Where are we with the Dark Matter search?

By observing the movement and the distribution of stars and galaxies, we learn that about 24% of the Universe is made of Dark Matter – an unknown type of matter whose origin is one of the main mysteries still kept by Nature. The world’s scientists are testing experimental methods to identify the particles of this elusive matter. How long will it stay in the “dark”? How can the LHC experiments participate in the race for discovery?


Figure 1: Dark Matter particles produced at the LHC would presumably escape undetected by the experiments. However, the event should be accompanied by some "missing momentum", which could be a signature of Dark Matter. Within the framework of a simple model for the production of Dark Matter, the CMS analysis significantly complements the sensitivity of direct search experiments. In particular, CMS is sensitive in the low-mass region below 3.5 GeV (the regions above the curves are excluded). Source: CMS Collaboration, arXiv e-print: 1204.0821.

The name itself indicates its “invisibility”. Identifying Dark Matter is no easy task. So far, its existence is only inferred by cosmological observations of the movement and distribution of stars and galaxies that can only be explained if we assume that “something new” is out there. This “something” cannot be ordinary matter otherwise we would see it with our current devices.

Given the issue at stake, a large part of the scientific community is involved in finding the best ways to investigate this mystery. And, although we do not have any final answer yet, it seems like the solution is no longer so far away. At least, if Dark Matter is made of “Weakly Interacting Massive Particles” (WIMPs) that scientists have so far identified as the most likely candidates. “This is a very exciting time for Dark Matter research,” says Marco Cirelli from CERN’s Theory Unit. “Experiments are producing data at an amazing pace. Some of them are reporting tantalising hints, while many others are increasingly constraining the parameters to regions where we could still find WIMPs. The situation evolves literally day by day. This is often quite confusing but it may actually be good: confusion might precede a breakthrough.”

Experiments try to find Dark Matter in essentially two ways: directly, by observing the recoil of nuclei that might be caused by Dark Matter particles hitting atoms of ordinary matter; and indirectly, by observing transformations, for example the annihilation into gamma rays, of Dark Matter particles. In the first category, we find experiments such as Xenon, DAMA, CoGeNT and CRESST; in the second category we find the space experiments Pamela, Fermi and AMS. As for the LHC experiments, they could observe new particles – such as those predicted by supersymmetry or by extra-dimension theories – that are very strong candidate components of Dark Matter. “In principle it is possible to combine data from the LHC and other experiments,” says Marco Cirelli. “However, this requires choosing a well defined theoretical model to describe the interactions between Dark Matter particles and the quarks that make up protons circulating in the LHC.” This is not a trivial exercise, but it has recently been accomplished by CMS and ATLAS, as explained on figure 1.

Figure 2: The excess of 130 GeV gamma rays from the centre of the galaxy, recently observed in data from the Fermi space observatory (peak in the red curve). It could be a signature of Dark Matter particles of the same mass. Although this result is very interesting and tantalising, it needs further investigation to be fully understood and correctly interpreted. Source: Christoph Weniger, arXiv e-print: 1204.2797 (in press on JCAP).

News on Dark Matter is also coming from space. Recent data from the Fermi space observatory has shown a signal that could be interpreted as a signature of Dark Matter particles: an excess in the number of gamma rays at 130 GeV coming from the centre of the galaxy (see figure 2). Such a signal could be produced by the annihilation of the elusive particles. “It is too early to say anything conclusive about this,” clarifies Marco Cirelli. “More data is needed and scientists need to understand the origin of the signal and whether the same observation can be made in different regions of the Universe.”

In this complex scenario, there is one thing scientists seem to agree on: the recently discovered boson and its relatively low mass seem to point to new physics that should definitely not be out of the reach of the LHC experiments. “If Dark Matter is really made of WIMPs, we should be able to identify them over the next few years of experimental activity,” confirms Marco Cirelli. And if WIMPs do not exist? Well, in this case, a very large portion of the experimental approach to provide a solution to the mystery will need to be redesigned…

by Antonella Del Rosso