Criticism of the DAMA/LIBRA collaboration's conclusions regarding dark matter detection

From Cosmic Variance: 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 controversy surrounding the DAMA/LIBRA collaboration points to the fact that the search for evidence about the nature of dark matter (and WIMPs) is still wide open... particle physics has a ways left to go in unraveling this (despite claims made by the Italian/Chinese DAMA/LIBRA collaboration).

Selections from Juan Collar's guest post on Cosmic Variance regarding the claims made by DAMA/LIBRA collaboration (some editorials left intact):

* 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.

* 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.

* 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.

* 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.

* More rigorous versions: 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.

* 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.