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.
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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.
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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.
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Ref: arxiv.org/abs/0907.1026:
Observation of Shadowing of the Cosmic Electrons and Positrons by the
Moon with IACT
