Thursday, January 28, 2010

Dark Matter Claims Face Challenge

Recent claims of possible dark matter origin for excess high energy electrons and positrons from Fermi, HESS, ATIC, and PAMELA may be down the tubes. It seems that a new model has shown a more conventional explanation for these excesses and thus, causing more doubt that these were signatures from the presence of dark matter {link available for free only for a limited time}.

Galactic electrons are thought to originate in the explosion of supernovae, and conventional models predict that they lose energy as they pass through the Milky Way's magnetic field. The annihilation of proposed dark-matter particles would also create electrons, and some theorists had interpreted the recent experimental detections of surplus high-energy electrons as evidence for this process.

But starlight also scatters the electrons. Petrosian says that starlight suppresses the energy of most electrons in a way that makes it seem as if there is an excess of certain high-energy electrons. The Stanford group's models show an excess that is similar to that reported by NASA's Fermi Gamma-ray Space Telescope; the High Energy Stereoscopic System (HESS), a ground-based detector in Namibia; and the Advanced Thin Ionization Calorimeter (ATIC), a balloon-borne detector that flew over Antarctica.
But by tweaking parameters in their model, the Stanford group can also mimic the PAMELA results. Like the electrons, the positrons are also thought to originate near supernovae — although through secondary collisions of protons. By increasing the density of gas and the number of photons near these supernovae — both possible scenarios given that supernovae occur in gas-rich star-forming regions near lots of stars — the model predicts high-energy positrons similar to those reported by PAMELA.

The exact reference to the paper is:

L.Stawarz et al., Astrophys. J. v.710, p.236 (2010).

This certainly throws a huge damper on those theorists who think they've seen tantalizing evidence of dark matter beyond just astronomical observations.


1 comment:

Doug said...

Local pulsars are also viewed as a possible conventional explanation. Of course, the fundamental problem with any claims involving these cosmic rays, conventional or otherwise, is that the model for how they propagate through the galaxy is very uncertain.