The Nobel Prize in Physics, the second award of this year’s Nobel week, has been awarded to Francis Halzen. The physicist proposed searching for neutrinos in the ice at the South Pole and led the construction of the IceCube observatory. It was the first to detect high-energy neutrinos arriving from beyond the Solar System.
Ultra-High-Energy Particles
The highest-energy neutrino ever reported by the IceCube collaboration was detected on March 31, 2019, but the findings were not published until March 2026 in Physical Review Letters. According to the materials accompanying the press release on NobelPrize.org, the particle’s energy was 11.4 petaelectronvolts—more than 11 million billion electronvolts. This is approximately 1,600 times the energy of a proton in the Large Hadron Collider’s beam, where that value reaches 7 teraelectronvolts.
Physicists at CERN offer an interesting analogy to help explain a particle’s energy. They compare the energy of a single particle at one teraelectronvolt to the kinetic energy of a mosquito in flight. Using this analogy, the record-breaking neutrino detected in the Antarctic ice carried as much energy as more than 11,000 mosquitoes.
The first particles of this kind were found almost by accident in 2013. At the time, researchers were searching for neutrinos with energies on the order of an exaelectronvolt—a thousand times higher—but instead detected two particles with energies of around one petaelectronvolt. A year later, the observatory’s team demonstrated that the recorded events could not be explained solely by processes in Earth’s atmosphere.
Distinguishing neutrinos arriving from distant space is difficult. Each year, the detector records approximately 100,000 neutrinos, most of which originate in Earth’s atmosphere when cosmic rays strike it. Only about a hundred are estimated to be of astrophysical origin.
Why Neutrinos?
Space is permeated by particles that natural accelerators, such as supernova explosions or plasma jets near supermassive black holes in active galactic nuclei, propel to energies up to a million times higher than those achieved in laboratories on Earth. Most of these particles are protons. Magnetic fields in our Galaxy and intergalactic space bend their paths, making it impossible to identify their sources from their arrival directions.
The processes that accelerate protons also produce neutrinos. These particles have no electric charge, so cosmic magnetic fields do not deflect them, and dust and gas do not stop them. Gamma radiation from the same sources, by contrast, weakens along the way or cannot pass through dense clouds at all.
The same property makes observation difficult. Neutrinos barely interact with matter. Every second, approximately 65 billion neutrinos from the Sun pass unnoticed through every square centimeter of our bodies. Detecting enough of these rare cosmic particles therefore requires a detector with a volume of one cubic kilometer.
Ice Instead of the Ocean
Francis Halzen of the University of Wisconsin–Madison first presented the idea of using natural Antarctic ice instead of water in 1988, together with his colleague John Learned. The detector would record flashes of light produced when a neutrino collides with an atomic nucleus. A similar deep-ocean muon and neutrino detector, DUMAND, was being installed in the ocean near Hawaii, but the project was shut down in 1995. Light emitted by marine organisms and radioactive impurities in the water reduced the sensitivity of measurements there.
The ice at the South Pole offers other advantages. At depth, there is permanent darkness and no living organisms, while the area itself is geologically stable. The nearby Amundsen–Scott South Pole Station was already operating with an established supply system.
The first attempts to lower sensors into the ice at the South Pole were made in 1992, and the results were disappointing. Air bubbles in the upper layers scattered the light and blurred the particles’ tracks. Below 1,400 meters, however, the ice proved exceptionally transparent, with flashes traveling as far as 300 meters—much farther than expected. The farther the light travels, the more sensors detect it, and the more precisely the neutrino’s arrival direction can be determined.
The IceCube observatory was completed in 2011, with Francis Halzen remaining the project’s scientific leader throughout its development. Its 5,160 light sensors are attached to 86 cables at depths ranging from 1,450 to 2,450 meters.
The Search for Sources Continues
One candidate source of cosmic neutrinos is the active galaxy M77, located 46 million light-years away in the constellation Cetus. Seventy-nine neutrinos arrived from its direction, but these data are not yet sufficient for a definitive conclusion. The first evidence of high-energy neutrinos from our own Galaxy was published in 2023. This year, the IceCube collaboration reported that the signal’s statistical significance had reached the threshold physicists consider a discovery. These results have been posted on the arXiv preprint server and have not yet undergone peer review.
This is the fifth time the Nobel Prize in Physics has been awarded for neutrino research. Previous laureates were recognized for the discovery of the muon neutrino, the first detection of the particle, observations of neutrinos from the Sun and a supernova, and neutrino oscillations. This year’s series of awards began yesterday with the Nobel Prize in Medicine for optogenetics, while the chemistry laureates will be announced on October 7 at 12:45 p.m. Kyiv time.
The search for an answer to the question of which objects accelerate cosmic rays will continue at the IceCube-Gen2 observatory. It will encompass eight cubic kilometers of ice at the South Pole. Meanwhile, neutrino telescopes are also being built in the Mediterranean Sea, off the coast of Canada, and in the South China Sea.
