Current Hints of Dark Matter (4/13)

Here’s a brief overview of six hints of dark matter — signals (two in the sky, and four underground) that just might be from dark matter particles doing something interesting — that are being explored, as of April 2013.  One or two of them might turn out to be true, but not all six can be, since they contradict each other.  And this shouldn’t worry you, because this situation is perfectly normal with forefront science; research is difficult, and most hints of something exciting turn out to be mirages — statistical flukes, previously unknown quirks of a measurement technique, or even pure mistakes.  With the Higgs particle, for instance, we had several false alarms (one as recent as July 2011) before the alarms finally rang true in July 2012.  So we have to be patient and cautious, yet hopeful; discoveries are rare, but they do happen.

Dark Matter Overhead

First, there is a hint from the Fermi satellite (pointed out by Christoph Weniger) that photons [particles of light] of a particular energy (about 135 GeV, i.e. about 143 times the mass-energy [i.e., the E=mc2 energy] of a proton) are streaming out of the center of our galaxy. This could potentially be a signal of dark matter particles (which, moving slowly around our galaxy, would be especially numerous at the galactic center) in the process of colliding, annihilating, and turning into photons.

Click here for more detailed explanation of how this works.  Briefly: Energy conservation assures that the energy in two annihilating dark matter particles (which is mostly in mass-energy, since dark matter particles move slowly inside a galaxy) is converted into the motion-energy of two photons — and thus the energy of each photon is equal to the mass of the dark matter particle times c².

Should we worry that this signal isn’t what it seems to be?  A minor issue is that a standard “WIMP” (a Massive Particle that Interacts with matter via the Weak nuclear force — roughly speaking) couldn’t produce this signal without giving other signals that we would also see (such as lots of photons at lower energy.) But WIMPs are more popular than they deserve to be (see, for example, this paper), and other types of dark matter particles, ones that various particle theorists have imagined over the years, could potentially do the job.

A much more serious concern is that not only is the signal coming from the galactic center, it also appears to be coming from the limb of the earth and maybe from the sun. You wouldn’t expect that from dark matter annihilation. And the fact that this signal shows up in strange places where it isn’t expected might mean that the whole business actually reflects a subtle problem with Fermi’s photon detector. Nobody knows for sure at this point.

Here’s another example. The AMS experiment attached to the International Space Station recently claimed to have made a big “discovery” (though actually, as most press articles forgot to mention, what they really did was confirm something that the PAMELA experiment already discovered back in 2008 — indeed I don’t know why Dennis Overbye didn’t emphasize this more in his New York Times article on AMS, given that he himself had written about the PAMELA experiment’s discovery back in 2008.)  What PAMELA discovered, and AMS confirmed and explored in much greater detail, is that in outer space, one finds an excess, compared to what one would have expected, of high-energy positrons (the anti-particles of electrons). The extra positrons are observed to have energies from 10 GeV on up to at least 350 GeV, where AMS’s data currently fades out.

Now maybe — maybe — those positrons are from the annihilation of dark matter particles! But if so, they can’t be the same type of dark matter particles that FERMI might be seeing from the galactic center. Any dark matter particles responsible for AMS’s signal would have to have mass above 350 GeV/c², in order to produce positrons of 350 GeV, while if there are dark matter particles making FERMI’s photons, they’d never give positrons with energies above 135 GeV.  [Again this is just energy conservation; if your two annihilating dark matter particles have mass of 135 GeV/c² each, and are moving slowly enough that their motion-energy is small, electrons and positrons produced in the annihilation can't have energy above 135 GeV each.]  So FERMI and AMS can’t both be seeing effects of dark matter; at least one of them is seeing something else.

As was suggested all the way back in 2008 (and as the AMS people were careful to admit), the positrons PAMELA saw then, and that AMS sees now, might be created by an astrophysical effect, such as a nearby pulsar (a rapidly spinning star with a powerful magnetic field, which can serve as a natural particle accelerator and become a source for extra electron-positron pairs.) And as everyone has known since 2008 (and as the AMS people were careful not to admit), a simple garden-variety neutralino from supersymmetry (or any other WIMP) could never give you such a big signal unless there was a previously unknown force able to enhance the annihilation rate. And even then, you wouldn’t get these positrons without getting other signals we also should have seen, unless you imagine this is from a very non-garden-variety type of dark matter — let’s call it a “jungle-variety” type of dark matter, not a bad name, because a whole host of particles are hidden away and hard to see. Jungle-variety theories are really cool, but the dark matter particles in such theories are not the simple supersymmetry WIMPs that the press articles about AMS all mentioned. (Talk about mixing your applets and your orangoutangs...)

Dark Matter Underfoot

Let’s move on! Anyone remember DAMA (now DAMA/LIBRA)? They’ve been claiming evidence of dark matter for over ten years! And they definitely have a real signal of something! Maybe it’s dark matter, and maybe it’s not.

You see, one clever way to look for dark matter is to let it find you. Just put a chunk or a vat of a carefully chosen and purified material in a mine deep underground. (Going deep underground drastically reduces effects from cosmic rays, high-energy particles from outer space.) Since dark matter must go right through ordinary matter, rarely leaving any trace, dark matter particles stream right through the rock and down into the mine and into the chunk or the vat. And if you’re very very patient, one of those dark matter particles might hit an atomic nucleus inside your material, and that little love tap may be just loud enough for you to detect it, if you’ve designed a very clever experiment. This is what DAMA, XENON, CoGeNT, CRESST, CDMS, and a host of other experiments are doing and have been doing for a long while now.

In an underground mine, dark matter passing through the rock sometimes traverses the chunk of material that makes up a dark-matter detector.  Extremely rarely, a dark matter particle might hit an atomic nucleus.  The recoiling nucleus will produce a tiny but detectable amount of sound, light, and electric charge. The problem is to distinguish this from various effects from radioactivity.

In an underground mine, dark matter passing through the rock sometimes traverses the chunk of material that makes up a dark-matter detector. Extremely rarely, a dark matter particle might hit an atomic nucleus. The recoiling nucleus will produce a tiny but detectable amount of sound, light, and electric charge. The problem is to distinguish this from various effects from radioactivity.

This is harder than it sounds. Radioactivity — the process by which an atomic nucleus changes from one type to another, spitting off a high energy particle or two along the way — can mimic the effects of a dark matter particle. [A process that mimics your ``signal'' --- the thing you're trying to detect --- is called a ``background''. For some conceptual discussion of signals and backgrounds, you can read the first part of this otherwise outdated article.] Backgrounds in this dark matter detection business are often much larger than the signals, and the experimenters need to understand all of the possible backgrounds very, very well if they’re going to detect something so small.

But now — getting back to DAMA specifically — here’s something you can do that’s diabolically clever. As the earth moves round the sun during the year, its velocity relative to the average velocity of the dark matter particles is changing. It’s similar to how, if you bike around a circular track on a windy day, sometimes the wind is brutally in your face, and sometimes it is comfortably at your back. So just as the strength of the wind changes as you circle the track, the speed of any dark matter “wind” changes strength during the year. And if the probability that a dark matter particle interacts with a nucleus depends on the relative velocity between the two (as is true in many examples of what dark matter might be), then the number of dark matter collisions that an experiment will measure will vary up and down, cycling once a year. So rather than just looking for signs of a few collisions, which might just be radioactivity that you’ve misinterpreted, perhaps you should look for variations of the number of collisions during the year!  If you can convince yourself that radioactivity and other backgrounds themselves can’t cycle during the year, then any oscillation of this type is a smoking gun for dark matter.

Just as a biker in a strong wind feels an extra-strong wind while biking into the wind, and a weaker wind while biking in the other direction, so the earth, as it orbits the sun, moves more or less rapidly relative to the nearby dark matter particles as the year goes by.  This can lead to a rate for dark matter collisions that cycles up and down once a year.

Just as a person biking into the wind feels a stronger wind than one heading the other direction, so the earth, as it orbits the sun once a year, moves first more and then less rapidly relative to the sea of dark matter particles in the sun’s vicinity. This can lead to a rate for dark matter collisions that cycles up and down once a year.

Unfortunately, although this sounds great, the backgrounds can in fact cycle during the year after all, perhaps because minor temperature changes cause more or less radioactive gas to float around inside the mine, or something like that. So although DAMA/LIBRA’s data definitely shows an oscillating number of candidate dark-matter/nucleus collisions, it’s still not entirely clear this is due to dark matter. So far no one’s been able to verify their signal, but no one’s been able to show it’s a false alarm either.

DAMA/LIBRA is not alone. Over the past year or so, the CoGeNT experiment reported that they see an excess of possible collisions too, which, like DAMA/LIBRA, have a rate that oscillates up and down during the year.

That’s not all. Last year, the CRESST experiment also reported seeing a bunch of candidates for dark-matter particles hitting an atomic nucleus inside their detectors. There are several possible effects that could give candidates of this type, but — they say — when they add up all of those effects, they should see something like 42 candidates; but in fact they see 67, an excess of about 4 standard deviations, which is very strong evidence that something’s amiss.

Finally, the newest hint: the CDMS experiment has reported that they have seen three candidates for dark matter collisions in their chunks of silicon. [They have both silicon and germanium detectors; the new result is for data taken some years ago in the silicon chunks. Since a silicon nucleus is much lighter than a germanium nucleus, silicon is more responsive than germanium to a collision with a light-weight dark matter particle.] This is really very interesting! But, as they themselves are careful to say, hardly definitive yet.  What’s almost certain is that it’s not from a background they expected.  It’s not so obvious at first glance; the backgrounds they know about should only have generated about half an event on average, and the probability of getting these three events is about 5% — not very unlikely at all, when you consider how many different unlikely things can happen in an experiment. But when they account for the energies of these candidate collisions, the probability drops to 0.2%. Now that’s starting to get serious! But remember: all this means is that either (a) they’ve discovered dark matter, or (b) they’ve discovered a previously unknown background that gives them a false signal.

Now when we take these last four experiments all together, the news is both good and bad. The good news is that all four of these experiments — DAMA/LIBRA, CRESST, CoGeNT and CDMS — are consistent with a dark matter particle that is somewhere in the vicinity of 10 GeV/c², more or less.

The minor bad news is that the four measurements are inconsistent with each other; the interaction probabilities the experiments infer, for a given dark matter particle’s mass, aren’t all the same, and differ by as much as a factor of ten.  This is illustrated in the figure below (taken from the CDMS paper), showing that the four allowed bands associated with the four experiments’ observations don’t generally overlap with one another.  So this would suggest that at least two of the experiments must be false alarms.

The various hints of dark matter discussed here (annotated version of plot given in CDMS's recent paper.)  The plot shows allowed regions  and disallowed regions (with 90% confidence) as a function of the dark matter particle's mass and interaction rate with matter.  DAMA/LIBRA, CRESST, and CoGeNT allowed regions are shown in yellow, brown and pink.  The new CDMS result is in light and dark blue; the black star is the best guess but I don't advise paying too much attention to it.  Notice there is no point where three of the four allowed regions overlap.  Meanwhile XENON10 (a special low-energy analysis, under review) and XENON100 exclude all regions above the light green and dark green lines, including all four of the other experiments.  Signals seen by FERMI and by PAMELA/AMS are for much heavier dark matter particles.

The various hints of dark matter discussed here (annotated version of plot given in CDMS’s recent paper, http://arxiv.org/abs/1304.4279.) The plot shows allowed regions and disallowed regions (at 90% confidence) as a function of the dark matter particle’s mass (horizontal axis) and interaction rate with ordinary matter (vertical axis). DAMA/LIBRA, CRESST, and CoGeNT allowed regions are shown in yellow, brown and pink. The new CDMS result is in light and dark blue; the black star is the best guess, though I don’t advise paying too much attention to it. Notice there is no point where three of the four allowed regions overlap. Meanwhile XENON10 (a special low-energy analysis, under review) and XENON100 exclude all regions above the light green and dark green lines, including all four of the other experiments. Some assumptions go into the precise position of these lines and regions.  For these assumptions and the meaning of other lines, see the CDMS paper.  Photon signals at FERMI and positron signals at PAMELA/AMS suggest much heavier dark matter particles.

The major bad news is that there’s another experiment which should (naively) be more sensitive to this type of dark matter particle than any of these four experiments. I refer to XENON100. For most of these signals, XENON100 should have seen lots of candidate events, typically tens or more. But they see only two. And so, nominally at least, all of these signals are ruled out by XENON100, and/or by a special analysis of the data from its predecessor, XENON10. Perhaps it can be argued that the CoGeNT and CDMS experiments’ signals are just barely ruled out, and so perhaps they’re still really worth taking seriously.

Yet one more sobering fact (NYU’s Neal Weiner emphasized this in his talk last week in Princeton) is that in all of these underground experiments, a failure to account for a small background will typically show up as a few extra low-energy collision candidates, which will then closely resemble what you’d expect for a low-mass dark matter particle. In other words, lightweight dark matter is what an oops! will look like.

As University of Chicago professor Juan Collar himself (leader of the CoGeNT experiment) said at a conference at CUNY’s graduate center in New York City about two years ago, the saga of dark matter searches is likely to be a long story of discovering one unexpected background after another — and it’s a story which could go on quite a while before dark matter is actually found, if ever, by one of these experiments. Indeed, this is reflected in the many false alarms that we’ve seen in recent years. [Interestingly, Collar stopped talking this way when CoGeNT started seeing a signal that could be interpreted as dark matter. But we remember what you said, Juan. We remember.]

Meanwhile, this type of puzzle is what theoretical physicists live for. A conundrum!  A challenge! Invent a theory of dark matter that is such that CDMS and CoGeNT can easily observe its effects but XENON100 cannot! The experiments work differently — CDMS and CoGeNT are made from chunks of silicon and germanium respectively, while XENON100 uses — surprise! — a vat of xenon. There are already many papers on the matter.  The most likely outcome is that XENON100 is right and CDMS and CoGeNT are seeing backgrounds of some type. But perhaps it will turn out otherwise.

To sum up: we have at least six hints of dark matter, mostly inconsistent with one another. The new CDMS hint is roughly consistent with CoGeNT; but if they’re both really seeing dark matter, why didn’t XENON100 see a big signal? Well… all of these experiments are working hard to improve their methods and their measurements, so if any of these hints are really signs of dark matter, we should start seeing more impressive evidence within a year or so.

47 responses to “Current Hints of Dark Matter (4/13)

  1. I like very much your statement about inventing a theory to ” swallow ” a contradiction !! but this might mean that we will never be able to know reality .

    • Torbjörn Larsson, OM

      Yes, because competition between theories is not a factor…. oh, wait. it is! Seems gravity is a real process after all.

  2. Nitpick re. seasonal variation of dark matter: The most diagnostic difference is not a change in the rate of events, but a change in their energy spectrum. When going “into the wind” one will see fewer low energy events and more high energy events, and vice versa.

    During my abortive stint as a postgrad, I had the dubious pleasure of reading one of DAMA’s seasonal variation conference papers and was less than impressed. They did appear to exclude H0, but presented that as evidence of confirming H1, which they had certainly not done. Tellingly, the energy spectrum of the discrepancy did not match predictions. To put it another way, they appeared to have found *something*, but not what they implied they had found.

  3. Thank you, very instructive. As a layman I found myself terribly misinformed by mainstream media (European) again. The good thing: this website has done a brilliant job in developing my “sixth sense” for bad science journalism.

  4. Hi Matt,

    Can you tell me why no one seems to talk about neutrinos being part of the background in the detection of dark matter (or is it so obvious it doesn’t need to be said?) Without looking any thing up, I thought that neutrinos from ‘outer space’ were detected underground for the same reasons that dark matter is looked for there. I think they used heavy water. But wouldn’t neutrinos interact with silicon or germanium? Puzzled.

    • Neutrinos do need to be accounted for. But relatively speaking, they are a small effect for now. Eventually, when the experiments become more sensitive, they will become the dominant background, and the limiting factor in most approaches to direct dark matter detection.

      I think this paper is relevant: http://arxiv.org/abs/0706.3019

  5. Marshall Eubanks

    I myself like the seasonal (and illegal) dumping of radioactive waste by the Mafia as an explanation of the DAMA seasonality. Backgrounds can indeed be unexpected…

  6. Tienzen (Jeh-Tween) Gong

    This is a very good summary for the current status of the dark matter research.

    Yet, I personally think that this is more of a linguistic issue than a physics one. What is the definition of dark matter in those researches?

    As you have showed that the mass of [(u, u, d), the ID of a proton] accounts for only 5% of its mass. That is, the 95% of proton’s mass is “dark”, not clearly identified by its ID. Linguistically, there is a “dark mass” for proton.

    The Planck data should be taken as a “fact”, that is, 95% of the mass (energy) of the universe is dark. And, this Planck data can be “numerological” described with an iceberg model. Of course, “numerology” can be quickly discounted as non-science, but it can still be a hint of how a physics frame should be like. Especially, when this numerological description works on more than one physics fact, that is, for both the proton’s mass and the Planck’s data.

    For the iceberg model, it goes way beyond as numerology but has ontological meaning, that is, “all” existences must be iceberg-like.
    a. All relativism is a subset of the iceberg-model. There cannot have a beauty if no ugly; no long if no short, etc. .

    b. The Empire State Building cannot be a reality if there is no big earth and big air space around it. That is, its mass (existence) cannot be counted as the mass of its visible buildings. It carries a big “dark mass”.

    c. Our lives (yours and mine) are chain-locked to big space with our digest track as that chain. When that track is severed from that outside space, our live- existence ends. Thus, our body mass is a bit larger than the number on a scale.

    Of course, someone can choose to view all the above as the non-science nonsense. But, when a particle theory is exactly as this iceberg-model, the above becomes the criteria for all theories.

    Thus, the issue is whether we need any “additional” dark mass to account for either proton’s or the Universe’s mass. Are we looking for dark mass to fit the Planck’s data? Or, are we looking for dark “particle” which makes up both proton’s and the Universe’s dark mass?

    There is a significant difference between the “dark mass” and the “dark particle”.

    • It’s not a linguistic issue. One should not confuse the name for the thing that it signifies. “dark matter” is a name for that thing which dominates the mass of every galaxy yet does not produce any electromagnetic radiation and does not have a significant probability of colliding with ordinary matter.

      • Tienzen (Jeh-Tween) Gong

        The term of “linguistic issue” is a heavily-loaded one, but I will not discuss it here. For an unloaded version (might be similar to your usage), it simply tries to point out the difference between two terms, the dark mass vs the dark matter.

        As the dark mass (including the dark energy) is very much a fact for this universe, it (dark mass) nonetheless needs not to be the result of the dark matter, as some other models can describe it. If we make “dark mass = dark matter” (while not “100%” accurate in physics), it will be linguistically wrong in addition to being wrong in physics.

      • Tienzen (Jeh-Tween) Gong

        If I did not make my view clear in the previous comments, I am trying to clarify it one more time.

        a. Dark mass is a “Fact” in physics.

        b. Dark mass is an essential part of every “existence (such as proton, life or the universe)”, demanded ontologically.

        c. Dark matter is a speculative idea, trying as an explanation for dark mass in some models.

        d. Dark mass needs no dark matter, as some other models account for dark mass without the need of any kind of dark matter.

    • Carl 'SAI' Mitchell

      We know what most of the proton’s mass is (gluons & quark-antiquark pairs). We don’t know what most of the mass of the universe is (galaxies rotate far faster than the amount of visible matter should allow. Likewise for galaxy clusters.) These topics are almost completely unrelated.
      “Thus, the issue is whether we need any “additional” dark mass to account for either proton’s or the Universe’s mass. Are we looking for dark mass to fit the Planck’s data? Or, are we looking for dark “particle” which makes up both proton’s and the Universe’s dark mass?” is a false dichotomy. We are looking for additional mass to fit the Planck (and other) data. It may be from a particle, but it’s highly unlikely that such a particle has anything whatsoever to do with protons. We are looking for such particles, and since they don’t seem to interact with the electromagnetic or strong forces much we hypothesize them as “dark matter.” If they had much of anything to do with protons they’d likely have a strong force interaction (that’s where the vast majority of the proton’s mass comes from) and thus wouldn’t be “dark.”

  7. What exactly is the difference between garden vatiety and jungle variety models? I always imagined that garden variety means just assuming some additional particles without too deep underlying theoretical motivation for them (they are quite ad hoc) …? What then are jungle variety models…?

    • By “garden variety” I meant a WIMP – one additional particle, typically one that is present because of minimal versions of supersymmetry, or something similar, with nothing else required.

      By “jungle variety” I meant particles that typically have additional, as-yet unknown forces that affect them, making their behavior unconventional.

      But needless to say, this is not a very rigorous classification. I could write a long article on this, but not today.

      • Thanks for this reply.

        I would certainly higly enjoy a longer article, should it spontaneously appear here some day :-)

  8. So if I understand it correctly, with these underground searches we will never be quite sure. Any signal could be just an unknown background. Also consistent results from different experiments could mean they all are wrong and inconsistent results might mean there are more types of dark matter particles. What about dark matter searches at LHC? Would it not be the most convincing evidence, if dark matter signal was found there? And there any implications of no SUSY found at LHC so far for the underground and cosmic dark matter searches?

    • The stated SUSY particle mass range exclusions at LHC are all well in excess of 10 GeV. But, while those results are convincing at the high end, it isn’t entirely obvious that LHC itself has much sensitivity in this mass range to all possible kinds of particles beyond that of prior experience like LEP and Tevatron (which, of course, also didn’t find any beyond the Standard Model Particles).

      Also, of course, all of the SUSY exclusions are model dependent. One can always come up with some quirk in the theory that would have put 10 GeV superpartners just under the experimental threshold of detection for all prior experiments. But, these theories look far more compelling when someone is using other means to actually see something in the relevant mass range than it does when they aren’t.

      • I don’t entirely understand these statements… in some important cases the LHC exclusions of undetected particles, like dark matter particles would be, go down to zero mass. So I’m not sure what you’re referring to.

    • You did not understand it correctly. As the experiments get better, one of these hints may stick, and turn into a discovery. It was the same with the Higgs boson; there were many false alarms, but then finally there was an alarm that wasn’t false. Multiple experiments saw the same thing; over time the evidence became more abundant and convincing; and now we’re sure a Higgs boson has been found.

      This is standard in ANY scientific investigation at the forefront of knowledge. Nothing special about dark matter or about these types of experiments. Non-experts often think it is obvious when you have made a discovery; in fact it is very rarely obvious, and it takes months or years before the evidence is convincing.

  9. Thanks for the overview!

    Frustratingly, Neil Weiner’s talk is the only one that doesn’t have slides available on the conference page. I would be very much interested in reading them! Did he say anything about how one might distinguish between an unknown background and a light WIMP?

    • Really, they’re not available? That’s unfortunate!

      The best ways to distinguish them are

      1) improve the experiment to reduce possible backgrounds further, take more data, and see if the signal becomes stronger

      2) have the result confirmed by an experiment which is sensitive to the same signal but has different types of backgrounds (for instance it would potentially be striking if XENON100 saw the same signal as CDMS, since the experiments are so different in character.)

      In the end we need to see a strong signal, present in and consistent between at least two experiments, and not contradicted by any other experiment.

      • Thanks for the reply.

        I just checked the site again, and the slides are now available!

        Regarding 2), is it at all possible that XENON100 is “over-excluding”, that is, classifying bonafide WIMP recoil events as background? Note, I ask this as someone who is almost 100% ignorant of how they actually classify the events. I’m sure this possibility has been looked over numerous times on their part.

        On a related note, I can’t wait to see what LUX finds.

  10. There are a couple more big problems with these results.

    First, in the time since they started searching for dark matter, astronomers doing simulations have determined that dark matter particles 10 GeV give or take (which is called “cold dark matter”), is too heavy to produce the kind of universe that we live in. Particles of this mass would produce far more satellite dwarf galaxies around galaxies like ours than our observed and would have the wrong distributions within galaxies to produce the galactic rotation curves and gravitational lensing patterns that astronomers observe. Simulations show that dark matter particles need to be at a sweet spot mass of just around 2 keV +/- about 1 keV (5 milliion times lighter than 10 GeV) to produce what we see and these lighter particles called “warm dark matter” aren’t remotely like what any of these experiments have detected.

    Even if these experiments are seeing some kind of beyond the Standard Model particle, they aren’t seeing the dark matter which explains myriad astronomy observations that we are actually looking for. If these are real, they might be some unstable supersymmetric particle, for example, but they aren’t the stuff that makes up well over two-thirds of the matter in the universe according to the lamdaCDM model (the Cosmic Microwave Background Radiation effects seen by Planck attributable to “cold dark matter” is actually indifferent to “cold” v. “warm” dark matter).

    Secondly, if these particles are about 10 GeV and are truly “weakly interacting” in the narrow sense of having weak force interactions via W bosons and Z bosons, then they would have shown up in precision electroweak mearements of W and Z boson decays in collider experiments decades ago (like LEP) since weak force bosons decay “democratically” when mass-energy permits the creation of particle-antiparticle pairs. Beyond the Standard Model weakly interacting particles of less than 45 GeV have been pretty definitively ruled out. Any weakly interacting massive particles need to be more than 45 GeV (and hence are ruled out by simulations that fail to reproduce what astronomers see as described above). So, one has to come up with an entirely new force other than the three Standard Model ones to describe their interactions with ordinary matter (generically, such particles are called “sterile”) with entirely model dependent properties which could easily be different from those assumed by the experimenters.

    • The issue of dwarf galaxies is not necessarily an issue for cold dark matter. First, quite a few ultrafaint dwarf galaxies have been discovered since the problem was first stated. And, second, it’s only reasonable to identify this as a problem if it can be demonstrated that all of the excess halos would actually be luminous. If not, they could be out there without us actually being able to find them. And, in fact, simulations which treat ordinary matter more realistically (using hydrodynamics rather than particle dynamics, at least up to the point where the non-linearities due to gravity make the hydrodynamic implementation computationally too intensive) seem to show that lighter “missing” halos should have very low luminosity. And, on top of this, there’s the issue of tidal stripping by large galaxies. Suffice it to say, reports of cold dark matter’s death have been greatly overstated. (Not, of course, that warm dark matter is ruled out either.)

    • I don’t think I agree with all these statements.

      My understanding of your first point is that it’s highly controversial. Could you please send me a reference which suggests that 2 keV +- 1 keV particles (an extraordinary claim of precision) are necessary? I remind you that serious astrophysicists/cosmologists (people like David Spergel) continue to write papers about dark matter at a much larger mass scale, including 10, 100, and 1000 GeV. I’ve never heard experts like Spergel or others in the field make the kind of flat-out, definitive statement you just made.

      As for your second point, you are correct about the W and Z (it is a point I skipped over for lack of time), but you forgot the possibility that the interaction in question occurs via the Higgs itself. Of course there could indeed be new forces involved, too. And of course the experiments don’t actually assume much about where the interaction comes from; they just require that one arises somehow, and they measure its strength, rather than assuming its strength. All of this is complicated by the possibility that the dark matter particle is a mixture of something that carries electroweak quantum numbers and something that does not, which also would get around your objection.

      Nevertheless, if your point is that the situation is complex and that I didn’t entirely do justice to the details, I have to agree; this was as much as I could handle and the article was already long.

  11. Could the LHC detectors be used to explore cosmic radiation while the beam is off?

  12. Matt, thanks for your very helpful summary of the current state of play. As far as I am aware the motivation for the possible existence of dark matter is from astrophysical observations (galaxy rotation curves, etc, etc), not any requirement from within the particle physics community. In the last decade or so it has been acceptable to speculate on modifications to theories of gravity to explain these observations (MOND and its relativistic developments).

    I know that these approaches have not been totally succesfull so far in their depth of explanation of all the details, but then lambda CDM has its problems too.

    What is your personal view on this? Would you tend to the view that either:

    a) Dark matter definitely exists, we just haven’t found it yet
    or
    b) This is getting silly, we should have seen something by now. Maybe MOND and its derivatives might just be (part of?) the answer.

    Personally I would love modified gravity in some form to be the answer, but I realise that it is a long shot.

    • Neither (a) nor (b), but something close to (a). Dark matter almost certainly exists, but almost is not the same as definitely; I leave open the door. But in the searches to detect dark matter through some non-gravitational effect, there’s no reason we need have seen anything yet. For one thing, it’s quite possible that dark matter only interacts with ordinary matter via gravitational effects, in which case none of these experiments will ever see anything at all. So we will never get to (b) using these experiments.

      However, we will get more and more astronomical and cosmological checks of dark matter vs. MOND vs. other options over time.

      I think, personally, that the evidence against something like MOND is very strong; the theory of Einstein gravity + dark matter + dark energy does a remarkably good job of fitting a wide variety of data. [And I say this as someone whose first scientific result was in the context of something like MOND; it's not as though MOND is a bad idea, it just doesn't work well, and the dark matter hypothesis works much better.]

  13. Over the time I got bored by all these dark matter announcements, one clearly senses that they cannot be very strong or else there would be more results and not just silence just like in cancer research. So, I found this summary very enlightening since it filled in some facts as to why these are not definitive. Thanks.

  14. Great article, thanks Matt.
    I recall reading in George Smoot’s book that when analyzing the CMB they had to account for red/blue shifting due to the earths orbit about the sun. But in addition they also had to account for 1) the suns rotation about the galaxy, 2) the galaxies proper motion within the local group and even 3) the local groups motion towards the great attractor. I would think that the DAMA team would also have to take at least #1 into account, assuming the DM is on average stationary with respect to the center of the galaxy..

    • Actually, they don’t have to account for #1, because (unlike for the CMB) they’re not sensitive to things that are essentially constant with time. They’re looking for a collision rate that grows and shrinks during the year; all that effect #1 (or #2 or #3) would do is change the average collision rate during the year, which is a hard thing to measure because it has larger radioactive backgrounds.

    • from memory – and this was 15 years ago – the DM halo was indeed considered to be centred on the galactic centre. It was modelled as a Boltzman gas, with an average particle velocity at a given radius equivalent to the galactic rotation curve – basically what you’d expect for an ideal gas in equilibrium in the galactic gravity well. There was some consideration of DM associated with the Local Group, but IIRC it was considered to be a small contribution, locally. Whether models have moved on in the past decade and a half, I’ve no idea :)

  15. Note that one thing to remember is that there is not a single shred of evidence form astrophysical data alone (whether its PLANCK,galactic rotation curves or Bullet cluster) that cold dark matter has anything to do with weak scale interactions. the only evidence from astrophysics is that it is cold and non-interacting(although some astrophysics are
    arguing for self-interacting dark matter. See this talk at STSCI https://webcast.stsci.edu/webcast/detail.xhtml?talkid=3180&parent=1)
    The only argument for WIMPS as dark matter is the “WIMP miracle”. But this maybe overblown,

  16. Markus Harder

    Is it possible that dark matter does not consist of a single type of particle, but is a “mix” of several different particles, with different energies? Just as ordinary matter consists of different kinds of particles?
    Are there physical reasons to assume that dark matter should consist of only a single type of particle?
    Of course, imagining a zoo of DM particles is not the simplest and most elegant answer, but Nature is not necessarily simple. As we have not yet really seen any DM particle, how could we be confident there is only a single type of them?
    In case there were several dark matter particles, couldn´t this possibly explain the observed situation that at different energies faint signals of DM (candidates) show up?

    • Marshall Eubanks

      My personal feeling is that that a DM zoo is the way to bet, but you have to start somewhere, so it makes sense to look at single solutions first. In any case, you will never get consensus around a DM zoo without either a good theory or strong experimental evidence in support of it.

  17. @Matt Strassler.
    Masses of subatomic particles are related to the degree of coupling = level of interaction through different forces and particles. By that token, isn’t a WIMP an oxymoron. unless of course it has interactions in the hidden sectors by which means it is not a WIMP

  18. Dear Matt, to me it seems that the community has to come to terms with dark matter consisting of active and sterile neutrinos with eV masses. This is a likely consequence of reactor experiments such as LSND and MiniBoone. Moreover, in my opinion it works good enough in the cosmos. But this will be sociologically unacceptable for the community, so many more WIMP stories to come first. Axions and warm dark matter are also acceptable, but not these neutrinos, I never understood why.

  19. This might seem really silly if you’re in the know, but is it possible that dark matter isn’t matter at all but that spacetime is not flat and has a terrain that is all twisted up in some places? Or does that just make no sense with our current understanding?

    • Torbjörn Larsson, OM

      Bot these things is seen in the CMB:

      – The sum peak and the acoustic peaks that shows the quantified existence of DE, DM and BM in one image. Easy to see for yourself.

      – The temperature fluctuation sizes that shows space (not spacetime) is extremely flat. Essentially it comes from knowing hydrogen gas behavior at the then temperature and density. [You can google Krauss youtube from 2009, where he walks you through seeing that in ~ 1 minute.]

  20. Marshall Eubanks

    @jemmywuk – The way I like to think about it is that Dark Matter is evidence for new physics, and that evidence is quite firm. What is not as clear is where the new physics is, in quantum field theory, which leads to investigations of WIMPs, Axions, etc., or in gravitational theory, which leads to MOND and TeVeS. The ideas for new gravitational physics are less popular, but the ability of MOND to describe galaxy rotation curves with one free parameter is very intriguing. (Whether MOND/TeVeS is entirely consistent with observational cosmology is debatable, and debated, but the MOND theorists I know would say it is in no worse shape than standard CDM, which has the cusp-core and related problems.)

  21. Pingback: Are Dark Matter Particles Lighter Than We Thought? | Sean Carroll

  22. Why is the conversion of neutrinos between the 3 types not thought to be due to interactions with dark matter?

    • I think the most important reason is that dark matter is too diffuse. There are interesting effects of neutrino oscillation inside the earth and inside the sun, where densities are much higher.

  23. This site was… how do you say it? Relevant!! Finally I’ve found something
    that helped me. Thanks!

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