Cosmic-ray results auger well for future

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Physicists excited by initial findings.

The first substantial results from the world's largest detector of cosmic rays are being presented this week at a conference in Mexico.

A small component of a large observatory.

The Pierre Auger Cosmic Ray Observatory is already the largest cosmic-ray observatory in the world even though it is only three-quarters finished. It sprawls across more than 3,000 square kilometres of the Argentine pampas near the city of Malargüe — an area a tenth the size of Belgium. Researchers from the 15 countries collaborating on the US$50-million project hope that after its 1,600 detectors and 24 telescopes have been commissioned around the end of this year the observatory may reveal the origins of the highest-energy particles bombarding Earth from space.

Among the results being presented at the International Cosmic Ray Conference in Merída, Yucatán on 3–11 July are new measurements of the energy spectrum of cosmic rays. The team confirms that above a threshold energy known as the GZK cut-off the number of cosmic-ray events falls sharply. Particles that start off with energies above this threshold are expected to be slowed down by interactions with the cosmic microwave background as they travel. Cosmic rays significantly above the cut-off energies would thus have to come from relatively nearby, which would imply that they were produced by strange new physics, such as the spontaneous decay of unknown massive particles.

An experiment in Japan called the Akeno Giant Air Shower Array sparked excitement when it failed to see the cut-off in data obtained up to 2004. Since then, though, data from the High Resolution Fly's Eye — a detector in Utah — have provided evidence for the reality of the cut-off. The new Pierre Auger data seem to confirm that the cut-off is real.

If there is no new physics in the form of particle decays or other exotic effects, though, there should be plenty of new astrophysics in understanding what sort of celestial mechanism might be pumping up incoming protons to energies millions of times higher than those produced by the most powerful Earth-based accelerators. One crucial step is to identify where in the sky they are coming from. Alan Watson of the University of Leeds, UK, a spokesman for the Pierre Auger project, says that the team has identified some interesting candidate regions of the sky, but won't reveal the details until it has obtained more data. “Unless I talk in my sleep, even my wife doesn't know what these regions are,” he says.

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Related external links

Homepage of Pierre Auger Cosmic Ray Observatory

The High-Resolution Fly's Eye

Akeno Giant Air Shower Array

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Hogan, J. Cosmic-ray results auger well for future. Nature 448, 8–9 (2007) doi:10.1038/448008b

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