Signals from dark matter?

[David B]

According to the BBC, some think ratios of positrons to electrons emanating from different parts of the sky are signals from dark matter.

Others think not, and that the positrons come from pulsars.

Time will tell, I suppose

http://news.bbc.co.uk/1/hi/sci/tech/7977102.stm

David

[Codec]

I can't say I really follow the physics, why positrons/electrons would be evidence of dark matter. Dark matter doesn't interact with normal matter (like electrons/positrons) except gravitationally.
There is plenty of evidence that something like dark matter exists, but not much that tells us what it actually is.

[Berthold]

Dark matter doesn't interact with normal matter...
So it was, in the original concept, conjectured to be the case with neutrinos. :D

[Ray Moscow]

Dark matter doesn't interact with normal matter...
So it was, in the original concept, conjectured to be the case with neutrinos. :D

Perhaps we can just say that no other interactions have been observed so far.

[lpetrich]

I can't say I really follow the physics, why positrons/electrons would be evidence of dark matter. Dark matter doesn't interact with normal matter (like electrons/positrons) except gravitationally.
There is plenty of evidence that something like dark matter exists, but not much that tells us what it actually is.
However, many dark-matter candidate particles do have nongravitational interactions with ordinary matter, but very weak interactions.

In any case, two annihilating dark-matter particles will produce a shower of other particles, including electrons and positrons, even if their interaction is gravitational-only. They'd combine to make a virtual graviton, which then turns into other particles. It's possible to do a rough, hand-waving estimate of the annihilation cross section in such a case, and it's TINY.

For a certain plausible candiate, the Lightest Supersymmetric Particle of the Minimal Supersymmetric Standard Model, it's something around 10-60 its expected nongravitational value.

[Schneibster]

The key point here is that they're high-energy positrons. I'm not sanguine they're from dark matter; I'd like to know what interactions could create such high-energy positrons.

[lpetrich]

The key point here is that they're high-energy positrons. I'm not sanguine they're from dark matter; I'd like to know what interactions could create such high-energy positrons.
Anything with enough energy per elementary particle.

Ram some electrons and/or nuclei together with relative kinetic energies greater than 1 MeV, and you will get electron-positron pairs. It happens all the time in particle-accelerator experiments.

[Codec]

Fusion reactions, such as those in our Sun, create enormous numbers of positrons (as a proton decays into a neutron).
1H + 1H → 21D + e+ + νe + 0.42 MeV
However they normally combine with an electron in short order so I doubt many escape from the core.

Pair production of course produces equal amounts of electrons and positrons.

[lpetrich]

Codec is right. The proton-proton reaction, which he gave us, releases 0.42 MeV of energy, and the electron has about 1/3 of that on average, or 0.14 MeV.

Checking various references on energetic electrons (beta particles) gives a scaled penetration distance in matter of 0.02 - 0.12 g/cm2 (divide by the density to get the actual distance). The Sun's central density is about 160 g/cm3, giving a distance of 1 - 10 microns.

Thus, the positrons produced in the Sun's interior won't come close to escaping from the Sun.

[Schneibster]

The key point here is that they're high-energy positrons. I'm not sanguine they're from dark matter; I'd like to know what interactions could create such high-energy positrons.Anything with enough energy per elementary particle.

Ram some electrons and/or nuclei together with relative kinetic energies greater than 1 MeV, and you will get electron-positron pairs. It happens all the time in particle-accelerator experiments.Well, yeah, that's kinda my point. Ain't many places gamma rays come from.