Courtesy: Discover Magazine (2008)
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!”
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.