ASTROPHYSICS:
Enormous Detector Forces Rethink Of Highest Energy Cosmic Rays

MERIDA, MEXICO--When, a decade ago, physicists in Japan reported seeing far more ultrahigh-energy cosmic rays than expected, some theorists interpreted the excess as a hint of exotic new particles--perhaps supermassive relics from the big bang that could be part of the mysterious dark matter whose gravity holds the galaxies together. But the controversial excess of super-energetic particles from space has a simpler explanation, researchers with a far larger detector array now say: It doesn't exist.

That conclusion, reported here^* last week, may be the most important early result from the Pierre Auger Observatory, which sprawls across the Pampa Amarilla in western Argentina. It's also a disappointment for researchers in the field of ultrahigh-energy cosmic rays. "It is less sexy than before, that's for sure," says Yoshiyuki Takahashi of the University of Alabama, Huntsville.

Still, plenty of mystery remains. Auger and other arrays do see some cosmic rays with the energy of a large hailstone, and physicists still can't say how or where in the heavens a single subatomic particle might gain such energy. But now that researchers see that the number of cosmic rays dives as expected at very high energies, explanations will likely turn from exotic particles to the astrophysics of stars and galaxies.

The purported excess sparked controversy years ago (Science, 21 June 2002, p. 2134). From 1990 to 2004, physicists with the Akeno Giant Air Shower Array (AGASA) west of Tokyo spotted roughly a dozen particles crashing to earth at energies of 100 exa-electron volts (EeV), about 100 million times higher than any particle accelerator has achieved. Physicists believe cosmic rays gain energy as they swirl in magnetic fields, and they couldn't think of any object in space both big enough and wielding a strong enough magnetic field to contain particles until they reach such staggering energies. So some speculated that the rays blast out of the decays of supermassive particles. The excess also clashed with an energy limit predicted in the 1960s. If each ray is a proton, then at energies above about 40 EeV it should interact with the photons in the after-glow of the big bang, the cosmic microwave background, in a way that saps its energy to 40 EeV within a distance of 300 million light years. If AGASA was seeing rays with energies above this "GZK cutoff," then they had to originate in the cosmic neighborhood.

Moreover, another group saw no excess. Whereas AGASA researchers detected 11 rays with energies greater than 100 EeV, physicists with the High-Resolution Fly's Eye (Hi-Res) detector at the U.S. Army's Dugway Proving Ground in Utah saw only a couple. The two detectors are very different, however. When a high-energy cosmic ray strikes the atmosphere, it triggers a cascade of billions of particles called an extensive air shower. AGASA used 111 detectors spread over 100 square kilometers of ground to measure the showers. In contrast, Hi-Res used twin batteries of specialized telescopes to detect the light produced when the shower causes nitrogen molecules in the air to fluoresce.

The Auger array uses both techniques. Covering 3000 square kilometers and comprising more than 1300 surface detectors and 24 fluorescence telescopes in four batteries, the almost-completed array has already collected enough data to rule out the excess. "If AGASA had been correct, then we should have seen 30 events [at or above 100 EeV], and we see two," says Alan Watson of the University of Leeds, U.K., who is the spokesperson for the Auger collaboration. The Auger data also show that very few of the most energetic rays are photons. As supermassive particles ought to decay readily into photons, that finding undermines exotic-particle musings, says Glennys Farrar, a theorist at New York University who joined the 300-member Auger collaboration in September.

Meanwhile, researchers working with Hi-Res, which stopped taking data last year, say the shape of their final spectrum of cosmic ray energies definitely proves the rays are running up against the GZK cutoff. "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 signature." Others aren't so sure. Auger's data suggests the highest energy rays comprise protons and heavier nuclei, which don't feel the GZK drag, Watson says. Instead of being slowed, the nuclei may never be accelerated to 40 EeV, he says.

Whatever its cause, the fall-off leads some to question the need to build a bigger array, as the Auger team hopes to do in the Northern Hemisphere. "Once you see the cutoff--even if you disagree about what it is--then building a bigger detector hardly gets you anything," because there are so few higher energy particles to capture, says Gordon Thomson, a Hi-Res member from Rutgers University in Piscataway, New Jersey. Members of the Hi-Res and AGASA teams are building a detector in Utah called the Telescope Array, which will be three-eighths the size of Auger. That may be just the right size, Thomson says.

Others say that only a bigger array can amass enough data to trace the fall-off in detail. "Now we understand that above the GZK cutoff there are ten times less cosmic rays than we thought 10 years ago, so we may need a detector ten times as big as Auger," says Masahiro Teshima of the Max Planck Institute for Physics in Munich, Germany, who worked on AGASA and is working on the Telescope Array.

The few highest energy, straightest flying particles will be crucial for determining whether high-energy cosmic rays emanate from particular points or patches in the sky, says James Cronin of the University of Chicago, Illinois, who, with Watson, dreamed up the Auger array in the early 1990s. Such "anisotropy" might reveal the rays' origins, and "if we can show an anisotropy, then that's a brilliant breakthrough," he says. Mapping the sky could take a decade--although Cronin and Watson hint they may have already seen something exciting that's not yet ready for release.