Looking for Dark Matter in the Moon's Shadow

By Sean Carroll | July 9, 2009 9:32 am

moonshadow.jpg 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.

  • Ellipsis

    I thought air Cherenkov detectors such as MAGIC could only operate decently on moonless or nearly-moonless nights, due to moonlight swamping the signal…

  • http://blog.denniswilliamson.us Dennis


    It’s done during the new moon!

  • Rules For

    But then sunlight would swamp the signal.

  • http://quantumnonsense.blogspot.com/ Qubit

    Next they will be using the earths gravity to to detect gravity waves!!!

  • jdporter

    The article says:
    “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, … exactly fits the bill. ”
    So apparently there is no moonlight swamping problem, at least for some instruments.

  • eric gisse

    Fantastically clever, but would the imagined count rates be definitive enough?

  • http://squarkonium.blogspot.com Squark

    Why is it that some detectors cannot distinguish between electrons and positrons? Is it related to Pomeranchuk’s theorem (which says that the ratio of the cross sections of a particle and its antiparticle on a given target approaches 1 at high energy)?

  • http://lablemminglounge.blogspot.com/ Lab Lemming

    Is the galactic centre close enough to the eccliptic?

  • Haelfix

    The obvious problem is the statistics. It will take a long time to get enough data to distinguish out all the myriad uncertainties.

  • Rules For

    Lab Lemming:
    The galactic center is in Sagittarius, which is along the ecliptic.

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  • confused

    eric gisse @ 6

    Fantastically clever, but would the imagined count rates be definitive enough?

    Dunno. Flux, directionality and mass-energy at the detector are all interesting; positron-bright areas of the sky that are not known to have astrophysical sources of positrons but which are suspected to contain dark matter overdensities (because of the presence of a black hole, for example) would be neat. Sagittarius A* is unfortunately in an area of the sky which is likely to be bright with positrons for other reasons, but there are likely to be intermediate-mass black holes in large globular clusters and at the cores of dwarf galaxies in orbit around the Milky Way. That means other nearby massive sub-galactic structures, some of which are well outside the plane of the baryonic-matter disc, are good places to look for an excess of positrons. Of course, so are more distant galactic cores, which are likely to contain supermassive black holes.

    A surplus of positrons closely associated with a dense high mass would favour hypothetical DM particles that are their own anti-particles (e.g. neutralinos) — denser collections of such particles will result in more annihilations.

    Other possibilities have been written down however, so the energy spectrum of the positrons becomes important in distinguishing among (as examples) DM-DM “hadrophobic” annihilation, DM-DM fusion that releases e+ as a daughter product, and some DM decay models, for example.

  • M

    “When the moon is near the position of the galactic center in the sky, it will block out some of those particles”. NO: directions of charged cosmic rays are randomized by galactic magnetic fields.

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  • Brian137

    The paper states that the moon creates a hole in the “isotropic CR flux.” The authors separate that flux into three components: e+, e-, and “diffuse rays.” I skimmed the article and found no stipulation that the moon needed to be between the Earth and the galactic center (I am, however, a notoriously bad skimmer). Regardless, the paper claims that three spatially separated shadows will result.

  • Jojo

    The idea of using the moon shadow to detect antimatter is not new. In 1996-97, the ARTEMIS experiment utilized the Whipple Imaging Atmospheric Cerenkov Telescope in an attempt to detect cosmic antiprotons. Pointing a Cerenkov Telescope at a full moon is tricky business.

    More info can be found on ARTEMIS can be found at:

  • http://www.carallelworld.blogspot.com/ carol gibson

    Antimatter is discussed in ancient documents, too.

  • http://viXra.org Xavier Terri

    Teoría Conectada: The best physics theory since 1687

    The new Copernican revolution

    I read Lee Smolin’s book ‘Las dudas de la física en el siglo XXI’, 2007, Ed. Crítica. Wonderful. I have seen that Lee Smolin are looking for a new big idea, the fundamental simply idea for the progress and unification of physics. Seems that Smolin got the conviction that both quantum mechanics and GR theorys don’t understand the deep nature of time (page 355). It is right for GR unless.

    Here is what i say:
    No more Lorentz’s Transformations. The new alternatives transformations (’relational transformations’) are deduced on ‘LA PARADOJA DE LOS GEMELOS DE LA TEORÍA DE LA RELATIVIDAD ESPECIAL DE EINSTEIN’, f.i., equations (22) and (23) with “C” and “D” given by (42) and (43) , pages (33) to (36). From them arises the ‘teoría relacional’, an alternative to special relativity.
    The generalitation of this ‘teoría relacional’, the unique possible classic alternative to GR, appears on ‘EXTRACTO DE LA TEORÍA CONECTADA’. 3 are the fundamental equations. (84), (171) and (172), pages (146) and (182). This 3=24 equations are necessary in order to eliminate the Newton-Einstein’s absolute space. There are not absolutes accelerations (neither absolutes velocities, of course).

    The DARK MATTER problem is solved in ‘Apéncice C’, page 205 (Exponential factor gets important at large distance from the center).

    What about a “quantum teoría conectada”? I believe that you can say something important about it.

    Xavier Terri
    Terrassa 2009 august 6

    P.D.: Smolin is completely certain when he tell that the great historic mistake comes from Descartes-Galileo (‘Las dudas de la física en el s.XXI’, pages 355-356). This great historic error is the Principle of Inertia (movement is a relational concept. It is completely false that the movement of ALL free bodies is a straight line. Some of them move, respect the SAME reference system, in a curved way. Spacetime metric defines a specific relationship betwen bodies and reference system.Pure evidence beyond the reason: at night, see the stars). Also, Principle of Inertia leads us to the inertial-non inertial dichotomy. Where is INVARIANCE of physical laws? (pages 45 on ‘Paradoja’ and 141, eq. (77), on ‘Teoría Conectada’).


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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