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Cosmic Ray Moon Shadow Could Reveal Dark Matter

If a strange excess of positrons hitting Earth are created by dark matter, then the way that the moon blocks these impacts could help confirm the idea.

The earth is constantly bombarded by high-energy positrons and electrons. These bombardments generate showers of secondary particles that light up our skies at night, if you have the right equipment to see ‘em: so-called imaging atmospheric Cherenkov telescopes. The ratio of electrons to positrons is predicted fairly precisely by our models of the way that cosmic rays interact with objects in the Milky Way.

But here’s a conundrum. Various space-based experiments such as PAMELA have recently found an excess of positrons out there, particularly at energies above 10 GeV. That’s totally unexpected and difficult to square with the conventional model.

The PAMELA measurement generated excitement because the dark-matter brigade pounced on the result as evidence that dark-matter particles must annihilate each other, producing the excess positrons in the center of our galaxy. These guys were forced to put the champagne back on ice when other astrophysicists pointed out that the positrons could equally be created by particle cascades in the magnetospheres of nearby pulsars.

What’s needed, of course, is more measurements of positron/electron ratios, particularly at energies up to a few TeV that cannot yet be made by space-based experiments.

Can the growing number of imaging atmospheric Cherenkov telescopes help? On the face of it, that looks unlikely because there is no way to tell apart the showers created by positrons and electrons when they hit the atmosphere. At least until now.

Today, Pierre Colin and pals at the Max-Planck-Institut fur Physik, in Munich, have come up with an ingenious idea that should be able to tell them apart. Most of the electrons and positrons come from the galactic center. Colin and co point out that when the moon comes between us and the electron/positron source, it creates a shadow that is already used to calibrate imaging atmospheric Cherenkov telescopes.

But here’s the interesting idea: Colin and co say that the shadow of charged particles should be deflected by the earth’s magnetic field. The electron shadow should be shifted eastward and the positron shadow westward. These imaging atmospheric Cherenkov telescopes should therefore be able to spot the separate shadows, allowing the measurement of positron/electron ratios at energies up to several TeV–well beyond what space-based experiments can achieve.

What’s more, imaging atmospheric Cherenkov telescopes ought to be able to spot these shadows now as long as they can make measurements in the glare of the moon. One such instrument, called MAGIC, built by the Max-Planck-Institut fur Physik at Roque de los Muchachos, in the Canary Islands, exactly fits the bill.

The measurements will still be tricky, however, particularly of the positron shadow, which may well be superimposed on the shadow created by positively charged atoms in the cosmic ray spectrum. However, Colin and co think that they ought to be able to pick out the electron shadow with just 50 hours of observing (although that may take several years, given that the shadows occur only at certain times of the year).

That’s an ingenious idea that may well give astronomers a way of determining what role dark matter plays, if any, in the creation of these excess positrons.

Ref: Observation of Shadowing of the Cosmic Electrons and Positrons by the Moon with IACT

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