More than once I wrote in this blog about crazy theoretical ideas to explain the DAMA modulation signal. There is a good excuse. In this decade, DAMA has been the main source of inspiration to extend dark matter model building beyond the simple WIMP paradigm, in particular inelastic dark matter was conceived that way. This in turn has prompted to tighten the net of experimental searches to include signals from non-WIMP dark matter particles. More importantly, blog readers always require sensation, scandal and blood (I know, I'm a reader myself), so that spectacular new physics explanations are always preferred. Nevertheless, prompted by a commenter, I thought it might be useful to balance a bit and describe a more trivial explanation of the DAMA signal that involves a systematic effect rather than dark matter particles.
Unlike most dark matter detection experiments, the DAMA instrument has no sophisticated background rejection (they only reject coincident hits in multiple crystals). That might be an asset, because they are a priori sensitive to a variety of dark matter particles, whether scattering elastically or inelastically, whether scattering on nucleons or electrons, and so on. But at the same time most of their hits comes from mundane and poorly controlled sources such as natural radioactivity, which makes them vulnerable unknown or underestimated backgrounds.
One important source of the background is a contamination of DAMA's sodium-iodine crystals with radioactive elements like Uranium 238, Iodine 129 and Potassium 40. The last one is the main culprit because some of its decay products have the same energy as the putative DAMA signal. Potassium, being in the same Mendeleev column as sodium, can easily sneak into the lattice of the crystal. The radioactive isotope 40K is present with roughly 0.01 percent abundance in natural potassium. Ten percent of the times 40K decays to an excited state of Argon 40, which is followed by a de-excitation photon at 1.4 MeV and emission of Auger electrons with energy 3.2 keV. This process is known to occur in the DAMA detector with a sizable rate; in fact DAMA itself measured that background by looking for coincidences of MeV photons and 3 keV scintillation signals, see the plot above. That same background is also responsible for the little peak at 3keV in the single hit spectrum measured by DAMA, see below (note that this is not the modulated spectrum on which DAMA claim is based!). The peak here is exactly due to the Auger radiation.
Now, look at the spectrum of the time dependent component of the signal where DAMA claims to have found evidence for dark matter. The peak of the annual modulation signal occurs precisely at 3 keV.The fact that the putative signal is on top of the known background is VERY suspicious.
One should admit that it is not entirely clear what could cause the modulation of the background,
although some subtle annual effect affecting the efficiency for detecting the Auger radiation is not implausible. So far, DAMA has not shown any convincing arguments that would exclude 40K as the origin of their modulation signal.
Actually, it is easy to check whether it's 40K or not. Just put one of the DAMA crystals inside the environment where the efficiency for detecting the decay products of 40K is nearly 100 percent. Like for example, in the Borexino balloon that is waiting next door in the Gran Sasso Laboratory. In fact, the Borexino collaboration has made this very proposal to DAMA. The answer was a resounding no.
There is another way Borexino could quickly refute or confirm the DAMA claim. Why not buying the sodium-iodine crystals directly from Saint Gobain - the company that provided the crystals for DAMA? Not so fast. In the contract, DAMA has secured exclusive eternal rights for the use of sodium-iodine crystals produced by Saint Gobain. At this point it comes as no surprise that DAMA threatens legal actions if the company attempts to breach their "intellectual" property.
There is more stories that make hair on your chest stand on end. One often hears the phrase "a very specific collaboration" when referring to DAMA, which is a roundabout way of saying "a bunch of assholes". Indeed DAMA has worked very hard to earn their bad reputation, and sometimes it's difficult to tell whether at the roots is only paranoia or also bad will. The problem, however, is that history of physics has a few examples of technologically or intellectually less sophisticated experiments beating better competitors - take Penzias and Wilson for example.
So we will never know for sure whether the DAMA signal is real or not until it is definitely refuted or confirmed by another experiment. Fortunately, it seems that the people from Borexino have not given up yet. Recently I heard a talk of Cristiano Galbiati who said that the Princeton group is planning to grow their own sodium-iodine crystals. That will take time, but an advantage is that they will be able to obtain better, more radio-pure crystals, and thus reduce the potassium 40 background by many orders of magnitude. So maybe in two years from now the dark matter will be cleared...