Nobel Prize in Physics 2026 | Nobel Prize for Neutrino Observatory

Most of the IceCube Neutrino Observatory lies deep in the Antarctic ice. Photo: dpa/NSF/IceCube/Martin Wolf Neutrinos are neutrally charged elementary particles that are among the most common particles in the universe. They arise when fundamental particles such as protons, electrons or photons interact with one another, including during nuclear fusion inside the sun or in the Earth's atmosphere when cosmic rays collide with air molecules. However, they can also arise in the vicinity of black holes. Every second, billions upon billions of neutrinos race through the Earth without leaving a trace.

This year's Nobel Prize in Physics goes to the Belgian Francis Halzen, professor at the University of Wisconsin in Madison. The Academy recognizes his critical contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos originating from outside our solar system.

This is the fifth time that research into neutrinos has been honored with the Nobel Prize. Its story begins almost a hundred years ago, when the Austrian physicist Wolfgang Pauli predicted a new, electrically neutral particle in 1930. Regarding his theory, he is said to have remarked that he had done something terrible because he had postulated a particle that could not be detected. Because of its neutral charge and vanishingly small mass, a neutrino only interacts very rarely with matter. This makes measurements very complex and scientists gave the particles the nickname ghost particles.

It was not until 1956 that Clyde Cowan and Frederick Reines succeeded in experimentally proving the existence of neutrinos at a nuclear reactor.

The detector is one cubic kilometer in size and registers an average of one neutrino per day.

Soon afterwards, physicists realized that the neutrino could be an ideal instrument in astronomy. From the path a neutrino travels, conclusions can be drawn about its place of origin. In addition to the already known neutrino sources, scientists also suspect so-called cosmic accelerators such as supermassive black holes as the origin of neutrinos, which eject extreme energies in the form of radiation into space far outside our solar system.

Their radiation consists of particles such as protons or neutrinos or high-energy gamma radiation. However, due to their electrical charge, protons are deflected by magnetic fields along their path, so that their path no longer points back to the source. Gamma rays, in turn, can be absorbed by dust or radiation along the way. Therefore, information about the cosmic accelerators can only be derived from the neutrinos. Since neutrinos are not charged and only rarely interact with matter, they almost always pass through media undisturbed, so that when measured they point back to their origin. This enables astronomy that does not require light, based on neutrino radiation.

Measuring extragalactic neutrinos is made even more difficult by their high energies. They carry around a billion times more energy than neutrinos from the sun or the Earth's atmosphere and are correspondingly rarer. The detector developed by Halzen at the IceCube Neutrino Observatory is one cubic kilometer in size and registers an average of only one neutrino per day.

Halzen first proposed his neutrino detector in 1988. The detector uses ice at the South Pole as a detection medium. It lies about two kilometers below the surface. When neutrinos react with the components of the ice, additional elementary particles (electrons, muons, tauons) are created. These in turn emit electromagnetic radiation as they cross the ice. The so-called Cherenkov radiation is measured using highly sensitive sensors located in boreholes. The trajectory and thus the direction of origin of the neutrinos can be reconstructed from the light trail.

Around 20 years after the concept was first published, the IceCube neutrino observatory went into operation in 2011 and just two years after measurements began, the first indications of high-energy neutrinos appeared. As a precursor, the AMANDA project (Antarctic Muon And Neutrino Detector Array) started in the 1990s. The scientists thus proved that high-energy neutrinos can be measured using Cherenkov radiation. Halzen's IceCube research project has now grown into a large international collaboration that now includes more than 450 people from 14 countries. Decisive contributions also come from a total of eleven institutes in Germany.

A comparison with the Large Hadron Collider at CERN, the most powerful accelerator in the world, shows how extreme the high-energy neutrinos are. This works with particles with energies in the range of tera-electron volts, i.e. 10¹² electron volts. Some of the neutrinos registered in IceCube carry at least a thousand times more energy. The number of high-energy neutrinos measured proves that these neutrinos actually come from space. Because this is too high for neutrino radiation that only comes from our solar system and can only be explained by neutrino sources outside our solar system. However, a single source has not yet been identified. Around ten percent of the neutrinos come from the Milky Way, the rest of the neutrinos measured come from foreign galaxies. Which cosmic objects are ultimately hidden behind the neutrino sources is also still uncertain. Supermassive black holes are considered the most promising candidates. The extragalactic sources in the neutrino sky “shine” so brightly that they outshine those in the Milky Way. The proof that high-energy neutrinos also arise within our galaxy is one of the latest results from the IceCube Neutrino Observatory.

The laureate himself appeared surprised on the phone. The news reached him in Italy, where he was on his way to a committee meeting. He is currently working on an application and hopes that it will be approved by the Nobel Prize. Halzen also emphasizes that he was very lucky. Although many of his colleagues thought the idea of ​​a neutrino detector in polar ice was good, hardly anyone, himself included, thought it was feasible. The biggest risk was that no one knew whether a cubic kilometer was big enough. Halzen goes on to say that the award also goes back to his courageous colleagues from the very beginning and that the project could never have been realized without the critical objections of his colleagues.

The prize for Halzen and his IceCube neutrino observatory is a plea for basic research and for the fact that science does not always need an application. Equally important is research based on curiosity and knowledge in order to further decipher our world, nature and its laws. Future generations of researchers will have to decide where the knowledge about neutrinos will lead. Because the actual astronomy, says Halzen, is still to come. The observatory is to be expanded and its volume expanded to eight cubic kilometers. The next generation is scheduled to be completed by 2032. So who knows what we will learn about neutrinos in the next few years.

Francis Halzen was born in Belgium in 1944. He is a professor of physics at the University of Wisconsin-Madison in the USA and is the scientific director of the IceCube Neutrino Observatory.

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