Gianfranco Bertone, “The moment of truth for WIMP Dark Matter,” ArXiv, Nov. 17, 2010 [Nature 468: 389–393, 18 November 2010], tell us everything we want to know about WIMP particles as candidates for dark matter. Let us summarize the paper.
We know that dark matter constitutes 85% of all the matter in the Universe. Current cosmological knowledge indicates that Dark Matter cannot be made of ordinary matter, so new particles must exist. Particle physicists have proposed literally tens of possible Dark Matter candidates. But the most studied class of candidates is that of WIMPs (for weakly interacting massive particles), i.e. the neutralino (proposed in 1983), which arise naturally from supersymmetric theories that seek to extend the Standard Model of particle physics. If Dark Matter is made of WIMPs, we should be able to detect it, by indirect or direct methods.
Indirect detection consists in the search for the annihilation or decay products of Dark Matter particles, such as photons, anti-matter and neutrinos (with energies between 1 GeV and 10 TeV). Although in principle interesting, obtaining convincing evidence from astrophysical observations has proven a very difficult task. For example, a very clean signature of Dark Matter annihilations would be the observation of high-energy neutrinos from the center of the Sun (with energies between 102-104 GeV neutrinos). The problem is that most of the Supersymmetry parameter space will remain inaccessible to current neutrino telescopes (such IceCube, currently under construction at the South Pole). Fortunately, although indirect searches may appear to be not particularly suited to provide incontrovertible evidence for Dark Matter, they have the big advantage of not requiring dedicated experiments, and that some theoretical models are indeed within the reach of current and upcoming experiments in the next 5–10 years.
Direct detection consists in the detection of the recoil energy of nuclei struck by Dark Matter particles traveling through a detector, through the measurement of the light, the charge or the phonons produced in the target material by the scattering event. The sensitivity of direct detection experiments has gone down by more than 3 orders of magnitude in the last 20 years. However, Dark Matter has not been identified yet.
This figure summarizes the current situation of direct Dark Matter searches, compared with theoretical predictions, in the wimp-proton cross-section. The theoretical predictions depend on the specific model considered. The stars correspond to a set of benchmark models in a Supersymmetric theoretical setup called Minimal Supergravity (mSUGRA). The red contours to that of the constrained Minimal Supersymmetric Model (constrained because the general class of supersymmetric models allow much more rich phenomenology); the blue contours show the result in a general Supersymmetric model with 7 free parameters. A big portion of the parameter space where theoretical models lie will be probed by ton-scale experiments that are expected to start operations within 5–10 years. This is good news, as for this set of parameters we can perform the program described above.
Accelerators (like the Large Hadron Collider at CERN) will allow to test the existence of new particles at the TeV scale. The search is not easy. The most motivated theoretical extension of the Standard Model, the so called Minimal Supersymmetric Standard Model, has however about 120 free parameters; obviously, some assumptions must be made in order to reduce the number of free parameters and make quantitative predictions for the mass and couplings of supersymmetric particles. The number of free parameters in the Dark Matter sector of the most popular supersymmetric models are: (1) constrained Minimal Supersymmetric Model and Minimal Supergravity are theories with only 4 free parameters; (2) the phenomenological Supersymmetric Model is a 7 free parameters theory (DarkSUSY code); (3) a less constrained version of this model has 24 free parameters. There are large portions of the Supersymmetric parameter space within the reach of the LHC, and there are good chances to discover it at the LHC within 5–10 years.
The main problem with the LHC is that even if Supersymmetry is discovered, and the mass spectrum of new particles is determined with good accuracy, reconstructing the relic density of the neutralino will be challenging. Fortunately, particle astrophysics experiments can provide complementary information on the nature of Dark Matter. In fact, direct searches provide an effective way to reduce degeneracies in the parameter space of new theories, when reasonable assumptions are made on the distribution of Dark Matter particles in the Milky Way. Hence, a combination of LHC and direct detection data is a due requirement.
Let us stay optimistic, though. The plans to detect Dark Matter in the near future have been laid out carefully, and they deserve to be carried out with the outmost care, as a discovery would mark the start of a new era of physics, and it would represent the best reward to decades of painstaking searches.