LONG BEACH, CALIFORNIA--The "Oh-My-God" particle is not a fluke or experimental error. At a meeting of the American Physical Society here last week, researchers announced they had collected another seven observations of these mysterious, extremely powerful cosmic rays, bringing the total to 16. The finding means the mystery of the particles' origins is for real.
On an October evening in 1991, astronomers from the University of Utah in Salt Lake City detected a faint flicker of light in the sky. It was the signature of a cosmic ray, a high-energy particle from deep space. When such particles strike the atmosphere, they release a cascade of secondary particles that causes the air to fluoresce. By analyzing the direction and size of the faint glowing streak in the sky, the astronomers were able to figure out that it had been caused by a particle that packed a shocking 320 exaelectronvolts (EeV) of energy, the same punch as a 55-mile-an-hour fastball. Most cosmic rays are at much lower energies, like one ten-thousandth of an EeV.
Initially, astrophysicists thought the Oh-My-God event, as they refer to it, might be exceedingly rare, if it was real at all. Over the last decade, however, a Japanese detector has spotted about seven cosmic rays with energies over 100 EeV, and a Russian detector found one. And now, an improved version of the original Utah detector, completed in 1997 and dubbed HiRes, has recorded seven more particles with energies over 100 EeV--with one clocking in at an estimated 280 EeV, the second highest ever. In addition, HiRes detected 13 particles with energies over 60 EeV. "People are beginning to think these events are real," says University of Utah physicist Charles Jui.
Researchers don't understand what accelerates cosmic rays to such high energies; even extremely violent events like supernovae can't give particles more than about one thousandth of an EeV of energy. "This is a burning astrophysical problem that needs to be solved," says University of Chicago's René Ong. That may well happen within the next few years. A second detector was added to HiRes in October, increasing its sensitivity and giving it the ability to pinpoint a cosmic ray's direction and even its mass--data that Ong says may help determine the source of the strange particles.
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