Here’s an extremely clever and fun idea (via arxiv blog). A while back the PAMELA experiment claimed to see an excess of high-energy positrons in cosmic rays — a signal that could come from imperfectly-understood astrophysical objects such as pulsars, or might be produced by something more exotic like dark matter annihilations. Some damper on enthusiasm for this idea was introduced by new results from the Fermi observatory, but it wasn’t completely conclusive, since Fermi’s detectors can’t actually distinguish between positrons and electrons.
So now Pierre Colin and collaborators have hit upon a cute way to distinguish between electrons and positrons: treat the magnetosphere of the Earth like the interior of a giant particle detector. Ever since cloud chambers, physicists have put magnetic fields in their detectors to help distinguish between positively charged particles and negatively charged particles, which get pushed in opposite directions. Well, the Earth has a magnetic field, so maybe we can use that. The problem is that the positrons and electrons would still all hit a telescope such as MAGIC, so the fact that they were deflected by the magnetic field wouldn’t be very relevant.
But Colin et al. suggest a trick: using the Moon’s shadow. Let’s imagine that the excess positrons really are coming from dark matter annihilating in the galactic center. When the moon is near the position of the galactic center in the sky, it will block out some of those particles, casting a shadow on ground-based telescopes. That’s already interesting, but the fun part is that positrons and electrons will be deflected by the Earth’s magnetic field, so the positron shadow will be in a slightly different position than the electron shadow! Using that effect, it may be possible to distinguish between the signals.
I am completely unable to judge how feasible this actually is. But the idea is sufficiently imaginative, I’m sure rooting for it.