Are Dark Matter Particles Lighter Than We Thought?

The 2010’s is known among the cognoscenti as the Dark Matter Decade. At least among those cognoscenti who are optimists by nature. After years of effort, experimentalists have improved the reach of their detectors to the point where we might be close to directly detecting dark matter (DM) particles — at least if the DM falls into the Weakly Interacting Massive Particle paradigm, or comes close to it for some reason. (Not every dark matter model does; axions are the obvious counterexample.) Jennifer summarizes the current situation in the latest issue of Quanta; some previous updates are from Matt Strassler and Résonaances.

There are two things going on. One is that the experiments, which look for energy being deposited by a (rare but predictable) interaction between dark matter particles and atomic nuclei, are now cutting into large regions of the predicted parameter space for weakly-interacting dark matter. So if the DM is WIMP-like, we have a great chance of seeing it before the decade is out.

The other is that there are already some hints that we have seen something. But those hints are confusing. It’s unclear whether they amount to the first tentative glimpses of most of the matter in the universe, or just statistical fluctuations in the detectors.

Here’s a figure summarizing the situation, adapted from a paper earlier this year from the CDMS experiment.

Dark Matter limits

The horizontal axis is the mass of the DM particle in GeV (where a proton is about 1 GeV). The vertical axis is the strength with which the DM interacts with a proton or neutron. Lines are limits; anything above the line is supposedly ruled out. Colored regions are possible signals, if we optimistically interpret some of the data. The various limits come from CDMS’s Silicon detectors, CDMS’s Germanium detectors, a CDMS low-threshold analysis, EDELWEISS, XENON10, and XENON100. The possible signals come from CDMS’s Silicon detectors, DAMA, CoGeNT, and CRESST.

You can see why the purported hints are confusing. For one thing, they don’t really agree with each other (although they’re not too far apart). More importantly, the possible signals are apparently ruled out by some of the limits! XENON, in particular, seems incompatible even with CoGeNT and CDMS, while practically everything is incompatible with DAMA and CRESST. And no, you’re not reading the labels wrong; the recent CDMS results from their Silicon detectors are quoted both as a limit and as a signal. They see three events, where they would expect to see less than one. So the limits are what we can infer if those events are just a fluke, while the blue region is the best fit if they are actually dark matter.

Even though the various possible detections don’t completely agree with each other, they do share an intriguing property: they are pointing roughly to DM masses in the 5-15 GeV range. That is not where most people would have expected to find the dark matter. The mass isn’t precisely predicted, but typical WIMP models have masses in the 100-500 GeV range. So if this is indeed the dark matter, it’s noticeably lighter than people would have guessed. On the other hand, and in part because it’s not what was expected, it’s also a region of parameter space where the experiments are just a bit less reliable. It’s not too hard to imagine that there are backgrounds we haven’t completely taken into account, which would give the same kind of events that you might attribute to light dark matter. Rest assured that the experimenters are all over this issue.

Finally, there’s something potentially very intriguing about light dark matter. Remember that there’s about five or six times as much dark matter (by mass density) than ordinary matter in the universe. And almost all the mass of ordinary matter is in the form of nucleons (protons and neutrons). So if the dark matter particle is actually five or six GeV, it’s conceivable that there is precisely one dark matter particle per ordinary particle in the universe. And if that’s true, it’s irresistible to imagine that the origin of dark matter is somehow tied to the origin of ordinary matter — more particularly, to the asymmetry of matter and antimatter. If you could cook up a theory (and people have certainly been trying) where the dark particles carried anti-baryon number, the world would be a very interesting place. (Not that it’s not interesting already, but we would have an extra glimpse into just how interesting it is.)


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18 Responses to Are Dark Matter Particles Lighter Than We Thought?

  1. Gizelle Janine says:

    Dare I say I told you so? Not yet. 😀

  2. Albanius says:

    What do you think of the majorana fermion hypothesis?

    The report quotes co-author Robert Scherer:
    “the model makes very specific predictions about the rate at which it should show up in the vast dark matter detectors that are buried underground all over the world. These predictions show that soon the existence of anapole dark matter should either be discovered or ruled out by these experiments.”

    Can you shed any light on what experimental evidence would confirm or refute the hypothesis?

  3. Doc C says:

    Some interesting speculations for a slow news day. And now, back to our regularly scheduled programming…

  4. N. says:

    Dark matter is for the Dark Side, people. I would concentrate on MOND.

  5. JimmyDP says:

    ATLAS and CMS also have some limits on these WIMP-nucleon x-sections via their “monojet” searches, which are much stronger than the direct detection experiments in the low mass region (although some may argue not 100% model independent).

  6. N. says:

    Why. We all know that two of the basic forces are distance dependable. Why not gravity?

    Then again, the space – time density might affect the speed of light – this would account for the red shift, and we would be presented with a whole new cosmology.

    Well, that’s it for my new crackpot theory.:))

  7. Torbjörn Larsson, OM says:

    @N: Since the inflationary standard cosmology has beaten MOND everywhere now, I doubt anyone would “concentrate” on alternates to GR.

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  9. Bob Zannelli says:

    Fascinating post by Carroll. It would be very exciting if within a decade we could learn what a big chunk of the universe is made of. And then there’s dark energy.

  10. zzz says:

    Torbjorn, I guess you dont follow the european job market trend, but postdocs who have published on Mond in the last 5 years get a large rate of permanent jobs relative to the rest of this very competitive field, and the ones who didnt get tenure yet manage to get a continuous stream of postdocs. Surprised? Anyway, to get back to the science, remember that Mond is not about getting rid of any form of dm. What is Mond then, really? Well, it is taking the view that the regularities seen in galaxy dynamics are *fundamental* (i.e. linked to the lagrangian of your new field, call it the dm field if you like) rather than *emergent* (i.e. linked to feedback from the baryons, SN explosions, gastrophysics, etc. on plain simple stable neutral weakly-interacting dm particles). That’s all. That does not mean the evidence for dm on large scales, such as the cmb or galaxy clusters, should be ignored. It should not, and cannot, be ignored. But if you cook up a dm model that reproduces the observed Mond relations in galaxies from the fundamental interactions of your dm particle with baryons, you’ve got a Mond theory. If you’ve got a new field which behaves as particle dm on the largest scales but has new fundamental interactions leading to the Mond phenomenology on galaxy scales, you’ve got a Mond theory. That said, I of course believe it is useful to indeed try to find the solution from the “emergent” front, concentrating on feedback etc., I’d be happy if that works out, but for the moment it doesnt. Those who tell the contrary are either ignorant or dishonest. That’s why the “fundamental” front is also explored, aka Mond.

  11. Meh says:

    I feel like MOND is experiencing the same resistance that String Theory and black holes did in their early days. It’s important for those who support MOND to not become pricks about it. Lets not make everyone go “jesus, here we go with another belligerent crackpot” any time MOND is mentioned. I think it’s safe to say that General Relativity may need some slight amendments one day, since we didn’t really understand the dynamics of galaxies at the time. But until then…be patient and let the data speak for itself.

  12. JimmyDP says:

    @zzz MOND, or Modified Newtonian Dynamics, is a modification of our theory of gravity, weather that be GR or newtonian mechanics. As soon as you start adding new fields and particles you’re not working on a MOND anymore, at least not by the accepted definition of that term.

  13. zzz says:

    JimmyDP, modifying GR most often imply adding new fields. They could be massive, too. I can assure you that my definition of MOND is the one which is understood and accepted by researchers working on it today. I am one of them. Someone should some day make this wikipedia page clearer. Unexplained regularities in galaxy dynamics is the one and only empiricist motivation behind this research. If semantics is not clear, the problem is that we run into sometimes unnecessarily hostile arguments because both sides of the debate dont necessarily understand what the other side is talking about. I should add, to come back to the topic of this thread, that if a 5-15 GeV plain simple WIMP was conclusively detected, I would be the first to admit that the “fundamental” approach was not the right one, and that regularities in galaxy dynamics should indeed be understod in the “emergent” way.

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  15. N. says:

    Gastrophysics. I love that.

  16. Anton Szautner says:

    Sean’s remarks in that last paragraph resonate most strongly with me. The anti-baryon=dark matter hypothesis is an idea I originally toyed with back in the ’70s: matter-matter interactions occur between particles on the same ‘brane habit’, where attractive forces produce concentrations which deepen local gravitational fields according to nominal theory. Matter-antimatter (the latter alias ‘dark matter’) interactions on the other hand would occur between particles on opposed but adjacent parallel branes (which have the same origin in the common big bang) but at arbitrarily long distances or times from the origin the interactions between them deepen the gravitational potential wells by attractive interactions at right angles operating between the gap separating the opposed brane habits rather than just by concentration via attraction between individual particles inhabiting the same brane habit. I called them ‘temporal habits’ back then, where the word ‘habit’ denotes habitat in the context of time, wherein one habit is distinguished from the other as matter-antimatter domains with their arrows of time pointing in opposite temporal directions, yet both together in concert growing in spatial expansion away from their common ‘big bang’ origin as both obey the 2nd Law; the term ‘brane-habits’ better emphasizes the potential field of (attractive) interaction between them.

    In that scheme there need be no exotic particles such as wimps that co-inhabit our temporal or brane habit interspersed with ordinary matter; rather, ‘dark matter’ can be identified as ordinary baryons with opposite baryon number inhabiting an antimatter brane ‘located on the next floor’ (so to speak) which provide the apparent extra gravitational potential each side would induce in the other at large >galactic scales, without the particles in either ever coming in direct contact on the same brane habit. It also provides for a geometry that accounts for the matter-antimatter asymmetry apparent to observers who must necessarily be confined to one or the other habit. The scheme may provide insights into other problems.

  17. John Duffield says:

    Dark matter doesn’t have to consist of particles. And you don’t need MOND to “modify” gravity. The expansion of the universe does that. Galaxies are gravitationally bound. The space between the galaxies expands, but the space within does not. Conservation of energy surely tells you that the result is a gradient in spatial energy density. That’s what a gravitational field is. Einstein showed the way with “the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy”. There’s no WIMPs in there. But sigh, it seems that relativity has always been the Cinderella of modern physics. She has some ugly sisters.

  18. David Redfrost says:

    Something to ‘think’ about- maybe a better title for this piece would have been:
    “Are dark matter particles lighter than thought?”