Now, this is far from the first time such a hint has been found. The DAMA experiment is perhaps the longest-standing claim of discovery; that question, and why it's not widely accepted, is a whole blog post in itself. I want to start with this one because it is recent, and also most closely related to the work I have done in dark matter detection.
When we talk about detecting dark matter, what we mean is detecting it some way other than through its gravitational interactions. In this sense, we are implicitly assuming that dark matter, and not modified gravity, is the solution. There are several different and complementary ways to try and do this. The most straightforward is direct detection: looking for dark matter interacting with equipment here on Earth, usually by looking for it scattering off atomic nuclei. This is the approach taken by the already-mentioned DAMA. The problem is that the signal is fairly featureless and the backgrounds are severe, which is one factor behind the doubt over the signals claimed by DAMA, CRESST1 and CoGENT.
Another way to try and look for dark matter is indirect detection. The idea here is that dark matter out in the Universe will tend to fall into gravitational wells, such as in the Sun, or at the center of the galaxy. These regions will have a higher density of dark matter, and sometimes two dark matter particles will annihilate in those regions. We can then look for those annihilation products in various telescopes on Earth. This approach has the advantage of a certain degree of model-independence. Specifically, the most popular type of dark matter particle is the thermal WIMP, where the annihilation rate of dark matter in the early universe sets its density today. Inverting this, knowing the current density means that we know the annihilation rate.
Unfortunately, it's not quite that simple. (It never is.) There are many ways that this relationship can be modified; the annihilation rate can be suppressed today by the small dark matter speeds2 ; or it can be enhanced by interactions that are only effective at those same low speeds. Even if the total annihilation rate is the same, we don't know what it annihilates to. We can check all possible conventional final states, but if it annihilates to something we haven't found yet, we're out of luck.3
One possible thing that can be produced in dark matter annihilations is light. This seems a bit counter-intuitive, since dark matter is by definition dark. We can get around this in three ways:
- The dark matter can annihilate to something that in turn decays to photons. This is the most common case, as anything that decays to quarks will produce the lightest strongly interacting state, the neutral pion, which decays to two photons.
- The annihilation produces charged particles, which produce photons. This can either happen by the charged particles interacting with e.g. starlight, or simply through the fact that accelerated charges radiate.
- The annihilation is directly to photons, but is suppressed. This is possible since we can only place limits on the direct interaction, rather than rule it out completely.
- Assume that any photons with an energy less than 20 GeV are dominated by astrophysical background processes, and that the backgrounds at higher energies will have the same topologies.
- For five different theoretical models of the dark matter distribution, figure out where the signal would be strongest.
- Combining 1 and 2, for each theoretical model figure out where the signal to background ratio is most favourable, and focus the search on those areas.
(Edited to add tags.)
1 CRESST's home page is lower ranked on Google than their Wikipedia page. That's not good.↩
2 Small compared to the speed of light.↩
3 Strictly, if dark matter annihilates to something we haven't found, that new thing must in turn decay to stuff we have (or it would be dark matter). However, those decay products will have lower energy and be harder to find.↩