Francis Halzen to receive 2026 Nobel Prize in Physics for cosmic neutrino detection

This article was updated at 5:30pm EDT.

The Royal Swedish Academy of Sciences announced today that Francis Halzen of the University of Wisconsin–Madison will receive the 2026 Nobel Prize in Physics “for decisive contributions to the IceCube Neutrino Observatory and the discovery of high-energy neutrinos of astrophysical origin.”

In his comments at the press conference, Halzen, principal investigator of IceCube, said, “This was kind of an adventure where success was not guaranteed, and we were lucky to overcome the various challenges.”

Trillions of neutrinos pass through your body every second but leave no trace: Because they can travel across vast distances unscathed, they’re useful probes of distant objects. The highest-energy neutrinos, with energies of gigaelectron volts and higher, are flung from mysterious astrophysical particle accelerators beyond our solar system—and they may carry useful information about their cosmic sources.

In the rare instances when neutrinos collide with an atomic nucleus in a surrounding medium, they can produce a flash of blue light known as Cherenkov radiation. To have a shot at capturing interactions between neutrinos and nuclei, scientists recognized that they would need massive volumes of matter. “This is an idea that’s been around since the early 1960s,” says Spencer Klein, a physicist at Lawrence Berkeley National Laboratory who worked on IceCube alongside Halzen starting in the 2000s. Moisey Markov, a Russian physicist, was the first person to propose the idea of using large natural bodies of liquid water or ice to detect neutrinos.

Halzen and others realized that the large volume of clear glacial ice below the surface of Antarctica could provide a natural lab on the scale necessary to observe those bursts of light. He sketched the idea, shown in the figure, in a 1988 symposium paper coauthored with John Learned of the University of Hawaii at Manoa, and soon convinced other scientists to join the effort.

Though Halzen is a trained theorist, he piloted a project to test the viability of ice-based measurements, which began in the 1990s. By 2011, 86 holes had been drilled several kilometers into the ice cap and strings of photodetectors lowered to capture those flashes. Two years later, the facility reported its first detection of rare high-energy neutrinos. Today, hundreds of high-energy neutrinos are measured at the facility each year, and an international group of more than 400 scientists collaborate at the cubic-kilometer observatory. “We’ve been able to prove there are neutrinos from outside of the galaxy, from supermassive black holes, from billions of light years away,” says Lu Lu, a physicist at the University of Wisconsin–Madison, who has worked with IceCube for over a decade.

Carlos Argüelles, a physicist at Harvard University who is part of the IceCube collaboration and a former PhD student of Halzen’s, says that “large experiments just don’t happen unless there’s a driving force, and [Halzen] has been a driving force in the field for more than 30 years.”

Klein says, “It’s worked far better than we could have expected when we were getting started.” In addition to spotting astrophysical neutrinos from the Milky Way and active galactic nuclei beyond it, the facility has measured neutrino absorption in Earth and has made precision measurements of neutrino oscillations.

Astronomers built the first catalogs of knowledge from what can be observed with visible light. Now multimessenger astronomy brings together more types of signals—cosmic rays, gravitational waves, gamma rays, and high-energy neutrinos—to piece together what’s happening at the far reaches of the universe. New and updated neutrino detectors are planned across the globe to contribute to that ongoing work: IceCube-Gen2 Observatory (an extension of the existing facility at the South Pole), KM3NeT in the Mediterranean Sea, P-ONE in the northeastern Pacific Ocean, TRIDENT in the South China Sea, and Baikal-GVD in Lake Baikal.

The last time a Nobel Prize in Physics was awarded to just one recipient was in 1992. But Halzen is quick to give credit to his collaborators: “I hope this really reflects on the really courageous people who joined in the beginning of this project when . . . no really respected conservative scientists would have joined me. But many did, and that’s why I’m here.”

Neutrino science has earned a slate of Nobels over the past few decades. In 2015, it was awarded to Takaaki Kajita and Arthur B. McDonald for the discovery of neutrino oscillations. In 2002, half the prize was awarded jointly to Raymond Davis Jr and Masatoshi Koshiba for the detection of cosmic neutrinos. In 1995, half the prize went to Frederick Reines for the detection of the neutrino. And in 1988, the prize went to Leon M. Lederman, Melvin Schwartz, and Jack Steinberger for the neutrino beam method and the discovery of the muon neutrino.

Halzen received his PhD in 1969 from KU Leuven in Belgium for research on broken symmetries of hadrons. He was a scientific associate at CERN until 1971 when he moved to a visiting position at the University of Wisconsin–Madison. He joined the faculty the following year. He has been principal investigator of IceCube since 2001.

The prize will be awarded at a ceremony on 10 December 2026 in Stockholm, Sweden.

For Halzen, the work doesn’t stop with the Nobel Prize. When asked after the announcement what he would be doing with the rest of his day, he replied, “I’m working on a proposal, and I hope that this prize will help getting it approved.”

IceCube highlights in Physics Today

• In 2008, Halzen cowrote a PT article that described the fundamentals of the future IceCube observatory and explained how high-energy neutrinos could help answer various open questions about their astrophysical origins.
• A 2018 news story reported on IceCube’s identification of an extragalactic source of neutrinos.
• A 2023 news story detailed how IceCube helped establish that cosmic rays were the ultimate source of neutrinos in the Milky Way.
• Earlier this year, a news story outlined a recent major upgrade to IceCube aimed at improving its sensitivity to lower-energy neutrinos originating closer to home.

Other selected articles in Physics Today

• Record-setting cosmic neutrino breaks in a new telescope (April 2025).
• Cosmic tau neutrinos uncovered (June 2024).
• High-energy neutrinos originate in our own galaxy (September 2023).
• A star’s demise is connected to a neutrino outburst (August 2022).
• The elusive Glashow resonance was observed deep within Antarctic ice (April 2021).
• Neutrinos reveal Earth’s inner structure (November 2018).
• Astrophysical high-energy neutrinos (October 2018).
• Sterile neutrinos give IceCube and other experiments the cold shoulder (October 2016).
• More ice, more neutrinos (June 2014).
• Gigantic IceCube tightens limits on theories that predict dark-matter particles (May 2013).

Other resources

• AIP Oral History Interview with Francis Halzen
• High-energy neutrino astrophysics , by Francis Halzen (Nature Physics, March 2017.)
• South Pole experiment traps neutrinos from beyond the galaxy , by Francis Halzen (Scientific American, December 2015).
• IceCube: An instrument for neutrino astronomy , by Francis Halzen and Spencer R. Klein (Review of Scientific Instruments, August 2010).
• IceCube: Neutrinos associated with cosmic rays , by Francis Halzen (AIP Conference Proceedings, December 2009).
• High energy neutrino astronomy and its telescopes , by Francis Halzen (AIP Conference Proceedings, August 1995).

Read PT ’s past Nobel coverage .