World’s largest particle detector opens up a new branch of astronomy

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The IceCube Laboratory at the Amundsen-Scott South Pole Station, in Antarctica, hosts the computers collecting raw data. Due to satellite bandwidth allocations, the first level of reconstruction and event filtering happens in near real-time in this lab. Only events selected as interesting for physics studies are sent to UW-Madison, where they are prepared for use by any member of the IceCube Collaboration. Credit: Sven Lidstrom. IceCube/NSF

Zeuthen near Berlin, 22 November 2013. Within the eternal ice of Antarctica, scientists have observed the first solid evidence for high-energy neutrinos coming from cosmic accelerators beyond our own solar system. Between May 2010 and May 2012 the IceCube detector at the South Pole captured a total of 28 neutrinos with energies greater than 30 teraelectronvolts (TeV). Two of the neutrinos had an energy of more than 1,000 TeV — that’s more than the kinetic energy of a fly in flight — compressed into a single elementary particle. The international IceCube collaboration, in which DESY is the second-largest partner after the University of Wisconsin-Madison, now presents these observations in the current cover story of the scientific journal Science.

“This is the first indication of very high-energy neutrinos coming from outside our solar system,” says Francis Halzen, principal investigator of IceCube and the Hilldale and Gregory Breit Distinguished Professor of Physics at the University of Wisconsin-Madison. Neutrinos are elementary particles that have almost no mass and interact extremely seldom with other particles. They are unique messengers of the highest-energy events in the universe, because in contrast to light they can easily escape from extremely dense environments — such as the core of a supernova explosion or the interior of cosmic particle accelerators.

For example, neutrinos from the famous supernova 1987A reached the Earth approximately three hours before the flash of light, which first had to make its way out of the supernova. “However, the neutrinos that have now been detected by IceCube have energies that are millions of times higher than those coming from supernova 1987A,” emphasizes the head of the neutrino astronomy group at DESY in Zeuthen near Berlin, Dr. Markus Ackermann.

The advantage of the neutrinos as cosmic messengers is also a disadvantage. That’s because they fly through matter so easily that countless neutrinos penetrate the earth every second without leaving any trace. Very seldom does a neutrino collide with another particle. Gigantic detectors are needed in order to enable researchers to occasionally observe such a neutrino event. IceCube, the largest particle detector in the world, encompasses a whole cubic kilometre of the eternal ice of the Antarctic. Inside IceCube, a total of 5,160 sensitive detectors hang from 86 steel cables. These detectors, which are known as optical modules, are sensitive to the weak flashes of light that are generated by a neutrino collision. After a construction period of seven years, the gigantic detector was fully operational at the end of 2010.

The first hints of extra-terrestrial high-energy neutrinos came with the unexpected discovery in April 2012 of two detector events above 1000 TeV. The IceCube scientists nicknamed these two rare events “Ernie” and “Bert”. An analysis of those events was reported in the scientific journal Physical Review Letters. An intensified search, the results of which are presented now, turned up 26 additional events beyond 30 teraelectronvolts, exceeding the results expected for neutrinos produced in the earth’s atmosphere.

“Perhaps, we are currently experiencing the birth of neutrino astronomy,” says Ackermann. The analysis did not find any statistically significant clustering of the 28 events either in time or space - the number of events is too small. “We are now working hard on improving the significance of our observation, and on understanding what this signal means and where it comes from”, says collaboration spokesperson Professor Olga Botner of Uppsala University (Sweden). With an increase in the number of events the scientists hope to identify sources of high energy neutrinos in the cosmos.

The international IceCube team consists of 260 scientists from eleven countries. “The success of IceCube builds on the efforts of hundreds of people around the world,” emphasizes Botner. In Germany, apart from DESY nine universities participate in the collaboration: the Technical University of Aachen, the Humboldt University Berlin, the University of Bochum, the University of Bonn, the Technical University of Dortmund, the University of Erlangen-Nürnberg, the University of Mainz, the Technical University of Munich and the University of Wuppertal. Apart from a quarter of the optical modules, the German participants contributed a significant part of the receiver electronics on the ice surface. The German share of about 20 million Euro was funded by the Federal Ministry of Education and Research, the Helmholtz Association, the German Research Foundation, and by the budgets of the participating universities.


Deutsches Elektronen-Synchrotron DESY is the leading German accelerator centre and one of the leading in the world. DESY is a member of the Helmholtz Association and receives its funding from the German Federal Ministry of Education and Research (BMBF) (90 percent) and the German federal states of Hamburg and Brandenburg (10 percent). At its locations in Hamburg and Zeuthen near Berlin, DESY develops, builds and operates large particle accelerators, and uses them to investigate the structure of matter. DESY’s combination of photon science and particle physics is unique in Europe.



Reference
“Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector”; The IceCube Collaboration; Science, 2013; DOI: 10.1126/science.1242856


Science contacts
Dr. Markus Ackermann, DESY, Zeuthen, +49 33762 77415, +49 1763 2587672 (mobile), markus.ackermann@desy.de


Media contact
DESY press officer Thomas Zoufal, +49 8998-1666, presse@desy.de

Images

Download [2 MB, 5000 x 3600] This is the highest energy neutrino ever observed, with an estimated energy of 1.14 PeV. It was detected by the IceCube Neutrino Observatory at the South Pole on January 3, 2012. IceCube physicists named it Ernie. Credit: IceCube Collaboration	Download [2 MB, 5000 x 3600] This is the second highest-energy neutrino ever observed, with an estimated energy of 1.04 PeV. It was detected by the IceCube Neutrino Observatory at the South Pole on August 9, 2011. IceCube physicists named it Bert. Credit: IceCube Collaboration
Download [8 MB, 8333 x 4729] Artistic rendering of IceCube DOMs below the Antarctic ice. Inside IceCube, a total of 5,160 sensitive detectors hang from 86 steel cables.Credit: Jamie Yang, The IceCube Collaboration.	Download [1 MB, 2592 x 1944] The deployment of each of the 86 IceCube strings lasted about 11 hours. In each one, 60 sensors (called DOMs) had to be quickly installed before the ice completely froze around them. Credit: IceCube/NSF
Download [1 MB, 5184 x 3456] The IceCube Lab under the stars. The IceCube Laboratory at the Amundsen-Scott South Pole Station, in Antarctica, hosts the computers collecting raw data. Due to satellite bandwidth allocations, the first level of reconstruction and event filtering happens in near real time in this lab. Only events selected as interesting for physics studies are sent to UW–Madison, where they are prepared for used by any member of the IceCube Collaboration. Credit: Felipe Pedreros, IceCube/NSF	Download [1 MB, 3732 x 2488] An aurora australis illuminates the IceCube Lab. The IceCube Laboratory at the Amundsen-Scott South Pole Station, in Antarctica, hosts the computers collecting raw data. Due to satellite bandwidth allocations, the first level of reconstruction and event filtering happens in near real time in this lab. Only events selected as interesting for physics studies are sent to UW–Madison, where they are prepared for used by any member of the IceCube Collaboration. Credit: Keith Vanderlinde, IceCube/NSF