## November 25, 2004

### 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.

Posted by distler at November 25, 2004 12:08 AM

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### Re: Dark Exoticism

Well, a diffuse *dim* source of gamma rays at 511Mev has got to be pretty exotic right?

I mean, anything dim but that hot has to have very weak electromagnetic interactions, otherwise it would be bright and cool down quickly?

Then the fact that it’s not thermal spectrum but an out-of-equilibrium source of positrons makes it even more exotic?

A priori, wouldn’t we expect a dim diffuse source of positrons to be a heavy-but-slowly-interacting state - i.e., dark matter?

Any other explanations - aliens pointing positron beams at us?

Posted by: Ralph on November 25, 2004 9:03 PM | Permalink | Reply to this

### Point sources

Well, it’s not impossible that this has a more conventional explanation. There might be point sources of positrons, such as supernovæ, with a positron mean free path which happens to be “just right”: short enough so that they all annihilate within the galactic core, long enough so that it wipes out the “point source” origin of the positrons and yields a diffuse source of 511 KeV photons.

Posted by: Jacques Distler on November 26, 2004 10:29 AM | Permalink | PGP Sig | Reply to this

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