RWTH Aachen University: High energy cosmic neutrino confirms long predicted resonance in the weak interaction

On December 6, 2016, an extremely high-energy neutrino was measured in the IceCube Neutrino Observatory in Antarctica, which allows surprising conclusions to be drawn about fundamental particle physics. Even the energy of 6.3 peta electron volts is record-breaking and indicates an origin outside of our solar system. With this event, a particle reaction can be confirmed for the first time, which was predicted by Nobel Prize winner Sheldon Glashow in 1960. The research results of the international team were published today in the journal “Nature” under the title “Detection of a particle shower at the glass show resonance with IceCube”. RWTH scientists were also significantly involved in this.

Measuring neutrinos is a difficult undertaking that requires huge detectors with high sensitivity and measurement accuracy. The IceCube Neutrino Observatory is located at the geographic South Pole and is the world’s largest instrument of its kind. With the help of around 5000 ultra-sensitive light sensors embedded deep in the Antarctic ice, a volume of 1 cubic kilometer has been monitored around the clock since 2010 . A neutrino is registered about every ten minutes. These are mainly neutrinos that are created in the earth’s atmosphere. However, particularly high-energy neutrinos also arise in extreme environments in the universe, for example in the vicinity of massive black holes. Since the beginning of the recordings, several hundred cosmic neutrinos have already been detected with IceCube and in 2017 even convincing evidence of the quasar TXS 0506 + 056 as a cosmic neutrino source was found. Even for these neutrinos, however, the energy of the neutrino observed on December 6, 2016 is spectacular. “Neutrino events with such high energy cannot be overlooked in the detector, but they only happen every few years”; analyzes Professor Christopher Wiebusch from III. Physics Institute B at RWTH Aachen University and head of the Aachen IceCube group. IceCube has recorded only three events with more than 5 peta-electron volts (PeV) of energy in eleven years.

At the measured energy of 6.3 PeV, elementary particle physicists listen carefully: This is exactly the energy at which neutrinos interact with a hundredfold increased probability when there is a resonance process of anti-electron neutrinos with atomic electrons in nature. This process was predicted in 1960 by Sheldon Glashow, founder of today’s standard model of elementary particle physics and Nobel Prize winner, but has not yet been experimentally confirmed due to the high energy. Dr. Christian Haack: “We not only observed one of the highest energetic neutrinos ever measured, which is clearly of cosmic origin, but also clarified a question of elementary particle physics that has been open for decades. As a result, there is now a new experimental procedure, with which we can distinguish cosmic anti-neutrinos from neutrinos. This makes it possible to make statements about the cosmic particle accelerators. ”Haack did his doctorate on this at RWTH Aachen University and is currently doing research at the Technical University of Munich.

International cooperation

The measurement was largely made possible by the international teamwork of young scientists within the international IceCube collaboration; by Lu Lu from the University of Chiba in Japan (now University of Madison, USA), Tianlu Yuan from the University of Madison, and Christian Haack. Since the neutrino reaction took place just outside the observed ice volume, the unequivocal reconstruction took several years. In particular, the discovery of characteristic individual signals reaching into the detector by the RWTH team was a key to success. In this way, the direction of origin of the neutrino could be precisely determined and the evaluation of the resonance process confirmed. Terrestrial underground signals or incorrect measurements were ruled out with a very high degree of certainty. The possibility of another neutrino reaction could also be excluded to a few percent. Future measurements must now show whether the certainty can be further improved. Since the universe apparently provides neutrinos with sufficient energy, it is only a matter of time before further reactions of this type are observed. Glashow, meanwhile emeritus of the University of Boston: “So far it’s only been one, one day there will be more.”

The IceCube Neutrino Observatory

IceCube was built as an international collaboration of around 400 scientists at 53 research institutions in twelve countries and is also operated jointly by them. The University of Wisconsin – Madison (USA) and the National Science Foundation (USA) are in charge. With the support of the Federal Ministry of Education and Research (BMBF) and the German Research Foundation (DFG), more than 100 scientists from Germany are participating at the universities of RWTH Aachen, HU Berlin, RU Bochum, TU Dortmund, FAU Erlangen, JGU Mainz, TU Munich, WWU Münster, BU Wuppertal as well as the two Helmholtz centers German Electron Synchrotron (DESY) and Karlsruhe Institute of Technology (KIT).

There are big plans for the future: In the next few years, in a first step, the IceCube Upgrade, new photosensors will be installed centrally in the IceCube in order to improve the measurement accuracy. Thereafter, by the end of the decade, as part of theIceCube-Gen2 project, the volume of IceCube can be increased almost tenfold, which then increases the number of measured neutrino reactions accordingly. “With IceCube we have succeeded in demonstrating the glass show resonance for the first time, and with IceCube-Gen2 we will be able to use the reaction to precisely measure the flow of cosmic anti-electron neutrinos,” says Professor Dr. Marek Kowalski from DESY, who coordinates the preparations for IceCube-Gen2. “This will give us a completely new approach to understanding the little-known production mechanisms of high-energy cosmic neutrinos.”