Section 7: Dark Forces
WIMPs and axions are compelling candidates for dark matter particles, but neither one has been detected experimentally. While ever-more sensitive laboratory experiments are conducted, theorists constantly develop new models, sometimes inventing new possibilities for dark matter. A plausible third candidate for dark matter has recently emerged, called dark forces. The dark forces theory is really an extension of the supersymmetry theory we first reviewed in Unit 2. In addition to the heavy WIMP particles, the latest version of supersymmetry theory posits the existence of light particles called the Greek letter phi. If the exists, it is predicted to be more massive than two electrons, but less massive than 200 electrons. It would interact with other particles just like a photon, but with an interaction strength at least 1,000 times weaker.
The idea for dark forces arose when an Italian cosmic ray experiment called "DAMA/LIBRA" (DArk MAtter/Large sodium Iodide Bulk for RAre processes) observed energetic electrons and positrons unaccompanied by antiprotons. Ordinary WIMPs cannot explain this DAMA/LIBRA result, but in the dark forces version of supersymmetry, heavy WIMP particles would annihilate with one another and produce high-energy particles. The particles would then decay into energetic electron-positron pairs.
Figure 19: A technician works on detectors for the DAMA/LIBRA project, which stimulated the theory of dark forces.
Source: © DAMA/LIBRA. More info
The emergence of the dark forces theory has led to a series of new ideas for current and new experiments. If the theory is correct, WIMPs produced in high-energy collisions at the Tevatron and Large Hadron Collider would decay to several particles. Those particles would then decay to a large number of electrons, positrons, or muons, giving a clear experimental signature. At low-energy colliders, the would manifest itself in rare decays of known particles. In lower-energy electron-proton collisions, extra electrons and positrons in the decay products would indicate that the collision produced particles. Physicists would need to gather a huge amount of data to test dark forces. Because the interacts with one-thousandth the strength of a photon, only one event in a million might contain a .
Although the dark forces theory arose to explain cosmic ray experiments and the DAMA/LIBRA results, it would still be viable even if the experimental basis were shown to be a fluctuation or result of a known process. Like axions and supersymmetry, the dark forces theory as yet has no solid experimental basis. However, it is a perfectly reasonable description of dark matter in every respect and should be experimentally pursued.
Supersymmetry theory has suggested other possible sources of dark matter. They include the gravitino, the supersymmetry partner of the graviton, and the electrically neutral neutralino, a particle with very small mass. Like other dark matter candidates, they have so far defied experimental efforts to detect them.