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One promising puzzle piece for confirming dark matter now seems unlikely fit

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(Nov. 28, 2011) — Like jazz musicians who make up a melody as they go along, scientists often improvise even after an experiment is underway. One recent example of this comes from the Fermi Gamma-ray Space Telescope. Launched by NASA in June 2008, the $690 million telescope has since been working as advertised, providing scientists with the most complete look yet at gamma rays, the highest energy forms of light. But just two months after the launch, a tantalizing finding from a European experiment hinting at evidence of dark matter had Stefan Funk and Justin Vandenbroucke wondering if the telescope could be used to look at something for which it wasn't intended -- specifically, electrons and their antimatter twins, positrons, that are streaming across the universe in cosmic rays.

Using Earth as a Scientific Instrument

By finding a clever way to use Earth itself as a scientific instrument, researchers turned the Fermi Gamma-ray Space Telescope into a positron detector -- and confirmed a startling discovery from 2009 that found an excess of these antimatter particles in cosmic rays, a possible sign of dark matter.

Their problem was that the telescope, designed to detect neutral gamma rays, doesn't have a magnet for separating negatively charged electrons and positively charged positrons. So Funk, Vandenbroucke, and Stanford graduate student Warit Mitthumsiri, all at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University, started looking beyond the experimental hardware. They found a magnet a few hundred miles away from the telescope that might do the trick. It happened to be Earth itself, which, thanks to its magnetic field, bends the paths of charged particles raining more or less continuously from space. (The spectacular aurora visible at high latitudes is the result of charged particles being bent and funneled toward the poles and impacting Earth's atmosphere.)

After studying up on geophysics maps and calculating precisely how Earth was filtering out charged particles seen by the telescope, the researchers went ahead with their analysis, and wound up with somewhat dramatic results.

Their paper, submitted to Physical Review Letters and originally published on the internet physics archive on Sept. 2, confirmed the curious excess of antimatter positrons formally reported in the 2009 study in the journal Nature that had the physics world agog. However, Funk and Vandenbroucke's analysis is most noteworthy for what it didn't see -- a sudden drop-off of this excess in those cosmic rays beyond a certain energy level, as many theories predicted would happen if dark matter was involved. Their result casts doubt on the dark matter explanation, which is one reason why the paper started making news just four days after it was published online. The first scholarly paper on the implications of Funk and Vandenbroucke's work appeared on the physics archive soon thereafter. That paper declared that "the standard positron production scenario must be incomplete." In other words: Who knows where these excess positrons are coming from?

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The above story is reprinted from materials provided by The Kavli Foundation.

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Journal Reference:

  1. The Fermi LAT Collaboration: M. Ackermann, M. Ajello, A. Allafort, L. Baldini, G. Barbiellini, D. Bastieri, K. Bechtol, R. Bellazzini, B. Berenji, R. D. Blandford, E. D. Bloom, E. Bonamente, A. W. Borgland, A. Bouvier, J. Bregeon, M. Brigida, P. Bruel, R. Buehler, S. Buson, G. A. Caliandro, R. A. Cameron, P. A. Caraveo, J. M. Casandjian, C. Cecchi, E. Charles, A. Chekhtman, C. C. Cheung, J. Chiang, S. Ciprini, R. Claus, J. Cohen-Tanugi, J. Conrad, S. Cutini, A. de Angelis, F. de Palma, C. D. Dermer, S. W. Digel, E. do Couto e Silva, P. S. Drell, A. Drlica-Wagner, C. Favuzzi, S. J. Fegan, E. C. Ferrara, W. B. Focke, P. Fortin, Y. Fukazawa, S. Funk, P. Fusco, F. Gargano, D. Gasparrini, S. Germani, N. Giglietto, P. Giommi, F. Giordano, M. Giroletti, T. Glanzman, G. Godfrey, I. A. Grenier, J. E. Grove, et al. Measurement of separate cosmic-ray electron and positron spectra with the Fermi Large Area Telescope. Physical Review Letters, 2011 [link]

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