High-Energy Cosmic Neutrino Confirms 60-Year-Old Theory
An international research team involving scientists from RWTH has published its latest findings in the prestigious journal Nature.
On December 6, 2016, an extremely high-energy neutrino was measured at the IceCube Neutrino Observatory in Antarctica, allowing surprising conclusions to be drawn about fundamental questions in particle physics. The extremely high energy of 6.3 peta-electronvolts carried by the neutrino indicates its cosmic origin outside our solar system.
The event also confirms for the first time a particle reaction predicted by Nobel Prize winner Sheldon Glashow as early as 1960. The research results of the international team were published today in the journal Nature, in an article titled Detection of a particle shower at the Glashow resonance with IceCube. RWTH researchers were significantly involved in the research activities.
Measuring neutrinos is a difficult task 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 detector of its kind. With the help of about 5000 ultra-sensitive light sensors embedded deep in the Antarctic ice, the observatory has been monitoring one cubic kilometer of glacial ice since 2010. Every every ten minutes, the detector registers a neutrino event. The detected neutrinos are mainly generated in the atmosphere of the Earth.
However, particularly high-energy neutrinos are also produced in extreme environments in the universe, for example in the vicinity of massive black holes. So far, IceCube has detected several hundred cosmic neutrinos, and, in 2017, found convincing evidence that the TXS 0506+056 quasar is a cosmic neutrino source. However, even for cosmic neutrinos, the energy carried by the neutrino observed on December 6, 2016 is spectacular.
“Neutrino events with such high energy cannot be missed by the detector, but they only happen every few years," explains Professor Christopher Wiebusch from RWTH’s Physics Institute IIIB, who is head of the Aachen IceCube group. IceCube has recorded only three events with more than 5 peta-electronvolts (PeV) of energy in eleven years. One PeV is about 100 times more energetic than what the most powerful particle accelerators are able to generate, such as the proton beams of the Large Hadron Collider (LHC) at CERN in Geneva.
The measured energy of 6.3 PeV is of particular interest to particle physicists: It is exactly the threshold energy at which the probability of an interaction between neutrinos is increased by a factor of 100, when there is a resonance process of antineutrinos with atomic electrons in nature. This process was predicted in 1960 by Sheldon Glashow, one of the theorists responsible for today's Standard Model of elementary particle physics. Due to the high energy it requires, it was not possible experimentally to confirm this process so far.
As Dr. Christian Haack explains: “Not only have we observed one of the highest energy neutrinos ever measured, which is thus clearly of cosmic origin, but we have also answered a key question in elementary particle physics that has been open for decades. As a result, we now have a new experimental method with which to distinguish cosmic anti-neutrinos from neutrinos. This will allow us to gain further insights into cosmic particle accelerators.” Haack, now a researcher at TU Munich, completed his dissertation on the observation of high-energy neutrinos from the Galaxy and beyond here at RWTH.
The measurement was made possible by an international team effort of researchers in the international IceCube project, including Lu Lu, who was a postdoc at Chiba University in Japan during the analysis and is now at the University of Madison, USA; Tianlu Yuan from the University of Madison; and Christian Haack.
As the neutrino reaction occurred slightly outside the ice volume under observation, its reconstruction took several years to complete. The Aachen team’s discovery of characteristic individual signals extending into the detector, in particular, was a key to success. These made it possible to precisely determine the direction of origin of the neutrino and to confirm the analysis of the resonance process. Terrestrial background signals or a false measurement can thus be ruled out with a very high degree of certainty. The possibility of another neutrino reaction can also largely be ruled out, with a remaining uncertainty of within a few percent.
As Glashow, now emeritus professor at Boston University, says, “To be absolutely sure, we should see another such event at the very same energy as the one that was seen. So far there’s one, and someday there will be more.”
Expansion of the IceCube Neutrino Observatory
There are big plans for the future of IceCube: In a first step, the IceCube Upgrade, 750 advanced photosensors will be installed over the next few years to improve the detector’s measurement accuracy. Subsequently, the IceCube-Gen2 project will enlarge the detector’s volume to a gigantic eight cubic kilometers, increasing the detection rate of cosmic neutrinos by a factor of ten.
“With IceCube, we succeeded in detecting the Glashow resonance; with IceCube-Gen2, we will be able to precisely measure the flux of cosmic anti-electron neutrinos,” says Professor Dr. Marek Kowalski of DESY, who is coordinating the preparatory activities for IceCube-Gen2. “This will provide us with a whole new approach to understanding the little-known production mechanism of high-energy cosmic neutrinos.”
Approximately 400 physicists from 53 institutions in 12 countries make up the IceCube Collaboration. The project is led by the University of Wisconsin–Madison and the National Science Foundation of the Unites States. With the support of the Federal Ministry of Education and Research and the German Research Foundation, more than 100 researchers from the following German universities and research institutions are contributing to the project: RWTH Aachen University, Humboldt-Universität zu Berlin, Ruhr-Universität Bochum, TU Dortmund University, The University of Erlangen (FAU), Mainz University (JGU), TU Munich, the University of Münster, the University of Wuppertal, Deutsches Elektronen-Synchrotron DESY, and the Karlsruhe Institute of Technology (KIT).