Tuesday, December 6, 2016

Data Disfavors Self-Interacting Dark Matter Models

There are two leading dark matter particle theories: warm dark matter (involving particles of about 2 keV/c^2 mass that behave more or less like sterile neutrinos), and self-interacting dark matter (SIDM). A new paper concludes that one of those approaches, SIDM, is almost entirely ruled out by existing astronomy data.
Coupling dark matter to light new particles is an attractive way to combine thermal production with strong velocity-dependent self-interactions. Here we point out that in such models the dark matter annihilation rate is generically enhanced by the Sommerfeld effect, and we derive the resulting constraints from the Cosmic Microwave Background and other indirect detection probes. For the frequently studied case of s-wave annihilation these constraints exclude the entire parameter space where the self-interactions are large enough to address the small-scale problems of structure formation.
Torsten Bringmann, et al., "Strong constraints on self-interacting dark matter with light mediators" (December 2, 2016).

The conclusion of the paper notes that:
Models of DM with velocity-dependent self-interactions have recently received a great deal of attention for their potential to produce a number of interesting effects on astrophysical scales. We have shown in this Letter that these models face very strong constraints from the CMB and DM indirect detection. In the most natural realization of this scenario with a light vector mediator with kinetic mixing, these constraints rule out the entire parameter space where the self-scattering cross section can be relevant for astrophysical systems. These bounds remain highly relevant for a number of generalizations of the scenario, such as a different dark sector temperature and different mediator branching ratios. Clearly, future efforts to develop particle physics models for SIDM need to address these issues in order to arrive at models that provide a picture consistent with all observations in cosmology, astrophysics and particle physics.
So, at this point, dark matter particle theorists are pretty much stuck in a "warm dark matter or bust" position, as all alternatives are poorer fits to the data or are entirely contradicted by the data.

Then, scientists must determine if this one remaining decent contender in the dark matter particle arena can rival or outperform all of the several quite viable modified gravity theories that explain dark matter phenomena, some of which are particularly promising because they are inspired by quantum gravity corrections to general relativity.

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