Do strongly interacting dark matter particles interact with baryons?
A series of discrepancies between cosmic structure formation simulations and observations of cosmic structures motivated the idea of strongly interacting dark matter - a form of dark matter which, unlike the "traditional" dark matter which interacts very weakly with itself and the rest of the world, can interact with itself through the strong force. However, the strong force is also felt by ordinary matter, and the question arises: are strongly interacting dark matter particles allowed to interact with ordinary matter (baryons) as well, or is such a possibility already excluded by existing observations?
We have addressed this problem by examining what would happen if we "turned on" strong dark matter - baryon interactions in two distinct physical environments: the very early universe, and more specifically the era of big bang nucleosynthesis, and the present-day Galaxy.
During big bang nucleosynthesis, if dark matter interacts strongly with baryons it may break up light elements as they are trying to form, and delay the onset of nucleosynthesis. However, detailed calculations incorporating this process showed that the effect on the resulting abundances of elements is minimal - big bang nucleosynthesis is OK with strong dark matter - baryon interactions.
The present-day Galaxy presents another sensitive probe of baryon-dark matter interactions. The Galaxy is bathed in a continuous flux of highly energetic baryons, the cosmic rays. It also possesses large amounts of dark matter, which simply sits there minding its own business. If, however, an aggressive cosmic ray comes along and hits a dark matter particle on the head, then, assuming dark matter particles can feel the strong force and through it interact with baryons, a pion may be born. Pions are generally mentally unstable, and they fall apart shortly after their birth, in a spectacular nervous breakdown which results in the production of two gamma rays. Putting our gamma-ray goggles on, we can look for such gamma rays which would betray the presence of the aforementioned interactions. However, the gamma rays that we see are too few and can all be explained as a result of other processes (for example, the interaction of cosmic rays with ordinary intergalactic gas), so we conclude that the degree to which strong dark matter - baryon interactions are allowed is severely limited, at least when the particles involved have energies comparable to those of cosmic-rays.
Reference: Cyburt, R.H., Fields, B. D., Pavlidou, V. & Wandelt, B. 2002, PhysRevD, 65, 123503