When Francis Halzen addressed reporters early Tuesday morning upon receiving the 2026 Nobel Prize in Physics, he emphasized that the research project for which he received the honor - the massive Antarctic neutrino observatory called IceCube - began as a collaboration with UC Berkeley.
"I hope this prize will reflect … credit (on) this large collaboration, and especially some very critical people who made critical decisive contributions to this project over time," Halzen, a professor of physics at the University of Wisconsin, Madison, told the assembled media via phone. "I would like to point out that this project started as a collaboration of UW Madison and Berkeley."
That collaboration, called AMANDA (Antarctic Muon And Neutrino Detector Array), evolved over decades into the IceCube Neutrino Observatory, which in 2013 confirmed the existence of high-energy neutrinos with a million times the energy of neutrinos from the sun. Neutrinos are ghostlike elementary particles that, while abundant, rarely interact with anything else, making them extremely hard to detect. IceCube, made from detectors planted throughout a cubic kilometer of ice at the South Pole, was large enough to successfully find the rare high-energy ones. Thanks to this experiment and Halzen's leadership, we now know that high-energy neutrinos are bombarding us constantly from all directions, though scientists still don't know where they come from or how they're made.
Jane Scherr for UC Berkeley
The Wisconsin/Berkeley collaboration began in the 1980s between Halzen and late Berkeley physics Professor P. Buford Price, who at the time was studying cosmic rays - another type of particle whose origins, at the very highest energies, are unknown.
When Halzen conceived the idea of turning the clear ice covering Antarctica into a high-energy neutrino detector, Price and his team were intrigued.
"Buford was just an original thinker," said Steven Barwick, a UC Irvine professor of physics who at the time was a graduate student and then postdoctoral fellow in Price's lab. "He just liked doing new things, looking for new particles, looking for new detector schemes. I think this was just a perfect merging of those interests. And it was a perfect match to Francis' ideas, because Francis was very forward looking too. I think they just were kindred spirits."
The project may also have appealed to Price as a novel way to tackle the mystery of cosmic rays. It's hard to trace cosmic rays to their source because they are charged particles and are easily deflected by magnetic fields in space. But neutrinos travel in a straight line and might be a surrogate for cosmic rays, helping scientists track down their source.
"People in Prof. Buford Price's group … were among the first to take the whole idea of an ice-based detector seriously," according to Spencer Klein, a Berkeley research physicist and senior scientist at Berkeley Lab who now leads the two local contingents of the IceCube team.
Another key attraction for Price and his team, Barwick admitted, was that "honestly, a lot of us were just motivated because we didn't know what we were going to find. And that's always kind of fun. It was a cool project."
The plan was to embed cameras in the continent's solid ice to detect blue light produced when neutrinos collide with other atoms. The collisions produce charged particles, often muons - essentially heavy electrons - that are traveling faster than the speed of light in the ice and emit Cherenkov radiation, which is blue.
University of Wisconsin-Madison
Not long after Halzen and Price began working together, during a previously scheduled trip to the South Pole to install a cosmic ray detector, Barwick and others in Price's lab took the opportunity to drill holes in the ice to see whether it was clear enough to transmit light without scattering. The results were mixed - at the depths they looked, the ice was transparent enough for light to travel long distances, but not clear enough. Bubbles, like those in refrigerator ice cubes, scattered the light. In theory, however, at sufficient depth the bubbles should be squeezed out of existence. Subsequent tests in Greenland in 1991 proved this to be the case - photomultipliers in the ice could detect Cherenkov radiation produced by muons, though these muons were generated in the upper atmosphere by cosmic rays.
"Those tests in Greenland and the first tests in Antarctica were unsuccessful by some measures, but established that ice was really, really transparent," Barwick said. "And we knew that at some point the bubbles had to disappear. We just didn't know quite where."
That turned out to be about 1.4 kilometers below the Antarctic surface, where scattering was low enough that the team would be able to draw a straight line from the light to the muon to the neutrino and its source in the sky.
"They (the Price team) then became pioneers in AMANDA, … which showed that a South Pole neutrino detector was really possible," Klein said.
Halzen still had to gin up support and funding for the project. Despite proof of concept, many people still had doubts the team would detect anything in ice, as Halzen wrote in 2002 in a series of essays about AMANDA. Many people thought very clear ocean or lake water would be better.
Steven Barwick/UC Irvine
"Convincing donors of the soundness of our idea was no simple matter: I was only a theorist, after all, with no experience building anything, and my collaborators, at least in the beginning, were very talented but very junior physicists at the University of California, Berkeley," Halzen wrote. "Nevertheless, NSF was willing to give us the benefit of the doubt, and within a few years we had joined forces with eight other universities and three research laboratories in Belgium, Germany, Sweden and the United States."
AMANDA was a cylindrical array of 19 separate strings carrying 677 detectors mounted below the 1.4 kilometer depth Price's team had identified. AMANDA too detected atmospheric muons, showing the feasibility of the project. With further funding from NSF, the IceCube experiment kicked off in 2004 and incorporated the AMANDA arrays into a much bigger, 91-string array covering nearly one cubic kilometer of ice.
"AMANDA showed the detector was going to work. You just had to build it bigger," said Barwick, who was co-spokesperson for the experiment. "This was (part) of the genius of Francis - he had the perseverance and political savvy to convince the US and international funding agencies to invest in a telescope with much larger size."
Though Price moved on - among many other projects, he became excited by the prospect of studying the dust and bacteria embedded in Antarctic ice - Berkeley Lab scientists became much more involved, Klein said.
Courtesy of Spencer Klein
"The lab's enormous contribution was toward industrializing (for lack of a better word) the IceCube design, since it was clear that AMANDA's approach could not scale to a much larger detector," Klein wrote in an email. "Our specific contribution was to propose putting complex electronics in each optical module. This was revolutionary since these modules would not be accessible after deployment, and so would require satellite-level reliability, but without satellite-level costs."
The Berkeley Lab scientists demonstrated that the concept worked and went on to design and build all of the electronics for the optical modules - 5,693 boards in all. Klein initially got involved in IceCube data analysis and was part of the team that discovered the first neutrino seen by IceCube, dubbed "Big Bird". With an energy of 2 petaelectron volts - 2 quadrillion electron volts - it was then the highest energy event detected. More recently, Berkeley Lab researchers have focused on studying the properties of these neutrino interactions, which are thousands of times more energetic than those produced at accelerators.
In 2017, IceCube detected a high-energy neutrino from the direction of an extremely bright active galactic nucleus powered by a supermassive black hole, providing the first link to a possible source. The connection was indirect, however; when the galaxy flared, IceCube saw a shower of neutrinos. Combining neutrino detection by IceCube with observations from other telescopes - called multimessenger astronomy - may be the best way to pin down an actual source.
Barwick also became less involved in IceCube and more interested in designing the next generation of cosmic neutrino observatories. While experiments based on detecting blue Cherenkov light are planned for the clear waters of Lake Baikal in Russia, the Mediterranean and a couple of places in the Pacific Ocean, Barwick has turned his focus to detecting radio pulses from neutrino interactions. Such detectors would extend the search for astrophysical neutrinos to higher energies, Barwick said, and work in tandem with optical-based neutrino telescopes, which are best for the energies of neutrinos IceCube detected and for which Halzen received the Nobel Prize.
"It's just remarkable what you can do once you know how to do it," he said. "IceCube was the first to really show that it could be done, giving birth to neutrino astronomy. However, neutrino astronomy is still in its infancy. I don't think this is the last Nobel Prize in neutrino astronomy. It might be just the start of many."
