# Assuming SUSY is found to be incorrect, what would then be the most compelling candidates for dark matter?

From what I've read, the only remaining candidates appear to be either sterile neutrinos or MOND (MOdified Newtonian Dynamics -- it does seem to keep changing.)

Did I miss anything else plausible?

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Are you asking about what would happen if SUSY is shifted to such a high energy scale that it is useless for dark matter or do you want to know about what dark matter could be if there is no SUSY at all at any energy scale? –  Dilaton Nov 28 '12 at 12:49
The dark matter model has the serious problem that cannot explain lots of data. Therefore searching some candidate looks sterile... –  juanrga Nov 28 '12 at 18:52
@Dilaton: no SUSY at all at any energy scale –  Walt Donovan Nov 28 '12 at 21:39
There are plenty of logical alternatives that are independent of supersymmetry. Almost any weakly interacting particle that isn't too heavy to freeze out in the early universe will do (eg those arising from little higgs models for instance). Many of these haven't quite been ruled out yet at the LHC. Axions as one of the answers below is another popular and consistent alternative, more generally condensates of many forms. And there are other ideas as well, eg primordial black holes or modifying gravity etc. –  Columbia Nov 29 '12 at 3:31

A plausible candidate is axions. These are (hypothetical) very light, very weakly-interacting particles, and notoriously difficult to actually detect. They are however well-motivated; they were first proposed as a solution to the 'strong CP' problem, and also arise automatically in string theory compactifications.

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But for this scenario to be possible, I have heard that SUSY must not be completely incorrect but rather shifted to a higher (at the moment not observable and for dark matter useless) energy scale ...? –  Dilaton Nov 28 '12 at 12:49
@Dilaton: There must be a slight misunderstanding somewhere; axions (and axion dark matter) are logically independent of supersymmetry. –  Rhys Nov 28 '12 at 14:27
Hm, I just thought that if the axions come from ST for example, SUSY should not be completely absent at all scales? –  Dilaton Nov 28 '12 at 14:32
There is actually at least one axion search search on at Fermilab. A "shine a laser through a wall" rig using a couple of decommissioned tevatron magnets as the conversion regions. Cool stuff. –  dmckee Nov 28 '12 at 16:02
@Dilaton: Read up on Peccei-Quinn symmetry. –  user1504 Nov 28 '12 at 16:20

Sterile neutrinos are candidates with or without SUSY. Though it is hard to get both the temperature and the total mass right without introducing several flavors.

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I wonder if you have an opinion on the $\nu$MSM which has keV scale neutrino dark matter: arxiv.org/abs/1208.4607 –  Michael Brown Jun 11 '13 at 10:02

It has been argued that structure on subgalactic scales is incompatible with standard ideas about cold dark matter, and that instead it indicates "warm dark matter" made of particles with a mass of about 1 keV.

At the same time, there are possible observations of dark matter accumulating at scales of 10 GeV and >100 GeV, which is more in the usual range for CDM theories; but the data is a little complicated, and people are proposing models that are more complex than before - e.g. "A Theory of Dark Matter", in which there is a new force specific to dark matter, and "Double-Disk Dark Matter", in which most of the dark matter is a structureless, noninteracting halo, but a small fraction of it interacts and forms structures.

The logical combination of these ideas would be, that the dark matter is mostly a homogeneous cloud of stable keV-mass particles, with a subpopulation that interacts at the GeV scale. There are a number of models out there already, that have keV- and GeV-scale particles, but I don't see anything in the literature that really fits the preceding description. Apparently there hasn't been any crossover between "warm DM" and "interacting DM" research. But I bet it's coming.

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