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MDM (a minimal extension of standard model) and PAMELA

  1. Sep 3, 2008 #1


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    A proposed MDM goes beyond the standard model in a minimal way, so as to produce a candidate for dark matter. Here is story from Nature News:

    Here is a key excerpt:

    ...It now seems that some physicists have taken matters into their own hands. At least two papers recently appeared on the preprint server arXiv.org showing representations of PAMELA's latest findings (M. Cirelli et al. http://arxiv.org/abs/0808.3867 ; 2008, and L. Bergstrom et al. http://arxiv.org/abs/0808.3725 ; 2008). Both have recreated data from photos taken of a PAMELA presentation on 20 August at the Identification of Dark Matter conference in Stockholm, Sweden.

    "We had our digital cameras ready," says Marco Cirelli,...

    The ethics of Cirelli breaking the embargo imposed by Nature on the PAMELA satellite data is not the topic here. I disapprove, as perhaps most people do, Cirelli's action. That said, the issue here is what about this proposed MDM extension of the Standard Model?

    Does it predict an excess of positrons, over what would otherwise be expected, arising from dark matter particle collisions and dark matter annihilation? Does this fit the PAMELA data---rather a lot of positrons observed by a satellite in orbit. The article by Cirelli reproduces preliminary data---so you can see the plots. Here is the article:

    Minimal Dark Matter predictions and the PAMELA positron excess
    Marco Cirelli, Alessandro Strumia
    To appear in the proceedings of the Identification of Dark Matter conference (idm2008), Stockholm, 18-22 August 2008
    (Submitted on 28 Aug 2008)

    "We present Minimal Dark Matter and its univocal predictions for Dark Matter observables. During the idm2008 conference, PAMELA presented preliminary results showing an excess in the positron fraction: we find a good agreement, with a modest astrophysical boost factor."
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  3. Sep 3, 2008 #2


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    Here is a key quote from the Cirelli paper:

    ...The Minimal Dark Matter (MDM) proposal [2] originates from different motivations: focussing on the Dark Matter problem only, we add to the SM the minimal amount of new physics (just one extra EW multiplet c) and search for the minimal assignments of its quantum numbers (spin, isospin and hypercharge) that make it a Dark Matter candidate without ad hoc extra features, and without ruining the good features of the SM. As detailed in the following section, we do find one optimal candidate, and we here focus on it. Its only free parameter (the DM mass) is fixed from the cosmological DM abundance, so that any DM observable can be univocally predicted...

    He is saying that MDM differs from other extensions of the Standard Model in that, according to him, it goes beyond the standard model only as much as needed to get a DM candidate.
    And they cite this earlier MDM paper:
    Minimal Dark Matter
    Marco Cirelli, Nicolao Fornengo, Alessandro Strumia
    16 pages, 2 figures, published in Nucl. Phys. B753 (2006) 178-194
    (Submitted on 8 Dec 2005)

    "A few multiplets that can be added to the SM contain a lightest neutral component which is automatically stable and provides allowed DM candidates with a non-standard phenomenology. Thanks to coannihilations, a successful thermal abundance is obtained for well defined DM masses. The best candidate seems to be a SU(2)_L fermion quintuplet with mass 4.4 TeV, accompanied by a charged partner 166 MeV heavier with life-time 1.8 cm, that manifests at colliders as charged tracks disappearing in [tex]pi^\pm[/tex] with 97.7% branching ratio. The cross section for usual NC direct DM detection is [tex]\sigma_{SI} = f^2 \times 1.0 \times 10^{-43} cm^2[/tex] where f ~ 1 is a nucleon matrix element. We study prospects for CC direct detection and for indirect detection."
    Last edited: Sep 3, 2008
  4. Sep 5, 2008 #3


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    To go off topic a little: Nature has no problems with stuff being posted on arXiv first. The main problem would be if the data was used without its owners' permission. However, they did make the data public at a conference, and the conference presentation was apparently acknowledged.
  5. Sep 5, 2008 #4


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    Hi atyy,
    yes I saw where Ned Wright (no less) posted to point that out on Peter Woit's blog. I don't mind if we go off the nominal topic at this point, because in this thread there is not enough interest in discussing the main stuff. Personally I like Cirelli's approach of trying a barebones minimal extension, just enough to come up with a dark matter candidate. I was hoping someone (could you help?) would find some flaws in what Cirelli is doing, or show that it is not unique---that there are other similar approaches proposed by other people. I don't know enough about this.

    what about MDM? what's wrong with it or right with it? anybody have ideas?


    atyy about Nature magazine, I take your point (and Ned Wright gave an example of a really interesting Xray observation of the start of a supernova, pure serendipity). But I can't say I understand Nature mag's policy. If something goes up on arxiv, then any science journalist on his toes could spot it and potentially scoop Nature. So what are they embargoing?
  6. Sep 5, 2008 #5


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    Theres nothing wrong with minimal extensions, but there are many possibilities and variants, and its relatively easy to write them down. Most of them are already in computer algorithms anyway. How you discrimate between them is of course up to you.

    The papers main interest to proffessionals is in the data perse
  7. Sep 5, 2008 #6


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    obviously there are other minimal extensions of the standard model. we've been hearing about minimal supersymmetric extension for quite a few years----that's not the issue.

    I would like to hear about another like MDM, namely a minimal extension designed simply to produce a Dark Matter candidate. no other frills.

    Quite possibly there are a bunch. I'd like author's name or an arxiv link so I could see what Cirelli's competition in just this category looks like.

    So here's the question on the table, expanded a little. It has two parts:

    1. What about Cirelli's MDM? What's good or bad about it (as a proposal to make the slightest enlargement needed to get a dark matter candidate----the minimal dark matter model idea)?

    2. What's a link to some other minimal dark matter (MDM-type) proposal? So we can compare.
  8. Sep 5, 2008 #7


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    I'm sure there's plenty of interest in discussing this! It's just that it's difficult to make sensible statements in this field without thinking hard about it, so most of us interested observers (I'm a neurobiologist) can't reply immediately to your posts. But yes, thank you for all your updates on the various threads - because you really pick up stuff that is interesting rather than cranky! I follow your postings, even if I don't know how to reply, and I bet I'm not the only one!:smile:
  9. Sep 11, 2008 #8
    Hello, I am new to this forum (I found it typing my name on Google).
    Before posting anything I wonder whether authors of the works that are discussed here (like me) are allowed to chime in or it is preferable to avoid interference.
    Marco Cirelli
  10. Sep 11, 2008 #9


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    Dear Sir,
    In my opinion, this forum, as many others, is in vast need of more informed commenters.
    Please post if you have anything to add, correct, or contribute in any way.
    I am sure the majority of readers agree with me, but I don't run things around here.
    Jim Graber
  11. Sep 11, 2008 #10
    I am not a moderator and cannot tell you what forum policy is. What I can tell you however is that we have had the author of an Arxiv paper (Garrett Lisi) come in to discuss his work in the past and there was no problem with it.

    I for one would be extremely interested to hear any comment on your MDM paper which you may have.
  12. Sep 11, 2008 #11


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    The authors of the work are certainly welcome to comment. We just require that it isn't based on speculative work that hasn't been published, that's all.

    Thanks, and welcome.

  13. Sep 11, 2008 #12


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    It is definitely preferable to chime in! We feel honored. I, and probably others, are hungry for anything you can tell us about MDM, possible observations of dark matter, and any thoughts you have about other possible candidates. Please interfere as much as you have time for!
  14. Sep 14, 2008 #13
    Thanks for the welcoming!
    So, about the uniqueness of the MDM proposal: I will comment in two ways, depending on which meaning one gives to the word "unique".

    Q. Is the MDM model a unique approach, not tried by anyone else?
    A. No, there are certainly other minimalistic constructions that have been proposed recently. Let's loosely define minimalistic as a model in which DM is the central problem to be addressed and does not come from a whole new sector (there are probably better definitions). An example that comes to my mind is the Inert doublet model (see e.g. 0712.4206 for a short presentation).

    Q. What is the MDM approach, exactly?
    A. The approach of the MDM proposal tends to be even more minimal than the other constructions mentioned above, in the sense that we ask ourselves: "is there a way to obtain a DM candidate by just adding one single multiplet (a set of particles) to the Standard Model, banning every extra feature (like ad hoc symmetries that guarantee the stability or other auxiliary particles)?"

    Q. Does the MDM approach lead to a unique answer, then?
    A. Yes. You go home, you do your exercises and it turns out that the answer to the question you asked yourself is: "yes, a quintuplet (set of 5 particles) with hypercharge = 0 and mass of about 10 TeV is the only solution that fits". Now, of course if you relax the stringent constraints that you imposed to yourself, then other possibilities emerge. For instance, a doublet (set of 2 particles) works if there is an extra symmetry that keeps it stable. I am calling this "less minimal" but it is a matter of definitions and maybe tastes.

    About the good or bad features of MDM. I'll say some that come to my mind. Beware: of course I am biased...
    GOOD: the DM candidate is automatically stable, no ad hoc symmetries like R-parity to keep it from decaying.
    BAD: no gauge coupling unification, no hierarchy problem solution (i.e. no explanation of why the Higgs should have a light mass).
    GOOD: predictivity, we can compute every signal with no dependence on free parameters.
    BAD (or better: unlucky): the candidate is too heavy to be studied at the LHC.
    GOOD: for the moment it seems to agree with all available data, in particular the predictions for indirect detection in positrons and antiprotons (PAMELA) are met.
    BAD: to be more precise, these predictions are met if the boost factor is of order a few (up to 10). The boost factor is the enhancement in the predicted rate that dark-matter particles in the Milky Way, on average, would annihilate. Boost = 10 means that you need ten times more annihilations than expected on average. A boost factor is expected to be produced by the clumpiness of the DM galactic halo (if there are clumps, the DM density will be higher and they will annihilate more efficiently). But large values are unlikely.
    GOOD: most other models need boost = 100 or 1000 or even more...
    etc etc etc

    Thanks for your attention. I am happy to discuss further if I have been unclear.
    Cheers, Marco (marco.cirelli@cea.fr)

    PS for the "ethics" discussion, I prefer not to reiterate it here. I think we did nothing wrong and the issues are elsewhere. There are comments left http://www.andrewjaffe.net/blog/science/000368.html", for instance, that express my opinion.
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  15. Sep 14, 2008 #14


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    Your discussion of the MDM is indeed clear and helpful. Thank you. I cannot think of any question which you have not answered, but I hope others here will have questions and that there can be further discussion. In the meantime I propose that you name the particle after me. Since my name is Marcus, you may call it the marcon. :biggrin:
  16. Nov 1, 2008 #15


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    Christine Dantas just initiated a thread here at Beyond forum that relates to the PAMELA data:

    There was an early presentation of PAMELA data in late August, and a subsequent proposal by Marco Cirelli of an extension to the Standard Model to include a dark matter particle conformable with the observed positron excess. That is what this thread is about. Now, two months later, the PAMELA group has made their official release and publication. There is considerably more discussion, but I don't know that anything essentially new has come out. Any comment?
  17. Nov 1, 2008 #16
    Hi Marco---

    I just realized that this thread existed. I'm glad to have authors of papers commenting here :)

    Where does the scale 10 TeV come from? Is this just there to fit the LEP data (i.e. effective operators suppressed sufficiently)? It looks like you varied the mass of the dark matter to fit the positron spectrum---is this correct?

    Sorry for the naive questions, and I hope you come by again to comment!
  18. Nov 1, 2008 #17

    Can anyone explain what is the difference between MDM and the "superunified theory of dark matter" that is receiving some discussion in the aftermath of the new CDF results?
  19. Nov 1, 2008 #18
    In a word: supersymmetry.

    In three words: more new ****.

    The MDM model assumes only the minimal amount of new stuff and nothing else.

    The Weiner/Nima paper seems to introduce a new hidden sector gauge group under which dark matter interacts, and only makes sense in the context of SUSY.

    The Weiner/Nima model has a hidden sector gauge group which is higgsed just like our SU(2)xU(1). The new higgs needs to have a small mass (just like our higgs), which needs supersymmetry to protect its mass.

    If you don't like supersymmetry, then you have to take it out and explain why we have light scalars. This can (in principle) be done, but I'm sure it wasn't Nima/Weiner's intention. But I'm not Nima/Weiner, so I don't know :)
  20. Nov 2, 2008 #19
    Hi again,
    the 10TeV scale comes from matching the relic cosmological abundance of the DM particle. The mass of the particle is the only free parameter of the model (once one has assigned the quantum numbers under the SM forces) and it enters in the computation of the relic abundance in the early universe. Requiring that the relic abundance is the one observed in cosmology fixed the mass.
    By the way, it is not surprising that this scale is around TeV: it is a general property of WIMPs (Weakly Interacting Massive Particles) that particles with weak interactions and a typical TeV-ish mass give the correct relic abundance. MDM falls in this class.
    Then, the fact that it is actually multi TeV (even 10!) comes from some slightly more complicated effects (namely, large multiplets have larger "co-annihilation" cross sections, and the so-called Sommerfeld corrections).

    Such a large mass guarantees that effective operators are suppressed, so that, yes, LEP electroweak precision tests are passed. But it is not there for this purpose.

    No, we did not vary the mass of the particle to fit the positron spectrum from PAMELA. Our mass is fixed by the argument above, so we could only compare with the spectrum and see what happens. The freedom we have in this fit is instead on the overall normalization, related to the boost factor and other astrophysical uncertainties.

    PS I approve of course the "new name" of the particle...
  21. Nov 2, 2008 #20


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    So you actually have decided to name it the marcon! I must say that of course I feel profoundly honored by your choice. :wink:
    Last edited: Nov 2, 2008
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