Physicist Francis Halzen speaks during a presentation on the IceCube Neutrino Observatory at the Bergamo Scienza science festival in Italy on Sunday.Laura Pietra/Bergamo Scienza 2026/via The Associated Press
Francis Halzen, a University of Wisconsin-Madison professor who pioneered the detection of ghostly particles from outer space, has been awarded the Nobel Prize in Physics.
On Tuesday, members of the Royal Swedish Academy of Sciences named Dr. Halzen the sole winner of this year’s physics prize.
Dr. Halzen is best known for his work establishing the IceCube Neutrino Observatory at the South Pole. Using a one cubic kilometre volume of the Antarctic ice sheet as its detection medium, the experiment can spot flashes of light that occur when high-energy neutrinos from the distant universe occasionally collide with atoms in the ice.
Completed in 2011, the experiment ushered in a new era of particle astrophysics.
Winning the Nobel “was a great surprise,” said Dr. Halzen when reached by phone during the announcement.
“When I reflect on this moment I have to emphasize how lucky I was because when we started this project, everybody realized this was maybe a good idea but very few thought it would work, including myself.”
Eva Olsson, Ellen Moons and Mark Pearce announce Francis Halzen as the recipient of the 2026 Nobel Prize in Physics at the Royal Swedish Academy of Sciences in Stockholm on Tuesday.Christine Olsson/Reuters
Born in Belgium in 1944, Dr. Halzen received his PhD from KU Leuven university in 1969. In 1971, he began his research career in Wisconsin where he continues to lead the IceCube project.
The goal of IceCube is to detect neutrinos, particles that interact so infrequently with other forms of matter that they can easily pass through miles of solid lead as though through thin air.
Starting in the 1960s, experimental physicists began devising experiments to spot neutrinos produced by nuclear reactions in the sun’s core. Such experiments are typically located underground because the surrounding rock is invisible to neutrinos but screens out cosmic rays – electrically charged atomic fragments that are constantly raining down to Earth from collisions in the upper atmosphere.
Physicists want to catch neutrinos in the sea. Step 1: Make sure their machinery won’t drown
In 2015, Canadian physicist Arthur McDonald shared the Nobel Prize for leading a solar neutrino experiment built in a mine in Sudbury, Ont., which in the early 2000s helped to establish that neutrinos have mass.
Meanwhile, Dr. Halzen sought to extend the reach of neutrino detectors into the larger universe beyond our solar system.
His goal was to spot high-energy neutrinos that are produced in extreme environments, including massive stellar explosions and jets of material accelerated by the powerful magnetic fields that surround supermassive black holes.
High-energy neutrinos are far more rare than those from the sun. Spotting them requires a much larger detector than can fit inside a mine. Starting in 1987, Dr. Halzen and colleagues began exploring whether the giant sheet of clear glacial ice buried under snow at the South Pole would make an ideal place to build a giant neutrino detector.
Their initial effort was called AMANDA, for Antarctic Muon and Neutrino Detector Array. Built between 1995 and 2000, it consisted of hundreds of optical detectors. These were lowered into bore holes melted into the ice using hot water, which was then allowed to refreeze, locking the detectors in place. Passing neutrinos interacting with atomic nuclei in the ice generated telltale flashes of light that the detector could spot.
Though cosmic rays drowned out the neutrino signal from above, Dr. Halzen and his colleagues took this into account by only paying attention to particles moving upward from below, effectively using the bulk of Earth as a filter through which they could view neutrinos arriving from the northern sky on the other side of the planet.
AMANDA showed the approach could work, paving the way for IceCube, a much larger version of the experiment.
Over the years that it has been operating, IceCube has confirmed the existence of neutrinos arriving from distant galaxies and provided new information about high-energy events in the cosmos. It has also helped to pin down key properties of neutrinos themselves.
Nobel prize in medicine awarded to researchers who learned how to light up the brain
As leader of IceCube, Dr. Halzen has also helped to set a course for parallel efforts to spot high-energy neutrinos, including P-ONE, an underwater experiment in development off the coast of Vancouver Island.
Jodi Cooley, a particle astrophysicist and director of SNOLAB, the successor to Dr. McDonald’s facility in Sudbury, began her research career working on AMANDA. In an interview she said she considers Dr. Halzen one of her research mentors and expressed her joy at the news that he had won this year’s physics Nobel.
“He combined extraordinary scientific vision with an infectious enthusiasm for discovery, inspiring generations of scientists to pursue the bold idea that neutrinos could reveal a previously hidden universe,” Dr. Cooley said.
She added that Dr. Halzen also navigated an important transition when AMANDA grew into IceCube, becoming a much larger and costly experiment that required hundreds of researchers giving a large share of their careers to help the effort succeed.
“That takes real leadership,” Dr. Cooley said.
With Tuesday’s announcement, the physics prize has been awarded 120 times to 230 individuals since it was established in 1901.
The most recent Canadian winner was Geoffrey Hinton, a University of Toronto professor and pioneer of artificial intelligence who was awarded the physics Nobel in 2024 for is work on neural networks.