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The Sciences

Dark Atoms

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Almost a year ago we talked about dark photons -- the idea that there was a new force, almost exactly like ordinary electromagnetism, except that it coupled only to dark matter and not to ordinary matter. It turns out to be surprisingly hard to rule such a proposal out on the basis of known astrophysical data, although I suspect that it could be tightly constrained if people did high-precision simulations of the evolution of structure in such a model. In fact our original idea wasn't merely the idea of dark photons, it was dark atoms -- having dark matter bear a close family resemblance to ordinary matter, all the way to having most of its mass be in the form of composite objects consisting of one positively-charged dark particle (a "dark proton") and one negatively-charged dark particle (a "dark electron"). We thought about it a very tiny bit, but didn't pursue the idea and only mentioned it in passing at the very end of our paper. There is an informal rule in theoretical physics that you should only invoke the tooth fairy (propose an extremely speculative idea or hope for some possible but unprovable result) once per paper, so we stuck with only a single kind of charged dark particle. But once someone invokes the tooth fairy in their paper, anyone who writes another paper gets to invoke the tooth fairy for themselves. (That's just how the rule works.) And the good news is that it's now been done:

Atomic Dark Matter Authors: David E. Kaplan, Gordan Z. Krnjaic, Keith R. Rehermann, Christopher M. Wells Abstract: We propose that dark matter is dominantly comprised of atomic bound states. We build a simple model and map the parameter space that results in the early universe formation of hydrogen-like dark atoms. We find that atomic dark matter has interesting implications for cosmology as well as direct detection: Protohalo formation can be suppressed below $M_{proto} sim 10^3 - 10^6 M_{odot}$ for weak scale dark matter due to Ion-Radiation interactions in the dark sector. Moreover, weak-scale dark atoms can accommodate hyperfine splittings of order $100 kev$, consistent with the inelastic dark matter interpretation of the DAMA data while naturally evading direct detection bounds.

(Note that one of the authors has been a guest-blogger here at CV.) It looks like a great paper, and they seem to have done a careful job at chasing down some of the interesting implications of dark atoms. In fact the idea might be more robust than that of the one in our paper; the fact that dark atoms are neutral lets you slip loose of some of the more inconvenient observational bounds. And the last sentence of the abstract points to an intriguing consequence: by giving the dark matter particles some structure, you might be able to explain the intriguing DAMA results while remaining consistent with other (thus far negative) direct searches for dark matter. Stay tuned; that dark sector may turn out to be a pretty exciting place after all.

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