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Can SUSY be true if neutralinos have been ruled out to exist?

  1. Jun 19, 2010 #1
    The neutralino is the lightest supersymmetric stable particle, and is a good dark matter candidate. Some theories like MSSM predicts the LHC has enough energy to produce neutralinos, which can be indirectly detected by missing energy in collisions.
    Neutralinos are predicted to be abundant and stable based on SUSY-GUT big bang, enough to be dark matter.

    It may be the supersymmetry breaking scale is so high that LHC may not be able to produce neutralinos.

    The successor of Cryogenic Dark Matter Search CDMS II is Super-CDMS, with the sensitivity to detect neutralinos over its parameter space in most popular SUSY models. Hence, even at higher than LHC energies, the big bang should have produced neutralinos and these should be detectable. Ref below. In addition to Super-CDMS are other direct and indirect detection experiments that can detect neutralinos over its expected parameter space.

    R-parity is required in most phenomenologically acceptable SUSY models, no R-parity leads to predictions in conflict with experiment esp baryon number and lepton number conservation.

    So my question is if neither LHC, nor SuperCDMS, nor other direct and indirect experiments, such that say 3-bar or 4-bar, they can be said to not exist (in much the same way as SU(5) and SO(10) GUT experiments have been ruled out based on predicted proton lifetime)

    is SUSY still possible? If several independent lines of research (i.e LHC, SuperCDMS, other detection experiments, even Tevatron)
    rules out neutralinos as predicted by the minimal (MSSM), the next-to-minimal (NMSSM) and the nearly minimal (nMSSM), is there any reason to think R-parity conserving SUSY is a symmetry of nature?

    http://www.telegraph.co.uk/science/science-news/3351261/Large-Hadron-Collider-What-will-it-find.html [Broken]

    Eva Silverstein, Stanford University, USA

    'I'd be extremely puzzled if they don't find the Higgs, but wouldn't be devastated if they didn't come up with evidence for supersymmetry. Some of my intuition comes from string theory, an appealing candidate for a theory of all the forces of nature. According to many - perhaps most - versions of string theory, supersymmetry does not hold good at the energies probed by the LHC, so its discovery might require further explanation from this point of view. On the other hand, supersymmetry fits well with some existing observations, and it will be spectacular to finally learn whether it arises.'

    String theorists like Eva Silverstein can rationalize away any null result from LHC on the grounds LHC isn't enough energy, and the SUSY breaking scale is above EW scale, but what about neutralino non-detection via SuperCDMS?


    http://arxiv4.library.cornell.edu/abs/1005.0761 [Broken]

    SUSY dark matter in light of CDMS II results: a comparative study for different models
    Authors: Junjie Cao, Ken-ichi Hikasa, Wenyu Wang, Jin Min Yang, Li-Xin Yu
    (Submitted on 5 May 2010)

    Abstract: We perform a comparative study of the neutralino dark matter scattering on nucleon in three popular supersymmetric models: the minimal (MSSM), the next-to-minimal (NMSSM) and the nearly minimal (nMSSM). First, we give the predictions of the elastic cross section by scanning over the parameter space allowed by various direct and indirect constraints, which are from the measurement of the cosmic dark matter relic density, the collider search for Higgs boson and sparticles, the precision electroweak measurements and the muon anomalous magnetic moment. Then we demonstrate the property of the allowed parameter space with/without the new limits from CDMS II. We obtain the following observations: (i) For each model the new CDMS limits can exclude a large part of the parameter space allowed by current collider constraints; (ii) The property of the allowed parameter space is similar for MSSM and NMSSM, but quite different for nMSSM; (iii) The future SuperCDMS can cover most part of the allowed parameter space for each model.
    Last edited by a moderator: May 4, 2017
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  3. Jun 20, 2010 #2


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    What are the physical reasons to expect SUSY in the few TeV range covered by the LHC? Or is this just whishful thinking?

    Of course the MSSM has a lot of free adjustable parameters, more complex theories have even moe, so I wonder how reliable these predictions are.
  4. Jun 20, 2010 #3
    You can always evade parameters by pushing up mass scales, that's no problem, but the first issue I want to address is the fact that SU(5) and SO(10) have been ruled out, because they haven't. MINIMAL models are easy to rule out. The minimal version of the standard model has been ruled out many times before, most recently when neutrino masses were discovered, but also when the charm quark was discovered, and then when the bottom quark was discovered. The point is you just change what minimal means. So, yes, non-supersymmetric, minimal SU(5) is probably ruled out by proton decay, and supersymmetric minimal SU(5) is pretty tightly constrained. But non-minimal models are still very much alive and well.

    Now, for the dark matter bounds at CDMS. 18 months ago, or so, there was a lot of excitement about ``inelastic dark matter'', in papers by Nima Arkani-Hamed and Neal Weiner. The point they made in the paper is that people always try to fit these results to the stupidest model they can make, which doesn't mean much. You can build infinitely many models of low energy SUSY, so what does it mean to say ``The parameter space of these three is cut in half by the current experimental constraints'' ? Don't get me wrong: there is some value in papers like these. But to think that this one experiment rules out low energy SUSY is a bit naive. The LSP could be in the hidden sector, as in the Nima dark matter models. The LSP could also be a gravitino of a few eV mass if SUSY breaking is gauge mediated---these authors have assumed gravity mediation, which means that the nutralino is the LSP. In the case of gauge mediated SUSY breaking, the neutralino is not the LSP.

    So one result by CDMS, while interesting, doesn't say much about SUSY or the MSSM or the xMSSM (where x is a letter of your choosing) other than to put limits on specific parameters.
  5. Jun 20, 2010 #4
    The easiest way to understand the TeV scale as motivation for SUSY is to look at the higgs mass, in a SUSY model. There is a tree level relationship between the higgs mass and the Z boson mass which implies

    [tex] m_{h^0} \lesssim M_{Z^0} \approx 90 \text{GeV}[/tex]

    This looks like trouble until you realize that this expression gets corrected by loops. When you do the renormalization, you find something like

    [tex]m_{h^0}^{\text{loopy}} \sim \frac{m_t}{16\pi^2} \log \frac{m_{\tilde{t}_1}m_{\tilde{t}_2}}{m^2_{t}}[/tex]

    The [tex]m_{\tilde{t}_1},m_{\tilde{t}_2}[/tex] are the mass eigenstates of the top squarks, where we typically define

    [tex]m_{\tilde{t}_1} m_{\tilde{t}_2}\equiv M_{SUSY}^2[/tex]

    So in order to get a higgs mass of the correct size (>115 GeV) we need Msusy ~ 1 TeV or so. Of course, this depends on a lot of things: in the nMSSM there is an additional singlet for the higgs to mix with, so it gets new decay channels, which allows the higgs mass to be smaller.

    Let me also say that there is an independent argument, using the mass of the LSP, which I can't remember. Essentially, you guess that the LSP is some weakly interacting particle (i.e. bino/wino/higgsino mix) with a characteristic weak cross section. Then, you do the non-thermal relic density calculation for cold relics, and to an order of magnitude, you find

    [tex]m_{LSP}\approx 1\tex{TeV}[/tex]

    I want to emphasize that this argument is completely independent from the above argument about the higgs mass, and would be a petty remarkable coincidence.

    Finally, the most compelling argument (or least compelling argument, from the experimentalists point of view) is about grand unification. If you assume a SUSY scale of 1 TeV (i.e. all SUSY particles with masses near 1 TeV), grand unification works very precisely. Shifting this number significantly changes the sizes of the threshold corrections you need at the GUT scale to make unification work. The most important particles in this respect are the gauginos. You typically need them to be in the few hundered GeV to few TeV mass range to get acceptable unification.
    Last edited: Jun 20, 2010
  6. Jun 20, 2010 #5
    Thus far the Fermilab Tevatron has found nothing, nor CDMS-II.

    So if the LHC does not find neutralinos or gauginos or other SUSY partners, and it does have the energy and luminosity and detection sensitivity to explore this range, a null result would mean that GUT does not work very precisely?

    Since a null result at Tevatron hasn't diminished investment in SUSY as a research program,
    What would be a ramification of a null result with respect to SUSY and GUT LHC?
  7. Jun 20, 2010 #6


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    "What would be a ramification of a null result with respect to SUSY and GUT LHC? "

    Not much.

    Ruling out weak scale supersymmetry would leave the primary question of particle physics unsolved --the hierarchy problem. Presumably another answer would be found instead (hopefully). Whatever form that answer takes, likely changes the nature of what the GUT scale looks like and coupling constant unification and so forth. So unfortunately no such strong statement like you want to do, can in fact be made. Further GUT scale physics is compelling with or without SuSY, its slightly neater under the MSSM, but it wouldn't be the end of the world if it wasn't an exact fit (I think I explained this before once).

    Meanwhile SuSy itself would of course still be utilized by theorists and gravity people, since in a very real sense it is like the theorists best toy laboratory to unsolved questions. In general, whenever you have a hard problem in field theory that you want to answer, but don't know how to do it, supersymmetrizing the problem makes it tractable. Qualitatively you get similar physics, except that you can now actually solve a bunch of problems that you previously couldn't.
  8. Jun 20, 2010 #7
    supergravity implies existence of neutralino does it not? What would be the implications to supergravity theories if the future SuperCDMS, with the sensitivity to detect neutralinos over its entire parameter space, does not find neutralinos or other Susy candidates?
  9. Jun 21, 2010 #8


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    Ben answered the question already.
  10. Jul 4, 2010 #9
    The SuperCDMS has the sensitivity to detect neutralinos over its predicted parameter space.



    "SuperCDMS can discover neutralino dark matter"

    Given that previous CDM searches are null, searches for magnetic monopoles, cosmic strings, proton decay, axions, deviations inverse square law, higher dimensions, are all null,

    this is my question:

    if SuperCDMS does not discover neutralinos (nor other independent research programs like LHC) and SuperCDMS has the sensitivity to detect them if they do exist (which the links say they do, and these are lnks from Stanford and Berkeley)

    if Neutralinos do not exist, could SUSY still be true?
    If neutralinos do not exist, could super gravity be true?
    if neutralinos do not exist, could SUSY-GUT SU(5) or SO(10) be true?

    My suspicion is that SuperCDMS like search for proton decay and axions, magnetic monopoles, etc will come up negative.
  11. Jul 5, 2010 #10
    How many times are you going to ask this question? Asking it again won't change the answer.
  12. Jul 19, 2010 #11
    I'm not questioning abou one result of CDMS but a hypothetical combined results of both the newer more sensitive SuperCDMS and LHC, perhaps along with other experiments, out into future 2016.

    Bets have been placed on whether the LHC sees SUSY in any form, and of course the SuperCDMS has the sensitivity to detect LSP neutralino over its entire parameter space, if it really exists. Even if LHC does not have enough energy to produce neutralinos, due to SUSY breaking scale being above LHC energies, the Big Bang certainly did.

    So there are SUSY models such as gauge mediated SUSY breaking in which the neutralino, actively searched for by both LHC and SuperCDMS may not be detectable as it is not stable.
    Both SuperCDMS and LHC are being built now, and reliable data may pour in in 2013-14 timeframe.

    So SUSY could still be true even if the neutralino, the LSP in many models, is not found from either LHC or SuperCDMS (or other detection experiments).
    Does SuperCDMS, LHC, or other direct or indirect dark matter experiments have the sensitivity to detect LSP in these gauge mediated SUSY breaking models?

    Would In the case of gauge mediated SUSY breaking, the LSP, gravitino, be detectable or produce able and if so, how?

    A future super-super CDMS IV could detect the parameter space for gravitino?
    Last edited: Jul 19, 2010
  13. Jul 19, 2010 #12
    If neutralinos are not stable, then doesn't this imply R-parity violation, leading to phenomenology in contradiction to observation (i.e rapid proton decay?)
  14. Jul 19, 2010 #13


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    No and No =)

    It is true I think that an unstable LSP (lightest superpartner) implies rparity violation, but that need not be the Neutralino. Generically the LSP can be anything.

    However, the 'lightest' Neutralino is on the short list of nice candidates for the LSP for phenomenology reasons b/c it is uncolored and neutral and hence a nice dark matter candidate.

    Also it is not true that SuSY+ Rparity violation is ruled out in nature by proton decay bounds. The parameter space is greatly reduced and the resultant models are in so far as I recall rather unlovely, but strictly speaking there are still numerous ways to evade that bound.

    You have to understand the paramater space for even the simplest SuSy scenario (the MSSM) is enormous, going into extended scenarios makes that space very difficult to even categorize.
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