This story was originally published by APS Physics.
Beneath 1.5 kilometers of ice near the geographic South Pole, the IceCube Neutrino Observatory is on the hunt for ghost particles.
As the world’s first gigaton neutrino detector, IceCube is used by astrophysicists to better understand cosmic objects by observing the neutrinos — abundant, nearly massless, and mysterious elementary particles — that they produce.
But building a massive neutrino detector in the middle of Antarctica was once just a “cute” idea, said Francis Halzen, the Vilas Research Professor and Gregory Breit Professor at the University Wisconsin–Madison and IceCube’s principal investigator.
“Our attitude was, ‘let’s study the ice, let’s design a hot water drill, let’s see if it works,’” said Halzen. “We started small and, to our own amazement, we overcame all these hurdles. And suddenly, we had the tools to build a kilometer cube detector.”
Halzen is the 2026 recipient of the APS Medal for Exceptional Achievement in Research, the society’s largest prize, for his “contributions to the field of neutrino astrophysics, especially leadership of the IceCube Neutrino Observatory and the discovery of high-energy astrophysical neutrinos and their sources.”
Halzen’s scientific career began at KU Leuven in Belgium, where he earned a master’s degree in math and physics. He also earned his Ph.D. at KU Leuven, where he completed his thesis on the broken symmetries of hadrons. In 1971, while working at CERN in Geneva, he received an invitation from a colleague for a six-month research visit to UW–Madison, where he’s been ever since.
Halzen became interested in neutrinos in the mid-1980s, when he started looking into the possibilities for building a neutrino telescope with his postdoc Enrique Zas, now a professor at the University of Santiago de Compostela.
The advantage of using neutrinos to do astronomy is that, unlike photons, neutrinos can travel through objects, “so maybe you could see things in the universe you couldn’t see any other way,” he said. “And that was, of course, the big appeal. In fact, in the almost 40 years of this AMANDA/IceCube adventure, nobody ever told us that this was uninteresting. It was really a question of the technology.”
IceCube’s technology relies on the same design found in both historic particle detectors, like the Irvine-Michigan-Brookhaven detector, and modern neutrino observatories like Super-Kamiokande: a massive Cherenkov detector. When a neutrino interacts with the detector’s extremely clear water, it leaves behind a charged particle. This charged particle travels through the water faster than light, which causes it to emit electromagnetic Cherenkov radiation that can be picked up by the many highly sensitive sensors that surround the detector.
To meet this kilometer-scale challenge, Halzen and his team started small. “Especially for what we proposed, [which was] to switch from water to ice, there were a lot of things that had to be realized: the ice had to be clear, the drilling methods we developed in Madison had to work, and we had to be able to reject the backgrounds,” he said.
IceCube’s experimental proving ground was the Antarctic Muon and Neutrino Detector Array, or AMANDA. Thanks to funding from the National Science Foundation, in the 1990s researchers began drilling holes in the ice thousands of meters deep using hot water “hoses,” then dropping cables lined with sensors inside before the ice froze back over.
AMANDA’s initial results were published in 2001, paving the way for IceCube’s construction, which began in 2004, again with NSF funding. AMANDA was formally incorporated into IceCube, whose final string was lowered into the ice in December 2010. IceCube’s first fully instrumented physics run began in May 2011.
When talking about IceCube at conferences and seminars, Halzen often begins with what he considers its most significant finding: “Neutrino astronomy exists.”
“After one and a half decades of development, and another decade of construction, there was no guarantee we would ever see anything,” he says. “Many people thought we wouldn’t, but we did. After two years of data, there were neutrinos from way beyond — not from the atmosphere, not from our own galaxy, but reaching us from across the universe with enormous energies.”
Halzen added that while it was “relatively easy” to spot these cosmic neutrinos with incredibly high energy levels, figuring out where they came from was the next question. IceCube has recently observed where they originate, however. Their results point to the potential locations for so-called “cosmic accelerators,” which are also the source of cosmic rays, high-energy particles discovered more than a century ago.
“That’s why everybody thought neutrino astronomy was interesting, because cosmic rays, wherever they are born, make neutrinos, but at the time we had no idea where or how,” said Halzen. “Now, by seeing neutrinos, we start to see the first cosmic accelerators, and that’s really exciting.”
IceCube is also a powerful tool for understanding the underlying physics of these mysterious elementary particles, Halzen said. “Compared to [Fermilab], we have very few neutrinos, but some are a million times the energy levels,” he said. “Our expectation is not only to see the universe — like many astronomers did in other ways — but to hopefully make some contributions to neutrino physics.”
And while many big experiments fall apart after operating for decade-long time scales, said Halzen, IceCube is still growing, with the collaboration now including more than 450 researchers from institutions across 14 different countries.
Work is also now underway on the IceCube Upgrade, which will deploy seven new strings along with new and improved sensor modules. With a larger and more sensitive detector, Halzen hopes that IceCube will be able to not only solve the cosmic ray problem but also make some “totally unexpected” discoveries along the way.
“We have these neutrinos of enormous energies, but all the physics that we are doing seems to be consistent with the standard model neutrino physics that we know and love, and we want to break that,” he said.
For Halzen, there’s so much to be excited about for this “booming” field to spend much time reflecting on the impact of his career. “I have no time to think about my legacy — I only look at the future,” he said. “I have been incredibly lucky, and I want to enjoy the rest of it.”