM Higgs boson searches so far are much less worrysome to SUSY supporters. When we eventually see a Higgs boson signal (and I concur with Gordy that the mass will be in the range he quotes) deciding whether it is a SM or SUSY Higgs boson will in fact turn out to be quite a tough nut to crack.

    Addendum: Gordon wishes to add a couple more remarks on the above, where he basically counters my comments. Enjoy:

    It will be easy to tell h is the supersymmetric one since superpartners will also be found.  The generic prediction that scalars are heavy also implied that no deviation from the SM should be seen in b physics.
    That's nothing less than we would expect from a SUSY advocate, wouldn't we ? ;-)

    Comments

    Would not finding any Higgs be compatible with SUSY?

    dorigo
    Hi Anon,

    no, SUSY requires Higgs bosons. In the decoupling limit one of them is like the SM one, but it has to be there, at masses below 135 GeV or so...

    Cheers,
    T.
    Is there a simple way to understand why the decays of gluinos to 3rd family final states dominate, if the squarks are heavy? What I'm wondering is: suppose there was a 4th SM generation. Would the decays of the gluinos to the 4th generation now dominate, making the gluino decays very complicated indeed?

    In my understanding, the case for SUSY is also affecting String Theory, as the latter implies supersymmetry (right?).
    While ruling out SUSY will require more data (and, possibly, even then some exotic version of SUSY may survive), on the other hand I wonder if the cross section of the creation of tiny black holes in LHC could be a more effective way of getting rid of Strings for good.

    I have read that this cross section should be very sensitive to the number of spacetime dimensions, increasing when multiple dimensions exist, to a point where creation of black holes should be an everyday event. Is it possible that a possible absence of black hole events in LHC proves String Theory wrong, or am I thoroughly misled?

    @filippo

    Yes, you are. Black hole production depends on the fundamental scale of gravity. Extra dimensions may help in lowering such a scale so much that black hole production gets enhanced. This however has little to do with string theory and for sure it cannot be used as an argument against string theory. As a matter of fact, not even against the existence of extra dimensions.

    @Chris,
    the gluino decay is very sensitive to the masses of the squarks, when the latter are heavy. Since the mixing of the squarks is in general larger for the third generation ones the lightest stop turns out to be the lightest one and its exchange gives the largest decay amplutdes. The resulting partial decay widths into qqbar neutralino, q qprime chargino go like 1/msquark^4.
    A fourth generation should not be relevant if the decay of the gluino into these quarks is not kinematically open, as I would expect. Of course, nobody really knows what to expect ;).

    Dear Tommaso,

    Seems, gradually folks have to come round the fact that they were on the wrong trail. Even small experiments show what gigantic machines are unable to, depends how and where you fish.

    Francis's entry http://tinyurl.com/44va5ol reproduces Kane's spectrum, and looking at it the last remark of Kane is clear. Only the bosonic part of the higgs sector gets a Beyond-LHC mass. The mixings of charginos, neutralinos, etc, should still be in the electroweak range, or at least sitting in the middle. This is technically intuitive, I guess, because they still need to be partners of the electroweak gauge bosons.

    Filippo:
    There is a very good reason why it Was called `SuperString Theory(SST)' in the 1980s. Tachyons emerged & unitarity is violated if SUSY is not built-in to string theory. Hence string theory proper does not imply SUSY, but it's utterly inconsistent without it. In other words SST is built on a house of cards named SUSY: If she falls, likewise for SST.
    Earlier this yr., searches out to 7 Tev CM energy saw No Evidence for mini-BHs, sadly, as they were projected by several groups to emerge near there. Gian is correct, as Tev-scale quantum gravity was originally proposed by ADD in `98 to solve the Hierarchy problem via large extra dimensions, without the necessity of SUSY.
    Gordy Kane is not alone, & hundreds of physicists who've built their careers on SUSY will be loathe to admit defeat, & will merely play a shell game, inching the bar to higher energies until hell freezes over.

    Jimbo, thank you (and Gian) for answering. In fact I was not implying correlation of mini-BH production to SUSY, rather wondering if non-observation of BH could be evidence against extra dimensions, and in turn against SST. Since I am not aware that SST can live without extra dimensions, I thought the two topics were connected, but perhaps it's a mistake. I'll try to understand better.

    In the best scenario you could constrain LARGE extra dimensions scenarios. You don't rule them out.
    BTW, string theory can live more or less in any dimension if you construct non-critical models.

    For some reason this reminds me of something Max Planck said about progress and funerals.

    dorigo
    No need to be that corrosive Thomas... Gordy was kind to explain his views for this blog, so he does not deserve the Planck quote!
    Cheers,
    T.
    So,Mother Nature is holding on to her secrets - no conclusive sign of the Higgs, MSSM in trouble, and no hints of New Physics yet. But LHC still is only running on half of its design energy. When it does get up to 14 TeV, how much will things change then - assuming that nothing new is found by the end of this year?

    dorigo
    EDBM, I have no idea whether anything will be found- but 14 TeV will allow to roughly extend the searches to twice as high sparticle masses, for similar integrated luminosities. Such a linear rise is only apparently trivial -one must know that the detector resolution to the final state bodies one wants to measure remains acceptable if you double their energy, in the relevant range. Other factors to weigh in are the PDF at the higher Q^2, the pile-up conditions of the higher-energy running, etcetera.

    Cheers,
    T.
    The actual quote is:

    A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it

    See wikiquote.

    So it looks like the recent results send most of the susy masses above the TeV scale (save for maybe the gluinos). But what about the weakly interacting susy particles (charginos, neutralinos)?

    As far as I can see, the recent LHC searches were mainly for squarks and gluinos. Assuming mSUGRA, this is enough - you can translate limits there to gaugino limits. If you leave out that assumption, you have two independent intermediate states though. The best direct limits on weak susy particles seem to still be from the Tevatron... or has there been done such a search at ATLAS/CMS?

    dorigo
    Hi Lubos,
    thanks for your link. A small correction: "more details" is a wrong way to put it, since Gordon wrote the above paragraphs for this blog specifically. It is not stuff contained in a talk of his.

    Cheers,
    T.
    lumidek
    Dear Tommaso,
    I would agree if you just said that it's been a great honor for you that Gordon Kane wrote two paragraphs of text specifically for your little blog that is otherwise dominated by theoretical research that cannot exactly be called "serious".

    But what you're saying is not right. "More details" is totally appropriate a description; all the information contained in the two paragraphs on your blog *is* contained in the talk; many people have known these things from papers by Kane et al. before the Madison talk; and the talk contains much more than the brief summary on your blog - for example the masses of all SM particles, Higgses, and their superpartners, as well as details about the decays in which the stuff will be seen, cosmological context and non-thermal implications of the models, the non-trivial novel solution to the hierarchy problem, and many other things.

    So please don't get carried away when you try to self-centrically imagine that you were told some secret exclusive information. You were not.

    Cheers
    LM

    Man, sour grapes are sour.

    Tommaso,

    to the experts it may seem obvious, but I ask nevertheless. If the Higgs is in the range 115-128 GeV, how comes that the LHC sees no effects of it at higher energies?

    Alternatively, if the SM Lagrangian of the Higgs is really correct, why don't they see effects at all energies? Shouldn't the deviations from a Lagrangian without the Higgs increase with energy? The LHC is a huge machine with high energy - the deviations it sees should be clear and strong. Why is it not so simple to see them?

    dorigo
    Dear Mia,

    besides the apparition of the resonance, and thus the definitive observation of the Higgs boson, there are many effects due to it that can be observed in electroweak physics. However, we cannot be sure that those effects are due to a Higgs boson and not to some other obscure mechanism. Take precision electroweak fits (done using all available collider data): assuming that the Higgs exists, these fits can even obtain a reasonably narrow value for the most probable Higgs mass; but this proves nothing yet.

    If we do not see the particle as a resonance, it will be very hard to say we are sure the model is correct.

    Best,
    T.
    Tommaso,

    thank you for your answer. But in all the huga amount of data that the LHC collects, it must be possible to compare the (1) predictions of the standard model without the Higgs, (2) the predictions with the Higgs, and (3) the data itself.

    It seems so bizarre to an outsider, that so few of these comparisons are possible. After all, there are so many effects of the Higgs. The graphs we see are all based on Higgs decas. Aren't there any other ways to check the Higgs?

    dorigo
    Hi Mia,

    in a nutshell, the predictions of the no-Higgs SM are that the vector boson scattering diagrams diverge at high energy. This will be testable soon. Other things can be tweaked and in principle different physics may be the source of what we observe. The non-unitarity of the VV scattering amplitudes was the original sin of the no-Higgs IVB theory in fact. There are Higgsless models which mend the unitarity problem at high-energy, but these have testable predictions too. Typically they require high luminosity to be put to the test, though.

    Cheers,
    T.
    So at the end, the question is not when do we get enough data for the Higgs, but when do we get enough data for the VV scattering amplitudes, is it?

    dorigo
    Well Alejandro I sincerely hope we see a Higgs boson and that's it! I do not think we will be able of measuring high-energy deviations of WW scattering at a level which can distinguish different higgsless models in the near future.

    Cheers,
    T.
    Dear Tommaso,
    The sparticle spectrum which Kane included in his talk, which is also posted on Lubos' blog, has the lightest chargino and neutralino degenerate, or almost degenerate. First question is why there is such a degeneracy and how unique is it? Second question is what are the experimental implications/signatures of this mass degeneracy?

    It seems reasonable; it says that susy is only midly broken in the massive gauge sector, does it?

    dorigo
    Excellent questions Anon. As far as the first one is concerned, I hope some SUSY expert, which I am not, can answer it. For the second, I think we need to plug the masses in a Monte Carlo to have definite predictions, but I believe that per se the degeneracy does not yield funny effects which can be easily flagged.

    Cheers,
    T.
    OK, I actually looked at one of their papers and the masses are actually slightly split by ~200 MeV. As a result, the lightest chargino is quasi-stable and should produce a short track inside the detector. I'm not sure how one would trigger on such an event though.

    dorigo
    Ah, I now see what you meant. Sorry for not having thought in terms of explicit detector signatures. Anyway, a short track is almost certainly not detectable (it requires at least some tens of centimeters to be reconstructed). A chargino-neutralino production process could still be triggered by other leptons, e.g. when the chargino is produced with a second-generation neutralino, and the latter yields a Z. Instead the generic chain initiated by squark or gluino production may or may not produce significant missing Et in the case of a degeneracy - I believe it produces less of it in any case, so this might be a annoying corner of the phase space to investigate.

    Cheers,
    T.
    Thanks Tommaso!

    SHY SUSY
    -- James Ph. Kotsybar

    A string theorist’s concept of heaven,
    she’s nicknamed SUSY and some think she’s bound
    to prove dimensions number eleven,
    but SUSY’s shy and she hasn’t been found.

    They say SUpersSYmmetry’s hard to find
    because she’s fragile, easily breaking,
    their critics argue she’s just in their mind
    and mathematics of their own making.

    Proponents think she’s a flimsy feather
    and yet say that she could be dark matter.
    If she can hold galaxies together,
    and the four forces fit on her platter,
    maybe she’s not so timid as all that.
    She’s certainly an awesome diplomat.

    dorigo
    Nice one James! Thanks for sharing it here.
    Cheers,
    T.
    dorigo
    Oh yeah, the tiger is a favourite of mine. I love classics. Nice job here!
    Cheers,
    T.
    I was looking at slide 24 (gluino production cross section vs gluino mass) in Gordy’s stringpheno 2011 talk: http://conferencing.uwex.edu/conferences/stringpheno2011/documents/kane.pdf
    and I’m not sure if the LHC will actually collect enough data by the summer to really test for gluinos. Let’s say they collect 10fb^-1. From the plot, if the gluino mass is at, say 900 GeV, the production Xsection is 10 fb, which gives 100 events. However, taking into account the detector efficiency (at most few % according to the slide) is it realistic to expect some signal?

    dorigo
    Hi Anon,

    I doubt it, but I would need to dig into past searches for similar signatures to build an informed opinion. If Gordy himself says the efficiency is at the few percent level, this makes it few events of signal, in a 10/fb dataset. Unless the signature is exceedingly clean (and gluinos usually give messy hadronic final states not impossible to mimic by QCD) there is little chance that they make a visible signal.

    Cheers,
    T.
    Thank you, Tommaso! I had suspected that Gordy was far too optimistic about finding the gluinos in his scenario by the summer. I guess there is still a pretty good chance once the energy goes up though.