On October 6, the Royal Swedish Academy of Sciences announced that the 2026 Nobel Prize in Physics will be awarded to Francis Halzen of the University of Wisconsin–Madison. The prize recognizes his leadership in realizing IceCube, the neutrino observatory at the South Pole, and in making possible the discovery of high-energy neutrinos of astrophysical origin. The idea of embedding light sensors in a cubic kilometer of ice created a new way to probe particle acceleration in the universe, which is hard to study with light alone. Detecting neutrinos that arrive from space, however, is a different challenge from identifying the objects that emitted them. IceCube's significance lies in achieving the former and, step by step, closing in on the latter.
Neutrino astronomy and the search for cosmic accelerators
Neutrino research has led to the Nobel Prize in Physics several times before. In 2002, Raymond Davis Jr. and Masatoshi Koshiba were honored for detecting neutrinos from the Sun and from a supernova, opening the way to observing the universe with neutrinos. In 2015, Takaaki Kajita and Arthur B. McDonald were recognized for discovering neutrino oscillation, in which neutrinos change type in flight, showing that neutrinos have mass.
This year's prize recognizes observation of the high-energy universe that lies beyond that earlier work. Whereas the neutrinos observed from the Sun and supernovae are in the MeV range, the astrophysical neutrinos IceCube detects reach the TeV (tera-electronvolt) to PeV (peta-electronvolt) range. A PeV is 1,000 times a TeV, and different energies probe different physical phenomena.
The main components of the "cosmic rays" that rain down on Earth are fast-moving protons and atomic nuclei. Because they carry electric charge, however, their paths are bent each time they pass through magnetic fields in space. Even if we measure the direction from which they arrive on Earth, tracing that direction backward does not necessarily lead to their source. This loss of directional information is a major obstacle to finding where in the universe, and by what mechanism, particles are accelerated to such extreme energies.
Neutrinos help here because they carry no electric charge and barely interact with matter. In the standard understanding, when accelerated protons and nuclei collide with surrounding gas or photons, they produce particles such as pions, and the decay of those pions creates neutrinos. Neutrinos are not deflected by magnetic fields and can escape dense astrophysical environments easily, so tracing their arrival directions backward can point to the places where particles are being accelerated.
Gamma rays are another important clue to particle acceleration, but they can be absorbed by surrounding matter and light, and they can also be produced by other processes involving electrons. If neutrinos can be observed at the same time, they help narrow down whether processes involving protons and nuclei are at work. Multimessenger astronomy, which combines several "cosmic messengers" such as light and neutrinos, aims to study the same object through different physical phenomena.
Reading neutrino directions from light in the ice
Completed in 2011, IceCube has 5,160 light sensors arranged along 86 cables in the ice at depths of 1,450 to 2,450 meters below the surface at the South Pole. The instrumented volume is one cubic kilometer. Catching neutrinos, which barely interact with matter, required a detector this large.
The sensors do not capture neutrinos directly. When a neutrino interacts with nucleons and other constituents of atomic nuclei in the ice, it produces charged particles such as muons, or showers in which many particles are generated in a cascade. When the speed of such a charged particle exceeds the speed of light in ice, Cherenkov light is emitted. This happens because light travels more slowly in ice than in a vacuum; it does not mean that any particle exceeds the speed of light in a vacuum.
Researchers analyze which sensors received how much light and when, and from that they reconstruct the neutrino's energy and arrival direction. Events in which a muon leaves a long track make it easier to estimate direction. Shower events, in which light is concentrated in a relatively small region, make it easier to measure the energy deposited in the detector. Because different event shapes are better suited to different measurements, there is value in analyzing several types together.
It was not known from the start that Antarctic ice would be an ideal detection medium. Francis Halzen and John G. Learned proposed using ice in 1988. In AMANDA, IceCube's predecessor, air bubbles in the ice at depths of roughly 800 to 1,000 meters scattered light strongly, hampering accurate reconstruction of particle directions. When sensors were lowered deeper, to 1,500 to 2,000 meters, it turned out that very clear ice with few bubbles was available, opening the way to using the ice as a huge detection medium.
The path from the 1999 IceCube proposal to the start of construction in 2004 and completion in 2011 was also a process of turning a theoretical idea into a working instrument through studies of the ice's properties and drilling technology. The Nobel Prize will be awarded to Francis Halzen individually, but IceCube's construction and scientific results have been built up by an international collaboration involving many researchers and engineers.
What the 28 events showed
In 2013, the IceCube Collaboration reported in the peer-reviewed journal Science 28 events found in data from May 2010 to May 2012. The energy deposited inside the detector ranged from 30 to 1,200 TeV. The background from muons and neutrinos produced by cosmic rays in the atmosphere was predicted to be 10.6 events, with an uncertainty of +5.0 and −3.6.
The collaboration did not judge each of the 28 events individually to have come from an astronomical object. The number observed exceeded the expected background, and when energy, arrival direction, and event type were examined together, a component emerged that was difficult to explain with atmospheric neutrinos and muons alone.
In the paper, two previously found events and the additional 26 were analyzed together, and under the reference atmospheric model the hypothesis that all were of atmospheric origin was rejected at a significance of 4.1σ. This value changes depending on what assumptions are made about particle production in the atmosphere.
Here, σ is a measure of how rare it would be for a signal as large as the one observed to appear by chance if only background were present. It is not a number that directly gives the probability that the neutrinos are of cosmic origin. Moreover, the energy deposited inside the detector does not necessarily match the total energy of the incoming neutrino. If a muon exits the detector, for example, part of its energy cannot be measured.
Still, it was highly significant that high-energy astrophysical neutrinos could be observed as a population. It became possible to study the energy distribution of neutrinos arriving from outside the solar system and the directions in the sky they come from.
At that stage, however, no statistically significant individual source had been found. Confirming that astrophysical neutrinos exist and identifying the specific objects that produce them remained separate challenges.
Clues to the sources come in more than one form
On September 22, 2017, IceCube detected a neutrino with an estimated energy of about 290 TeV. In its arrival direction lay the blazar TXS 0506+056, which was brightening in gamma rays. A blazar is an object in which a jet extending from the active galactic nucleus at a galaxy's center points almost directly toward Earth. Telescopes around the world that received IceCube's alert conducted follow-up observations at various wavelengths, and a 2018 paper in Science reported evidence that a blazar can be a source of high-energy neutrinos.
Since then, IceCube has accumulated evidence for different questions: detection of astrophysical neutrinos as a whole, excesses from specific directions, and neutrino emission spread along the Galactic plane. Organizing the main results by what each examined and how far each can be taken shows that the discovery has progressed in stages.
| Publication / study | Targets and period | Result | What this result alone cannot tell us |
|---|---|---|---|
| 2013, Science | All-sky events starting inside the detector, May 2010–May 2012 | Compared 28 events with background and detected a high-energy component difficult to explain with atmospheric origin alone | The sources of individual events or any specific object |
| 2018, Science | The September 22, 2017 neutrino and multiwavelength follow-up | The directional coincidence with the flaring TXS 0506+056 supports its potential as a source candidate | What fraction of all high-energy neutrinos blazars account for |
| 2022, Science | Search of 110 pre-selected objects in data from May 2011 to May 2020 | Estimated 79 signal events from the direction of NGC 1068; 4.2σ after correcting for the number of trials | Which region inside the galaxy produces the neutrinos |
| July 2026, preprint | Galactic plane analysis combining 12 years of data with event shapes characteristic of three neutrino flavors | Reported detection of high-energy neutrino emission from the Galactic plane at 5.7σ | The identity of individual particle-accelerating objects |
Because the observation periods and background hypotheses differ across these studies, one cannot compare significance numbers alone and rank which source is more certain. Even if a strong signal is detected from an extended region such as the Galactic plane, it does not necessarily follow that a single object within it can be identified.
The 2022 result for NGC 1068, also known as M77, illustrates this difference well. The collaboration estimated the number of signal events from the direction of NGC 1068 at 79, with uncertainties of +22 and −20. It did not identify each of the 79 neutrinos one by one as having come from NGC 1068; the figure is a statistical estimate of the size of a signal component added to the background.
Also, when many candidate objects are examined, the chance of finding a large excess somewhere by coincidence rises. The paper corrected for the effect of examining 110 objects, and the local significance of 5.2σ obtained for NGC 1068 was evaluated at 4.2σ after the correction.
At the center of NGC 1068 lies an active galactic nucleus shrouded in gas and dust. One candidate model for explaining the observations holds that neutrinos are produced in the "corona," a hot plasma around the central black hole, while the gamma rays produced at the same time are absorbed in the surroundings. The paper states explicitly, however, that it cannot directly identify which region inside the galaxy produces the neutrinos. It did not observe the interior of the black hole itself, nor does it show neutrinos escaping from it.
In the Galactic plane study released on July 28, 2026, multiple event shapes characteristic of three neutrino flavors were combined, and the modeling and calibration of the ice and the event reconstruction methods were also improved. Including the effect of trying multiple emission models, the study reports detecting high-energy neutrino emission from the Galactic plane at 5.7σ. As of October 7, 2026, it is available as a preprint on arXiv.
High-energy neutrinos have become a means of probing not only the activity of distant galaxies but also the processes within the Milky Way in which cosmic rays interact with interstellar gas and other material. The next challenge is to determine how much of the total observed neutrino flux each type of source accounts for.
Understanding the ice will shape the next discoveries
In 2026, the IceCube Collaboration announced that it had added five densely spaced cables in the lower central part of the existing detector, installing more than 600 new light sensors and calibration devices. Research institutions in Japan and Germany contributed substantially to the supply of light sensors.
The IceCube Upgrade aims not only to advance precision measurements of neutrino oscillation but also to study in greater detail how light propagates through the ice. If the scattering and absorption of light by the ice cannot be modeled accurately, errors will also arise in estimating neutrinos' arrival directions and energies.
The collaboration expects that the newly installed calibration devices will allow a more detailed understanding of the ice's properties and that reanalyzing the past 15 years of observational data will improve the precision of neutrino reconstruction. However, completing the installation of the devices and demonstrating the resulting improvement in precision are separate matters.
A still larger "IceCube-Gen2" has also been proposed. If realized, it would expand the volume of ice instrumented with sensors to about eight times the current volume. The IceCube Upgrade, whose installation was completed in 2026, and IceCube-Gen2, a future concept, are at different stages.
Increasing the number of neutrino sources and narrowing down the regions where particles are accelerated will take more than simply increasing the number of detections. It will require improving the precision of arrival-direction reconstruction and cross-checking against electromagnetic waves such as gamma rays observed at the same time. If the Antarctic ice itself can be understood more accurately and signals from the same object can be captured repeatedly, it will become possible to test more concretely where, and how, the universe accelerates particles to such extreme energies.