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The “Death Cries” of Dark Matter? April 4, 2013

Posted by Will Thomas in Current Affairs.
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The cosmic ray energy spectrum is in the news! The Alpha Magnetic Spectrometer experiment (AMS-02), mounted on the International Space Station, is reporting results about the prevalence of positrons in the cosmic radiation, which otherwise comprises mostly protons. This is being touted as newsworthy, because, if there is a drop-off in that prevalence at higher energies, it will corroborate certain theories of dark matter, which propose that the mutual annihilation of dark-matter particles generates positrons of energies up to but not exceeding levels corresponding to those particles’ high mass.   Similarly enticing results were reported by the PAMELA (Payload for Anti-Matter Matter Exploration and Light-Nuclei Astrophysics) experiment in 2008.  The sophistication of AMS-02 will hopefully be able to take those measurements further, but, unfortunately, we will have to wait a while for more definitive results from higher-energy parts of the spectrum.

The AMS mounted on the International Space Station. Credit: NASA

AMS-02 mounted on the International Space Station. Credit: NASA

What is intriguing about this story is that it really brings us back to where particle physics began over 80 years ago.  In 1930 Robert Millikan (1868-1953), the doyen of physics at the California Institute of Technology, set postdoctoral researcher Carl Anderson (1905-1991) to work on building a cloud chamber in order to measure the same thing AMS-02 is designed to measure, the energy spectrum of cosmic rays.  Millikan believed that measuring the spectrum would confirm his controversial (and incorrect) theory that cosmic rays originated as photons produced in the interstellar synthesis of elements, which then created secondary radiation when they encountered atmospheric nuclei.  Much in the way that every element emits a characteristic spectrum of light, Millikan figured that the energy spectrum of this secondary radiation would cluster into characteristic bands, observing which would, in effect, be like listening to the “birth cries” of the elements.1