A Comedown for Cosmic Rays

By Adrian Cho
ScienceNOW Daily News
3 July 2007

Really energetic particles should hit Earth only very rarely. After all, the number of cosmic rays pelting Earth decreases steadily as the energy of the rays increases. Above a specific energy, the rate ought to drop even faster. For example, if the rays consist mainly of protons, then at such tremendous energies they ought to break into other subatomic particles when they collide with photons in the afterglow of the big bang, the cosmic microwave background. That effect is known as the GKZ cutoff and it should limit the energy of cosmic rays to about 10¹⁸ electron volts, a million times the energy achieved with particle accelerators. But from 1990 to 2004, physicists working with the Akeno Giant Air Shower Array (AGASA) west of Tokyo spotted roughly a dozen particles with energies 100 times higher (Science, 14 August 1998, p. 891). That excess puzzled physicists, both because they could not explain how the rays got past the GZK cutoff or how they could gain so much energy in the first place.

However, a group working with a different detector claimed they saw no such excess. Physicists working with the High Resolution Fly's Eye (HiRes) detector at the U.S. Army's Dugway Proving Ground saw only a couple events at such energies. Researchers suspected the discrepancy arose from the very different methods used by the detectors to study the rays. When an ultra-high energy cosmic ray strikes the atmosphere, it creates a cascade of charged particles and light called an extensive air shower. AGASA used an array of particle detectors on the ground to measure the showers. In contrast, HiRes used specialized telescopes to detect the light produced when all those particles excite nitrogen molecules in the air and cause them to fluoresce.

Now, a new gigantic cosmic ray detector that employs both techniques has settled the issue. The almost-completed Pierre Auger Observatory on the plains of the Pampa Amarilla in western Argentina comprises more than 1200 ground detectors and 24 telescopes and covers an area of 300 square kilometers (Science, 21 June 2002, p. 2134). And the array has already collected enough data to rule out an excess in cosmic rays above 10²⁰ eV. "If the AGASA had been correct, then we should have seen 30 events [at or above 10²⁰ eV], and we see two," says Alan Watson, a physicist from the University of Leeds, U.K., and spokesperson for the Auger collaboration. The team will present its results here this week at the 30^th International Cosmic Ray Conference.

Meanwhile, researchers working with HiRes, which stopped taking data in last year, say that they have definitely observed the GZK cutoff in their final spectrum of cosmic rays. "It looks very much like what everyone has been predicting," says Pierre Sokolsky of the University of Utah in Salt Lake City. "It's the classic GZK cutoff." Watson disagrees because, he says, details of the Auger data suggest the highest energy cosmic rays aren't all protons, but also include heavier atomic nuclei, which shouldn't be stopped by the cutoff.

Physicists still cannot explain how even a few cosmic rays reach such tremendous energy. The key to answering that question may come in identifying exactly where in the sky the rays emanate from. Auger hasn't seen any evidence for such point sources yet. But the experiment will likely run another decade, Watson says, and researchers have already noticed a couple of curiosities in the sky, which he's not willing to discuss just yet.

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