U. study confirms cosmic ray cutoff is real
The new research also casts doubt on a famous discovery by University of Utah scientists, the 1991 finding of a cosmic ray that supposedly carried a billion-billion electron volts of energy. Now it seems it may have been an ordinary cosmic ray distorted by atmospheric conditions.
"Cosmic ray" is a misnomer. They're not rays like beams of light, but subatomic particles ejected into space. According to the U., lower-energy cosmic rays stream from stars like our sun, while more powerful ones are released by some cataclysm such as an exploding star or a black hole gobbling matter in its vicinity.
Cosmic rays continually rain onto Earth's atmosphere. Under good conditions they can be detected by the flashes they make when colliding with air, or they can be detected by ground units they hit.
The 1991 "record" was detected by the University of Utah Fly's Eye Observatory at Dugway Proving Ground. The new report by scientists at the U. and around the world was developed from work at its replacement, the High Resolution Fly's Eye, or HiRes, observatory also at Dugway.
The study, "First Observation of the Greisen-Zatsepin-Kuzmin Suppression," published Friday in the prestigious journal Physical Review Letters, supports a 1966 prediction about the upper limit of intensity for cosmic rays.
Interestingly, according to a university press release, the limit was predicted by Kenneth Greisen of Cornell University while he was visiting the U. It also was predicted independently by Soviet scientists Georgiy Zatsepin and Vadim Kuzmin.
The 1991 super-cosmic-ray seemed to be over the limit. So did a series of detections by a Japanese observatory called the Akeno Giant Air Shower Array.
But nine years of study by HiRes showed that the prediction was right. Cosmic rays from far reaches of the universe lose energy when they collide with the Cosmic Microwave Background, material left over from the Big Bang. The study shows the cutoff is real, although it may not be absolute.
"Everybody's been looking for this for 40 years," said Pierre Sokolsky, a physics professor and spokesman for the HiRes Collaboration and dean of science at the University of Utah. "This is the first real definitive observation."
This doesn't mean there could not be particles of much higher energies, but they would be extremely scarce and could not be found without a far more sensitive detector.
It would have been fun to find a violation of the rule, but to see it actually working is "a pretty good feeling," he said.
So what did Fly's Eye see in 1991?
"I guess we have to believe at this point it was an artifact. We've not been able to replicate it, and our sensitivity has increased well beyond where we should have seen a large number of those things," he said.
Sokolsky speculated the earlier Utah report may have involved a low-energy cosmic ray hitting a layer of haze in the atmosphere, creating what looked like a high-energy cascade of particles. That would seem to "fake a high-level event," he said.
Charles Jui (pronounced Ray), professor of physics and, like Sokolsky, a co-author of the paper, said searching for evidence for or against the cutoff was the goal of HiRes. Data collected before were not abundant enough to reach a definite conclusion, but now enough observations have been made.
"People had hoped that maybe we would find some new physics," he said. The new study verifies standard physics and shows that the cutoff is real.
The finding also means that there are few sources of high-level cosmic rays in this vicinity in space, Jui said. "If there are local sources, we would probably see more ... at the highest energies."
E-mail: bau@desnews.com
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