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Mystery of cosmic rays deepens

April 23, 2012
Courtesy of the US National Science Foundation
and World Science staff

Mys­te­ri­ous par­t­i­cles rou­tinely pum­mel­ing Earth’s at­mos­phere from space don’t seem to come from cos­mic ex­plo­sions as many sci­en­tists have the­o­rized, a group of phys­i­cists has an­nounced.

A study by the re­search­ers found that these ex­plo­sions are fail­ing to pro­duce any de­tect­a­ble quanti­ties of a sec­ond type of par­t­i­cle that should accompany the oth­ers. The re­sults come from the Ice­Cube Neu­tri­no Ob­serv­a­to­ry, a mas­sive de­tec­tor de­ployed in deep ice at the U.S. Amundsen-Scott South Pole Sta­t­ion in Ant­arc­ti­ca.

The high-energy par­t­i­cles slam­ming in­to Earth’s at­mos­phere, known as cos­mic rays, were dis­cov­ered 100 years ago. They’re elec­tric­ally charged, sub­a­tom­ic par­t­i­cles that strike from all di­rec­tions with en­er­gies up to 100 mil­lion times high­er than hu­mans can ge­nerate us­ing at­om smash­ers.

The in­tense con­di­tions needed to ge­nerate these ti­ny but vi­o­lent pro­jec­tiles have fo­cused phys­i­cists’ in­ter­est on two po­ten­tial sources: the mas­sive black holes at the cen­ters of some ga­lax­ies, and dis­tant, ex­plod­ing fire­balls called gam­ma-ray bursts.

But in a pa­pe­r pub­lished in the April 19 is­sue of the jour­nal Na­ture, re­search­ers with the Ice­Cube ob­serv­a­to­ry de­scribed a search for ex­tremely light and hard-to-detect par­t­i­cles that should the­o­ret­ic­ally accompany those cos­mic rays if they come from gam­ma-ray bursts. The team checked out 300 gam­ma ray bursts ob­served be­tween May 2008 and April 2010.

Sur­pris­ing­ly, the sci­en­tists said, none of these light­er par­t­i­cles, called neu­tri­nos, turned up.

“Although we have not dis­cov­ered where cos­mic rays come from, we have tak­en a ma­jor step to­wards rul­ing out one of the lead­ing pre­dic­tions,” said Fran­cis Halzen, a phys­i­cist at the Uni­vers­ity of Wisconsin-Madison and the pro­ject’s prin­ci­pal in­ves­ti­ga­tor.

The find­ings are “sig­nif­i­cant be­cause for the first time we have an in­stru­ment with suf­fi­cient sen­si­ti­vity to open a new win­dow on cos­mic ray pro­duc­tion and the in­te­ri­or pro­cesses of” the ex­plo­sions, said Greg Sul­li­van, a phys­i­cist at the Uni­vers­ity of Mar­y­land and Ice­Cube spokes­man.

Ice­Cube ob­serves neu­tri­nos by de­tect­ing faint blue light oc­ca­sion­ally pro­duced when they hit ice. The par­t­i­cles are of a ghostly na­ture; they slip through peo­ple, walls, and the plan­et with barely a trace. To com­pen­sate for this prob­lem and catch a few neu­tri­no traces, the Ice­Cube de­tec­tor is im­mense. A cu­bic kil­o­me­ter of gla­cial ice, enough to fit the great pyr­a­mid of Gi­za 400 times, is out­fitted with 5,160 op­ti­cal sen­sors em­bed­ded deep in the ice.

Gamma-ray bursts, the uni­verse’s most pow­er­ful ex­plo­sions, are usu­ally first ob­served by satel­lites us­ing X-rays, gam­ma rays or both. The blasts are seen about once per day, and are so bright that they can be de­tected from half way across the vis­i­ble Uni­verse. The ex­plo­sions usu­ally last only a few sec­onds.


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Mysterious particles routinely pummeling Earth’s atmosphere from space don’t seem to come from cosmic explosions as many scientists have theorized, some physicists have announced. A study by the researchers found that these same explosions are failing to produce any detectable quantities of a second type of particle that should accompany the others. The results come from the IceCube Neutrino Observatory, a massive detector deployed in deep ice at the U.S. Amundsen-Scott South Pole Station in Antarctica at the South Pole. The high-energy particles slamming into Earth’s atmosphere, known as cosmic rays, were discovered 100 years ago. They’re electrically charged, subatomic particles that strike from all directions with energies up to 100 million times higher than humans gen generate using atom smashers. The intense conditions needed to generate these tiny but violent projectiles have focused physicists’ interest on two potential sources: the massive black holes at the centers of active galaxies and exploding fireballs observed by astronomers called gamma-ray bursts. But in a paper published in the April 19 issue of the journal Nature, researchers with the IceCube observatory described a search for extremely light and hard-to-detect particles that should theoretically accompany those cosmic rays if they come from gamma-ray bursts. The team checked out 300 gamma ray bursts observed between May 2008 and April 2010. Surprisingly, the scientists said, none of these lighter particles, called neutrinos, turned up. “Although we have not discovered where cosmic rays come from, we have taken a major step towards ruling out one of the leading predictions,” said Francis Halzen, a physicist at the University of Wisconsin-Madison and the IceCube principal investigator. The findings are “significant because for the first time we have an instrument with sufficient sensitivity to open a new window on cosmic ray production and the interior processes of” the explosions, said Greg Sullivan, a physicist at the University of Maryland and IceCube spokesman. IceCube observes neutrinos by detecting faint blue light occasionally produced when neutrinos hit ice. The particles are of a ghostly nature; they slip through people, walls, or the planet with hardly a trace. To compensate for this problem and catch a few neutrino traces, the IceCube detector is immense. A cubic kilometer of glacial ice, enough to fit the great pyramid of Giza 400 times, is instrumented with 5,160 optical sensors embedded up to 2.5 kilometers deep in the ice. Gamma-ray bursts, the universe’s most powerful explosions, are usually first observed by satellites using X-rays and/or gamma rays. The blasts are seen about once per day, and are so bright that they can be detected from half way across the visible Universe. The explosions usually last only a few seconds.