Musings

Dark Exoticism

Sonia Paban points me to a recent paper with a very heterodox view on Dark Matter. The jumping off point is the observation by the INTEGRAL $\gamma$-ray satellite of the “511 KeV line” from the center of our galaxy.

“511 KeV?” I hear you say, “Why … that’s … the mass of the electron!” Indeed it is. These $\gamma$-rays originate from $e^+e^-\to 2\gamma$ annihilation. Now, there are lots of point sources of positrons in astrophysics: supernovæ, blackholes, … . But what INTEGRAL finds is that the galactic bulge is a diffuse source of 511 KeV $\gamma$-rays. That suggests that the center of our galaxy harbours a diffuse source of positrons.

What could it be? Dark matter is clustered in the center of our galaxy. Boem et al postulate that the dark matter is a scalar of mass 1-100 MeV. And there’s a process, involving the exchange of some very heavy intermediate particle, by which these can annihilate into $e^+e^-$. The cross-section must be incredibly tiny; otherwise we’d have seen them already. Indeed, the whole scenario sounds like it ought to have been already ruled out in a half dozen different ways. But Boem and Fayet claim it squeaks by existing collider and astrophysical limits.

Anyway, since these scalars are so light, the positrons that are produced by their annihilation lose energy through ionization and annihilate, nearly at rest, before they leave the galactic core.

All in all, a pretty far-fetched scenario, but one sure to provoke discussion as we sit down to our Thanksgiving turkeys.