Dark, and hidden - more options for DM candidate particles

Thus far in these posts, we’ve discussed WIMPs, WISPs and axions. Whilst these- particularly WIMPs and axions- represent the prevalent candidate regimes for direct detection experiments, they are by no means the only theoretical possibilities for DM candidates.

And, as does occur in physics, ideas which at one time are considered only as theoretical, do indeed enter the province of experimental study (cast your mind back to the 2012 announcement of the finding of the Higgs boson).

Classifying the universe that we see - and all its constituent particles, forces and their interactions- as the visible sector, it may be suggested that there are invisible sectors, containing particles which interact with the visible sector only to the slightest amount and which, therefore, are thus far undetectable and unknown to us. This is not inconsistent with our current extent of knowledge of particle physics; it’s just that we currently have no way of detecting these particles. Hence, the appellation “hidden”.

That is, the dark-sector particles do not interact with the electro-weak and strong forces (or do so at an as-yet undetectably low level); however, the hypothesis is that they are able to interact gravitationally with visible-sector particles. These are the conditions that we ascribe to dark matter particles- gravitational interaction, but very little interaction besides.

Also, we require that DM particles be stable on cosmological time-scales; if a candidate DM particle is not stable on such time-scales, it may not be the best candidate. Some theories suggest that WIMPs may not be stable in this way, and that lighter (and thereby more stable) dark-sector particles may make better candidates.

So the theoretical discussion continues… with SUSY (supersymmetry) and WIMPs, WISPs and axions in the visible sector, and String/M-theory examining the hidden sector(s). And while this particular regime of candidate DM particles remains in the realm of theory at the present time, investigation into ways to detect such possible particles experimentally is under way.