16.05.2013

IceCube sees strong signs for first cosmic neutrinos

Pioneering results of the world’s largest particle detector

At the IceCube Particle Astrophysics (IPA) conference in Madison, U.S., the international IceCube collaboration presented the latest results of their search for astrophysical neutrinos. In the period between May 2010 and May 2012, the neutrino detector Ice Cube identified 28 neutrinos with energies above 30 teraelectronvolts (TeV). Two of them reached energies higher than 1 petaelectronvolt (1 PeV= 1000 TeV). With this, the astroparticle experiment, comprised of more than 5 000 digital optical modules suspended in a cubic kilometre of ice at the South Pole, made substantial progress in the hunt for extra-terrestrial neutrinos.

The IceCube detector is made of more than 5000 optical modules in the Antarctic ice.

“One petaelectronvolt is an energy level more than thousand times higher than the highest energy neutrino yet produced in a man-made particle accelerator,” said IceCube scientist Dr. Christian Spiering from DESY, former scientific coordinator of the project. “The number of 28 events is barely compatible with the results expected for neutrinos produced by cosmic radiation in the earth’s atmosphere.”

The IceCube detector includes 86 strings, each with 60 glass spheres with highly-sensitive light sensors deployed in depths between 1.45 and 2.45 kilometres. With these, scientists are able to detect neutrinos resulting from galactic explosions or other cosmic processes. The neutral particles rarely interact with matter, making them ideal messengers that provide insights into events happening in the depths of the universe. When a neutrino is stopped when it penetrates the polar ice, it produces tiny blue flashes which are picked up by the detectors. However, neutrinos might also originate from the interaction of cosmic radiation with the earth’s atmosphere. Hundreds of thousands of such “atmospheric neutrinos” have so far been recorded by IceCube, but these neutrinos mostly have significantly lower energies.

The first hints of extra-terrestrial high-energy neutrinos came with the unexpected discovery in April 2012 of two detector events above 1 PeV. The IceCube scientists nicknamed these two rare events “Ernie” and “Bert”. An analysis of those events was reported in April 2013 in a paper submitted to the journal Physical Review Letters. An intensified search, the results of which were presented yesterday, turned up 26 additional events beyond 30 teraelectronvolts, this again exceeds the results expected for neutrinos produced in the earth’s atmosphere.

“Perhaps, we are currently experiencing the birth of neutrino astronomy,” said Dr. Markus Ackermann, head of the neutrino astronomy group at DESY in Zeuthen. “Within the coming years, we are expecting more scientific breakthroughs with IceCube. Moreover, after the discoveries with IceCube, the possible extensions PINGU and IceCube++ will allow us to go one step further to precision measurements in neutrino physics and astronomy with neutrinos.”

The neutrino telescope IceCube is the world’s largest particle detector. It fills a cubic kilometre of deep Antarctic ice containing ultrasensitive light sensors. They record the tracks from neutrinos coming from outer space, to get information about distant galaxies from these astronomical messengers. The international IceCube team consists of 260 scientists from 36 research institutions in 8 countries, 9 institutions coming from Germany. 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 € was funded by the Federal Ministry of Education and Research, the Helmholtz Association, the German Research Foundation, and by the normal budgets of the universities.

Participating institutes from Germany (apart from DESY):

RWTH Aachen (Group of Prof. C. Wiebusch)
Humboldt-Universität zu Berlin (Group of Prof. A. Kappes)
University Bochum (Group of Prof. J. Tjus)
University Bonn (Group of Prof. M. Kowalski)
TU Dortmund (Group of Prof. W. Rhode)
University Mainz (Group of Prof. L. Köpke)
Technical University München (Group of Prof E. Resconi)
University Wuppertal (Group of Prof. K. Helbing)