Antarctic detector confirms observation of cosmic neutrinos
IceCube detection of elusive particle could open new era in astronomy
A group of researchers using a massive, National Science Foundation (NSF)-funded instrument buried deep in the ice at the geographic South Pole have announced a new observation of high-energy neutrinos, confirming they found particles from beyond our solar system--and beyond our galaxy.
Researchers with the IceCube Collaboration, an international scientific group headquartered at the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin-Madison, made the announcement in a paper published Aug. 20 in the journal Physical Review Letters. The new observation confirms a discovery of high-energy neutrinos made by IceCube in November 2013.
Neutrinos are subatomic particles, and billions of them pass through Earth every day, but they are incredibly difficult to detect. The particles are never directly observed, but the IceCube Neutrino Observatory, a cubic-kilometer-sized detector sunk into the ice sheet at the South Pole, allows researchers to see the byproducts of neutrino interaction with the ice. The instrument's array, IceTop, and its denser inner subdetector, DeepCore, significantly enhance its capabilities, making it a multipurpose facility.
The observatory records 100,000 neutrinos each year, most of which are a type called "muon neutrinos" generated when cosmic rays interact with the Earth's atmosphere. In contrast, the researchers were trying to find just a few dozen neutrinos generated elsewhere.
To find those, the IceCube Collaboration used an old strategy for a neutrino telescope: It looks through the Earth, using the planet itself to filter out the large background of atmospheric muons. By observing neutrinos coming from the Northern Hemisphere, they confirmed their cosmic origin, as well as the presence of extragalactic neutrinos and the intensity of the neutrino rate.
"Looking for muon neutrinos reaching the detector through the Earth is the way IceCube was supposed to do neutrino astronomy and it has delivered," said Francis Halzen, IceCube principal investigator and the Hilldale and Gregory Breit Distinguished Professor of Physics at the University of Wisconsin-Madison. "This is as close to independent confirmation as one can get with a unique instrument."
The IceCube Neutrino Observatory was built with an NSF Major Research Equipment and Facilities Construction award, with assistance from partner funding agencies around the world.
The Division of Polar Programs in NSF's Geosciences Directorate and the Division of Physics in the Mathematical and Physical Sciences Directorate continue to support the project with a Maintenance and Operations grant, along with international support from participating institutions and their funding agencies. The University of Wisconsin, Madison is the lead institution, and the international collaboration includes 300 physicists and engineers from the U.S., Germany, Sweden, Belgium, Switzerland, Japan, Canada, New Zealand, Australia, U.K., Korea and Denmark.
The Division of Polar Programs manages the U.S. Antarctic Program, through which it coordinates all U.S. research on the southernmost continent and in the Southern Ocean and provides the logistical support for that science
"This is an excellent confirmation of IceCube's recent discoveries, opening the doors to a new era in particle physics," says Vladimir Papitashvili, astrophysics and geospace sciences program director in the Division of Polar Programs. "And it became possible only because of extraordinary qualities of Antarctic ice and NSF's ability to successfully tackle enormous scientific and logistical problems in the most inhospitable places on Earth."
Neutrinos travel throughout the universe almost undisturbed by matter, pointing directly to the sources where they were created. The highest energy neutrinos are expected to emanate from the most extreme environments in the universe: Powerful cosmic generators, such as black holes or massive exploding stars, that are able to accelerate cosmic rays to energies over a million times the energies achieved by human-made accelerators, such as the Large Hadron Collider at CERN.
"Cosmic neutrinos are the key to yet unexplored parts of our universe and might be able to finally reveal the origins of the highest energy cosmic rays, including the rare 'Oh-My-God' particles," says IceCube Collaboration spokesperson Olga Botner, of Uppsala University.
This observation agrees well with previous results by the IceCube Collaboration, and also confirms the higher rate of neutrinos from beyond our solar system. Even though scientists are still counting them by the handful, IceCube results are close to the maximum rate based on theoretical estimates from potential cosmic ray sources being efficient generators of neutrinos.
The observed high-energy neutrinos are a brand-new neutrino sample, with only one event in common with the first results announced in 2013. That earlier observation searched for high-energy neutrinos that had interacted with the ice inside IceCube.
The new high-energy neutrino sample, when combined with previous IceCube measurements, allows the most accurate measurements to date of the energy spectrum and neutrino-type composition of the extraterrestrial neutrino flux. Those results are published in an accompanying paper in The Astrophysical Journal.
The National Science Foundation (NSF) is an independent federal agency that supports fundamental research and education across all fields of science and engineering. In fiscal year (FY) 2017, its budget is $7.5 billion. NSF funds reach all 50 states through grants to nearly 2,000 colleges, universities and other institutions. Each year, NSF receives more than 48,000 competitive proposals for funding and makes about 12,000 new funding awards.
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