Sunday, 6 May 2012

Dark Matter Found (or not)

So earlier this week I offered a brief overview of the Dark Matter problem.  (See also the Font of All Knowledge for more.)  Today I want to talk about a paper from two weeks ago relating to a possible discovery (or more accurately, hint of a signal).

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:

  1. 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.
  2. 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.
  3. 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.
The last case is a very promising one.  When we are looking for the annihilation products of dark matter, we are hindered by astrophysical backgrounds.  Anything that can be produced by dark matter can also be produced by stars, pulsars, black holes etc.  And we don't know the exact number of these objects, so we don't know the exact size of the background.  To identify a signal over background requires something characteristic about the signal that can not easily be faked.  When dark matter annihilates directly to light, the light it produces has a single energy, a single frequency; and this would stand out as a hard-to-fake feature.

The bad news is that this process will be suppressed compared to other things the dark matter can annihilate to.  Still, we have a number of experiments looking for such a signal, and setting progressively stronger limits.  One of the newest experiments is Fermi, a space telescope (satellite) that has been running for a few years now.  It is designed to focus on the energy range where we expect a dark matter signal, and has already found a number of interesting astrophysical objects and greatly expanded our understanding of galactic radiation in this energy interval.  So far, the experimentalists running Fermi have placed limits on dark matter, but not claimed any signal.

Fermi is required to publish all their data under the terms of their funding agreement.  This is scientific openness and transparency taken to its limit.  It also allows theorists to play around, and try to find something before the experimentalists do.  This brings me to this recent paper by Christoph Weniger, a theorist at the Max Planck institute in Munich.   He has looked mono-energetic photons in the Fermi data, and claims a hint of a signal.

How can he make such a claim, if the experimentalists haven't found anything?  The key step he took in his analysis was to focus his search on specific regions in the sky.  The limits placed by the Fermi collaboration are based on using all of their data, no matter where the photons came from.  As already noted, however, we expect dark matter to congregate in the centre of the galaxy, and so the strongest signal should come from there.  By focusing on that region, we can reduce the backgrounds by more than we reduce the signal, improving our chances.

To be specific, Weniger took the following steps:
  1. 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.
  2. For five different theoretical models of the dark matter distribution, figure out where the signal would be strongest.
  3. 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.
The different regions look like this:
The centre of these plots correspond to the centre of the galaxy; red points are more favourable.  Note that there is a certain amount of overlap between the different regions, and in particular the bottom three are roughly subsets of the ones above them.

Having chosen these regions, Weniger repeats the analysis of the Fermi team looking for a signal.  Each possible signal is checked to see if it is statistically significant from zero.  He gets the following results:
The horizontal axis is the energy, and the vertical axis is proportional to the statistical significance.  There are several peaks, but the ones at about 130 GeV are most significant.  It is not quite significant enough to claim discovery, but it is enough to consider interesting.  Also notable is that this analysis, if we can trust it, gives us a hint about the dark matter distribution, by seeing which of the theoretical models gives the strongest outcome.  In particular, the two most extreme regions, which either focus solely on the galactic centre or look at the furthest distances from it, give weak signals.

The last thing to note is that this is a theorist's analysis.  Even other theorists won't accept this till it is repeated by an experimentalist.  And already we have a paper that contests the dark matter explanation.  I actually wanted to talk a bit about that paper too, but I think this post is long enough already.

(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.

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