New experiment cannot reproduce long-standing dark matter anomaly

Close-up of the COSINE detector  [Credits: COSINE collaboration]

To correctly fit observations, physicists’ best current theory for the universe needs a new type of matter, the so-called “dark matter.” According to this theory, our galaxy – as most other galaxies – is contained in a spherical cloud of this dark stuff. Exactly what dark matter is made of, however, we still don’t know.

The more hopeful physicists believe that dark matter interacts with normal matter, albeit rarely. If they are right, we might get lucky and see one of those interactions by closely watching samples of normal matter for the occasional bump. Dozens of experiments have looked for such interactions with the putative dark matter particles. They found nothing.

The one exception is the DAMA experiment. DAMA is located below the Gran Sasso mountains in Italy, and it has detected something starting in 1995. Unfortunately, it has remained unclear just what that something is.

For many years, the collaboration has reported excess-hits to their detector. The signal has meanwhile reached a significance of 8.9σ, well above the 5σ standard for discovery. The number of those still unexplained events varies periodically during the year, which is consistent with the change that physicists expect due to our planet’s motion around the Sun and the Sun’s motion around the galactic center. The DAMA collaboration claims their measurements cannot be explained by interactions with already known particles.

DAMA data with best-bit modulation curve.
Figure 1 from arXiv:1301.6243

The problem with the DAMA experiment, however, is that the results are incompatible with the null-results of other dark matter searches. If what DAMA sees was really dark matter, then other experiments should also have seen it, which is not the case.

Most physicists seem to assume that what DAMA measures is really some normal particle, just that the collaboration does not correctly account for signals that come, eg, from radioactive decays in the surrounding mountains, cosmic rays, or neutrinos. An annual modulation could come about by other means than our motion through a dark matter halo. Many variables change throughout the year, such as the temperature and our distance to the sun. And while DAMA claims, of course, that they have taken into account all that, their results have been met with great skepticism.

I will admit I have always been fond of the DAMA anomaly. Not only because of its high significance, but because the peak of the annual modulation fits with the idea of us flying through dark matter. It’s not all that simple to find another signal that looks like that.

So far, there has been a loophole in the argument that the DAMA-signal cannot be a dark matter particle. The DAMA detector differs from all other experiments in one important point. DAMA uses thallium-doped sodium iodide crystals, while the conflicting results come from detectors using other targets, such as Xenon or Germanium. A dark matter particle which preferably couples to specific types of atoms could trigger the DAMA detector, but not trigger the other detectors. This is not a popular idea, but it would be compatible with observation.

To test whether this is what is going on, another experiment, COSINE, set out to repeat the measurement using the same material as DAMA. COSINE is located in South Korea and has begun operation in 2016. They just published the results from the first 60 days of their measurements. COSINE did not see excess events.

Figure 2 from Nature 564, 83–86 (2018)
Data is consistent with expected background

60 days of data is not enough to look for an annual modulation, and the annual modulation will greatly improve the statistical significance of the COSINE results. So it’s too early to entirely abandon hope. But that’s certainly a disappointment.