Guest Post: Juan Collar on Dark Matter Detection

By Sean Carroll | April 21, 2008 9:45 am

You may have heard some of the buzz about a new result concerning the direct detection of dark matter particles in an underground laboratory. The buzz originates from a new paper by the DAMA/LIBRA collaboration; David Harris links to powerpoint slides from Rita Bernabei, leader of the experiment, from her talk at a meeting in Venice.

The new experiment is an upgrade from a previous version of DAMA, which had already been on record as having recorded a statistically significant signal of the form you would expect from the collision of weakly interacting massive particles (WIMP’s) with the detector. The experiment uses a challenging technique, in which their focus is not on eliminating all possible backgrounds so as to isolate the dark-matter signal, but to look at the annual modulation in that signal that would presumably be caused by the Earth’s orbital motion through the cloud of dark matter in the Solar System: you expect more events when we are moving with a high velocity into the dark-matter wind. Other workers in the field have not been shy about expressing skepticism, but the DAMA team has stood their ground; as Jennifer notes in her report from the recent APS Meeting, the DAMA collaboration home page currently features a quote from Kipling: “If you can bear to hear the truth you’ve spoken/ twisted by knaves to make a trap for fools,/ ……………you’ll be a Man my son!”

Juan Collar To help provide some insight and context, we’ve solicited the help of a true expert in the field — Juan Collar of the University of Chicago. I got to know Juan back in my days as a Midwesterner, and a trip to his bustling underground experimental empire was always a highlight of anyone’s visit to the UofC physics department. You can hear him talk about his own work in this colloquium at Fermilab; he’s agreed to post for us about his views on the new DAMA result, and more general thoughts on what it takes to search for 25% of the universe. I promise you won’t be bored.

——————————————————————-

My dear friend Sean has me blogging: hey, I’ll try anything once. On the subject of the recent DAMA results no less, as per his request. I am normally a bit of a curmudgeon but… Sean, you really want the worst of me out there permanently on the internets, don’t you?

I’ll try to keep this to the point. A bard I am not, nor the subject invites any poetry. I have therefore chosen brief eruptions of flatulence as the metric and style for this piece. The result of indigestion, you see. I’ll start with the most negative, so as to end up on a brighter note:

  • The modulation is undeniable by now. I don’t know of any colleagues who doubted these data were blatantly modulated already back in 2003, when “the lady” (DAMA) decided to keep mum for a while. However, to conclude from something this mundane that the experiment “confirms evidence of Dark Matter particles in the galactic halo with high confidence level” or that there is “an evidence for the presence of dark matter particles in the galactic halo at 8.2 sigma confidence level” is simply delusional. There is evidence for a modulation in the data at 8.2 sigma, stop. Compatible with what would be expected from some dark matter particles in some galactic halo models, full stop. Anything beyond this is wanting to believe, and it smears on the rest of us in the field. Of course, of course… there is no other observed process in nature that peaks in the summer and goes through a low in winter, so this must be dark matter, right? (Occam is turning in his grave, rusty razor still in hand. He is thinking a remake of that opening scene in “Un chien andalou“, with help from this little lady. I am channeling him loud and clear).
  • Someone should take the DAMA folks aside for a beer, make them see the following. If one day soon we are all convinced that this effect was DM-induced (see below for what that will really take), they will be recognized for one of the greatest discoveries in the history of science, without them having to look desperate or foolish today. Or making the rest of us in the field do, by association: thanks DAMA, for cheapening the level of our discourse to truly imbecilic levels. (Sean, if you edit this I will scratch the paint off your car. I may not write blogs, but I do read them: I know how to hurt you).
  • Deep breath. Having cleared the air some (or just made it toxic, whatever), it is not DAMA’s fault that there is a penury of signatures in this field of ours, laboratory searches for particle dark matter. The one possible exception to this is a detector with good recoil directionality and sufficient target mass to be truly competitive, but we don’t know of a good enough way to do this as of today (“good enough” folds in the price tag). People are still trying. The diurnal modulation in the DM signal that would be sensed by such a device is wickedly rich in features, extremely hard for nature to imitate with anything else. The annual modulation resides on the other side of this spectrum of complexity. It is the poor man’s smoking-gun to DM “evidence”. Inspected carefully, it is disappointingly feeble: different models of the halo can shift the phase of this modulation completely, turning expected maxima into minima and vice-versa, changing the expected amplitude as well. Add to this the fact that essentially every possible systematic effect able to pass for a “signal” can be yearly-modulated, for one reason or another. That’s the ones we can presently think of, and the ones yet to be proposed. To grow convinced that we have observed dark matter in the lab we’ll require a number of entirely different techniques, using a variety of targets, all pointing at the same WIMP (mass, cross sections), with additional back-up information from accelerator experiments and from gamma-ray satellite observations (so-called indirect searches). All of those lines crossing at one point, so to speak. This I (for one) will call “evidence”. I know of no single existing or planned DM experiment, including those I participate in, that would be able to make anything close to a bulletproof claim on its own. My advice to any overambitious individuals looking for a quick kill is to look elsewhere in physics. WIMP hunting is not it, no matter how important the discovery of these particles might be.
  • I try to teach my students that a good experimentalist does not need any critics: he or she is his/her own worst enemy. If you don’t feel a sincere drive to debunk, test and revise your own conclusions, you should be doing something else for a living. This intent is seemingly absent from the DAMA collaboration. Sure, some obvious environmental parameters are kept constant and logged. But this is simply not enough. Again we see, like the last time, that the subject of a modulation in the photomultiplier (PMT) noise contaminating the data, which is on everyone’s mind, is treated in a quite unsatisfying, suspiciously ad hoc fashion. What will it take for DAMA to release a plot like those displaying the modulation in the 2-6 keV regions, but instead in the 1-2 keV region or even lower? (wait, I can hear the excuses, again). If this modulation is WIMP-induced, we can predict how much of it should be visible in those lower-energy noise-contaminated bins, for different WIMP scenarios. If this is just modulated PMT noise, there is a good chance that a careful, dedicated look further down in energy would settle the issue once for all. I personally would love to see a 20-page report produced on this issue only. Why? Because as a graduate student I saw all sorts of modulations close to (or into) the noise in dark matter Germanium detectors, until my graduate advisor slapped some sense into me. Noise is man-devouring hydra. I find their reluctance to be exhaustive on this subject fascinating. The sad way a polar bear at the zoo, retracing its own steps in an endless loop, is fascinating.
  • A suggestion for anyone high up in the Italian INFN willing to listen: form a committee of experts and charge them with taking the hard throughout look at the stability of low-energy data that DAMA is not willing to deliver. There is much to gain and much to lose. This sort of “audit” could be perceived as a punishment or a reward, it is in the eye of the beholder. The importance of the claim certainly justifies it. Much time and effort by others could be wasted if this “DAMA effect” is totally bunkum. If it is not, we may be wasting time as we speak: the emphasis of other experiments should be reoriented. A five year wait to have a mere rehash of the same tepid analysis, one that brought DAMA an unprecedented level of international criticism and scrutiny, seems an unbearable thought.
  • If this was my experiment, I would stop acquiring more statistics. We get it, we know what an 8.2 sigma “evidence” means. It is plenty enough. Now go out and really try to debunk yourselves. Here’s one of many possible ways: concentrate on blank runs with low-background non-scintillating or low-scintillation materials (synthetic quartz, acrylic, undoped NaI, etc.) in place of the sodium iodine crystals. The materials should still be as close as possible, optically speaking, to the original scintillator, to allow for PMT cross-talk effects such as dynode glow, etc. Acquire data (PMT noise, Cerenkov light in the envelope, and other known nuisances in this case) and demonstrate that the modulation is absent then, that the effect was in the NaI scintillation. Another possible test: you are sitting on almost 1000 kg-yr of data. This should provide DAMA with a sensitivity to diurnal modulations smaller than ~0.1%. It then seems statistically possible to find weak additional DM effects originating exclusively from the rotational speed of the laboratory around the Earth’s axis (see footnote in astro-ph/9808058v2), a far more complex piece of “evidence”. Such effects depend on the sidereal day (as opposed to the solar day) and are hard to mask by anything not of a galactic origin. Try, doggammit, try to put your experiment to the acid test instead of serving yesteryear’s cold leftovers again! DAMA can now proceed to do with this free advice the same as with the rest received from others. Too crude to print in this distinguished forum.

Intermezzo and change of guard: I would hate for anyone to tell me what to do with my experiments. Apologies, milady.

  • Kudos to DAMA on more than a couple of fronts: they have really made it very hard for other experiments using the same target (ANAIS, NAIAD, etc.) to match their sensitivity. It seems urgent by now to repeat the experiment independently, using the same detection medium. DAMA has done an extraordinary job in removing radioactive contaminants from NaI, better than anyone else to date. I do go out and defend DAMA (believe it or not) when folks too far afield attempt to criticize the quality of the experiment itself. They have done a phenomenal job (the experiment is a class act, their reasoning and public relations…). Another area where they excel is in reminding us that the dark matter possibilities are actually many, and that not all doors are closed on a real effect. Not nearly. Through the years they have proposed and compiled dark matter alternatives capable of explaining their effect but not yet tested by other experiments. Nothing wrong with this, as long as you don’t confuse it with “evidence” for anything. This should encourage creative approaches in a field that is not particularly notorious for them: we are all looking for the same type of particle, focusing on a particular region of WIMP phase space, relying on the same mode of interaction. If anything, the history of particle physics teaches us that surprises abound: often, whenever a natural hypothesis prevailed (relatively heavy SUSY WIMPs or light axions in our case) incoming experimental data forced the community to regroup, rethink and come up with other explanations. These always look evident with the privilege of hindsight. We are a certifiable ship-o-fools, let us not forget.

Well Sean, you made me do it, and I am exhausted. How can you guys blog every other day? People are dying in them blog sweatshops, now I am sure of this.

CATEGORIZED UNDER: Guest Post, Science
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  • Count Iblis

    Despite all the (valid) criticism, I still think that the new results are extemely interesting. The new results do not deviate from the old results (except for one 2 sigma deviation in one bin if I remember correctly). Although both DAMA experiments were rather similar, for two separate experiments to yield the same results if they are caused by some experimental artifact sounds a bit strange to me.

    In a previous article the DAMA team did announce that they were going to look for diurnal modulations in the DAMA/Libra data. So, perhaps this is yet to come.

    I agree with the last point, also noted here. Even if traditional WIMP models turn out to be correct, the DAMA results could theoretically be caused by other particles that do not significantly contribute to the DM density.

  • http://startswithabang.com/ Ethan

    They claim to be looking for models that CDMS and Edelweiss are insensitive to, but you’re right, I don’t know how they’re going to search for a positive signal with the current setup. I agree with you, in fact I wrote about it here last week, with a follow-up here.

    But on the other hand, they don’t know how to shield themselves from this “background” of particles. If any other experiment didn’t know how to account for background, everyone would assume this is modulation of the background. How could they shield it better? Does anyone know? I know you talk about the importance of getting it right and I agree, but how do they do it? I like your suggestion about the low-energy data, but without seeing it, I have no idea how robust it is. Is this a case of them just publishing the data that supports their conclusions (and inferences)?

  • Shantanu

    Anyone know how many graduate students/postdocs have graduated/working on DAMA?
    It would be interesting to read some Ph.D thesis from DAMA.

  • http://backreaction.blogspot.com/ B

    Thanks!

  • M

    This sounds similar to the claim by another Italian group a few years ago that they’d seen an excess in gravitational wave events from the galactic centre (see gr-qc/0210053), which was pretty quickly debunked as having used flawed statistics. There seem to be a few people who like to generate undue controversy and seriously jump the gun when making extraordinary claims. And this can be to the general detriment of the field. I hope whoever referees the paper will have some strong demands for the authors to take note of.

  • Count Iblis

    DAMA accounts for the background in a few ways.

    One method they use works as follows. DAMA uses a few detectors. If two or more detectors produce a signal at the same time, that is assumed to be due to the background. Some fraction fo the background will yield such multiple hit signals. You can easily estimate what that fraction should be. So, you can take the multiple hit signals, look for annual modulation in that and then deduce what the contribution to the annual modulation from the background in the single hits are.

  • Sili

    I like this guy! He should be locked in the sweatshop more often.

    Are any of these similar experiments running on the Southern hemisphere?

  • Shantanu

    M, there is one difference between the DAMA claim and the Nautilus/Explorer claim.
    DAMA expet claims to see a signal at 8 sigma signiciance. whereas the Nautilus/Explorer
    only claimed to see an “excess”, but not a signal.

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  • smm

    I try to teach my students that a good experimentalist does not need any critics: he or she is his/her own worst enemy.

    Thanks for that line. I’ll be applying it liberally with my own work.

  • http://arunsmusings.blogspot.com Arun

    Yep, please transfer JC to the bloggers’ sweatshop ASAP!

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  • http://www.music.freakout.biz Andi Chapple

    hi –

    interesting post, interesting other posts linked to from the editorial bit, thanks all round. I’m not qualified to comment on the science, but I will ask: what is it about physics that makes so many of the professionals so rude to each other in public? I understand (I think) that Juan Collar is worried that this other team have made his job explaining what he’s doing to non-professionals a bit harder, but still, politeness never hurt anyone. it’s only the nature of the universe, for goodness’ sake – it’s not like your children are being starved to death or something.

    best wishes,

    Andi

  • Brian

    If a similar experiment were placed on the Moon or Mars and gathered consistent data, would that help eliminate the argument of seasonal effects?

  • Pop

    I’ve read some of the other blogs about this. I have a question. Does DM clump, which implies motion as well? All known matter seems to form in clumps with areas of low density. Or does DM spread itself evenly across the univers? The question is moot since we haven’t proven the existence of DM yet. But, in looking at these data it should be a point of consideration. If DM is uniform and static, seasonal variation will be more easily detected. If DM clumps and is in motion, seasonal variations would be dificult to detect. Determining these data do confirm DM, then it also shows that DM is uniform and static. No?

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Pop, dark matter definitely clumps — there is more of it in galaxies than outside galaxies, for example. The precise amount of clumping is still unclear. But that would still be consistent with a seasonal variation; whatever the clumping is, there is still an average velocity for dark matter within the solar neighborhood.

    Brian, doing the experiment on the Moon would undoubtedly eliminate certain possible systematic effects, while eliminating others. It would also be more expensive by several orders of magnitude.

  • Adam

    Then again, there are plans for dark matter detectors on the International Space Station – not the moon, but definitely out there.

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  • jls

    Thanks a lot for writing this guest post. It was clear and useful.

  • Ryan

    Shouldn’t physicists trying to prove the existence of dark matter be attempting to make it in a laboratory, or capture it? It seems that even if these “detections” succeeded, even the three part experimental result Mr. Cole described, it still falls short of satisfactory proof. Why aren’t dark matter researchers attempting to produce the stuff in laboratories? Figure out the physical process that produces this stuff and replicate it. Or, gather some up from the space around Earth. If 90%+ of the mass of the universe is this dark matter then there has to be some in the space around Earth. Get a spoon and collect some. The experimenters here said they were measuring the impact of Earth on cloud of dark matter. Why not measure the cloud directly? Prod it with a thermometer, shine light on it.

    There is not one drop of experimental evidence that this dark matter stuff is real. While it is true that Kepler’s laws don’t predict the motion of a spiral galaxy, it is a totally invalid conclusion to presume Kepler’s laws are right and the universe is wrong. It’s saying that instead of Kepler’s laws not applying to galaxies (we had no good reason to ever think they would), the laws of radiation and light do not apply to galaxies, that we can’t trust our lying eyes. To say it’s not the model that’s wrong but rather the universe is arrogant, myopic and unscientific. It betrays dogmatism and lacks creativity.

    If this dark matter stuff were real people would be trying to prove its existence by creating it or gathering it. Folks, just keep in mind there may not be such a thing as dark matter and energy, antimatter emitting black holes and neutron stars the size of solar systems. We’ve been looking for evidence of those things for decades now and not found one scrap of it. We’ve had to modify the heck out of the original theories to stop them from making incorrect predictions and at some point in the life of a hypothesis, when you’ve tested it so many times, and all it does is fail, and you modify away the failure, test again and it fails again; eventually you should start over with a fresh theory.

    Some cosmology theories make successful predictions.

    http://www.holoscience.com/news.php?article=66b0jzyh

  • ed hessler

    I’m not a physicist but my preference is not to make Dr. Juan Collar a regular but to save and use him (as well as others) for informed commentary to help the rest of us understand (a little) what is going on. In addition, such commenters can and often do write longer and more detailed comments on topics that can use focused rays of light.

    This was another of your good ideas, Sean and one of the many reasons this is such an interesting and useful blog.

    Thanks again.

  • Brian2

    This was an absolutely great article. Thanks, Sean and Juan. I have a question, though.

    Do we have any reason to believe that dark matter particles should interact with barionic matter by exchanging W or Z bosons? Maybe they only interact with familiar matter gravitationally. I would very much appreciate an informed response to this question. Thanks in advance to any and all who can help me with this query.

  • JeffF

    Brian2: It certainly could be that dark matter interacts only gravitationally. In that case these sorts of experiments are hopeless – the rate of scattering in a detector would be far too low. Nonetheless, there is a reasonably powerful argument that dark matter is likely to have weak (W and Z) interactions.

    If we suppose the dark matter is composed of some new stable particle X, then this particle should have been pair-produced in large amounts in the hot early universe. As the universe expanded and cooled, however, the X particles stopped being produced and the leftovers annihilated with one another and vanished. It turns out, however, that this annihilation process is imperfect in an expanding universe – if the forces which mediate the annihilation are weak enough, some leftover amount of X particles will remain essentially unchanged until the present day. This is the simplest possible way of explaining how we got our dark matter (though it’s not the only way).

    The miracle is that you get roughly the right amount of dark matter if X is a heavy particle whose annihilation is mediated by the W and Z bosons (you’d get much too much if the interactions were gravitational, far too little if they were mediated by photons or gluons). Better yet, most extensions to the Standard Model of particle physics (supersymmetry, extra dimensions, etc.) predict new particles with just these properties, and in many cases the lightest such new particle is stable. It’s not an airtight argument, but it sure seems like cosmology and particle physics may be independently arguing for such a particle: a Weakly Interacting Massive Particle (WIMP), possibly detectable in this sort of experiment (CDMS, XENON, DAMA, COUPP, …)!

  • Brian2

    JeffF,
    Thanx for the help.
    I have been following the results of these laboratory WIMP searches with great interest. I hope DAMA pursues the search for a diurnal oscillation, and I hope they tell us the results whether positive or negative. If they have a discrete record for each recoil, then would it just be a simple matter of resorting the data?

  • JeffF

    Brian2,
    The diurnal oscillation signature is an interesting one, and not one commonly discussed in the field (because it requires an extremely large data set). DAMA should have time stamps for each particle event so it should be a fairly simple matter to check, though I haven’t checked the numbers to see how significant the signal could be.

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  • http://axitronics.blogspot.com/ BDO Adams

    That was a very negative article about a heroic result in astrophysics. 8 sigma!, 3 sigma could
    be an artifact, 5 sigma is usually enough to claim a discovery, 8 sigma is some very strong evidence. And the signal phase and modulation, did look like the result the expected from CDM.

    Why the antagonism, maybe because the result is bad for SUSY, none of the other experiments
    find a significant evidence for CDM, and the combination of the DAMA, CDSII and other experiment
    rules out most basic models of neutralino CDM.

  • Count Iblis

    Why the antagonism, maybe because the result is bad for SUSY…

    That’s right. And add to that the fact that most other searches assume DM in the form of WIMPS that will scatter of nuclei and you get some fireworks :)

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .

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