Every year, the Dutch physics community gets together to celebrate the year in physics. These are some highlights from the meeting. Since it's a meeting, it's not possible to link to published work (a talk could cover multiple papers, or just parts of papers). Where possible, We've linked to the research group that presented the work.

This year, the search for dark matter seems to be dominating the minds of a lot of physicists. It's quite an intriguing issue. We have a lot of gravitational evidence for dark matter at length scales from single galaxies to galaxy clusters—and even the cosmic microwave background. The variety of evidence is such that it's difficult to imagine a suitable modification to the laws of gravitation that would satisfy all these constraints.

But actual dark matter remains elusive. I’ll discuss some details in a moment, but my take-home from the dark matter talks is that if it can be detected at all, we should see it relatively soon.

So how do we go about detecting dark matter? Francesca Calore from University of Amsterdam presented results from observations of cosmic and gamma ray production. The idea is that dark matter forms a halo of relatively slowly moving particles around the galaxy, where the particles occasionally run into each other. In doing so, they may destroy themselves and generate some high-energy particles of the type that we can detect. Following a number of possible decay paths, we get gamma radiation and/or cosmic rays.

Gamma rays are just a high-energy version of ordinary light, so they travel relatively unimpeded through the galaxy. That means we can turn our detectors to the sky and observe the energy, intensity, and direction of high-energy gamma rays. This data can then be compared to what might be expected from known astronomical sources.

This process is actually a good deal more complicated than you might imagine. First, you need to consider all the other possible gamma ray production processes and subtract those from the signal. Then, you need to create a model of the dark matter distribution and see if any observed excess of signal correlated with expected clumps of dark matter.

Finally, you need to examine the energy spectrum of the gamma rays and see if they have the right energy range and the right intensity at each energy (the shape of the spectrum) to match what might be expected from dark matter destruction.

This all probably sounds a bit strange. We don’t know what dark matter is, so we can’t know what energies the gamma rays should have, right? Well, not quite. For instance, we have a lot of particle physics data and a lot of other observations that have eliminated whole swaths of possible energies. If the excess appears in a region of the spectrum that is known to not originate from dark matter annihilation, we know the signal is spurious.

Taking all of this information into account, there is a signal that looks like dark matter. But—and this is a big but—not all possible background gamma ray production processes have been taken into account yet. It might be possible that there are faint gamma ray producers that just happen to coincide with the observed hot spots. These producers would be astronomical objects like galaxies that have not been cataloged over the course of ordinary observation.

To eliminate this explanation, the team is going to train radio telescopes on these possible sources and see if there's something there. If not, that is one more explanation eliminated. However, even then, there is still more work to be done.

In any case, at the moment we also see an excess of cosmic rays that might be due to dark matter. But the picture is even murkier here because it seems that there are too many cosmic rays. If this was all due to dark matter, then it would conflict with other measurements that have eliminated dark matter possibilities. It’s very clear that more work needs to be done in both cases, but these are still very exciting times.

Earthly detection

There are also many people who spend time at the bottom of mine shafts, hoping to detect dark matter. In this case, we rely on the fact that the Solar System is orbiting the galaxy with its plane at an angle to the galactic plane. The result is that the Earth experiences a dark matter head wind as the Sun drags it around the galaxy. However, as the Earth orbits the Sun, it experiences a stronger and weaker wind on an annual basis, similar to Earthly seasons.

This idea has been the subject of an extensive search, and the DAMA collaboration in Italy has been claiming dark matter detection for about a decade now. Indeed, their signal is so consistent that they are now over nine standard deviations from the noise in their data (for particle physics, five standard deviations are required to claim a new particle).

But no other detector has seen this signal. Indeed, Laura Baudis from the University of Zurich and Patrick Decowski and Andrew Brown from Nikhef in Amsterdam presented results from the XENON collaboration showing that they have eliminated many possible dark matter explanations for the DAMA signal (and with higher sensitivity than DAMA). Furthermore, they still have an entire run of data left to analyze. However, the real key will be an independent verification. In order to do provide this data, a replica of the DAMA experiment is being constructed in Antarctica.

The conflicting observations are not really a controversy at the moment. As in the case with astrophysical detection, you need to subtract a lot of background signals from these experiments.

The basic idea behind detection is that a dark matter particle can sometimes collide with a normal particle. The recoil can do one of several things: generate heat, generate light if the nucleus is suddenly moving faster than the local speed of light (light travels slower in matter than in vacuum, so this is not a violation of the laws of physics), or generate light by stripping an electron or two from an atom. All of these events can also be generated by background radioactivity, cosmic rays from space, and other potential annoyances.

Eliminating these events requires more than just understanding how often they happen, but also how strongly the material you are using as a sensor responds to the various different possibilities. It turns out that this process has a very large degree of uncertainty in it. The last run of the XENON detector is actually dedicated to reducing these uncertainties.

The XENON collaboration is on the verge of completing its upgrade: researchers are increasing the size of their instrument so that it contains one ton of liquid xenon in the detection volume (the previous incarnation used 62kg of xenon). At the same time, they hope to eliminate a large amount of background. At the moment, they record about one event per 10 kilograms of xenon per year, but they want to reduce that to one event per ton of xenon per year, which would be a remarkable achievement.

With more xenon, a better understanding of how xenon reacts to background radiation, and a reduced noise level, the XENON collaboration is looking forward to a bumper 2016. This is, however, not the limit of the researchers' ambitions. They are already in the planning stage to upgrade XENON to a multiton device. In the end, they want to be able to do dark matter spectroscopy, which will require yet another upgrade or two beyond that. Big plans and a bright future.