More than 600 feet below the surface of Antarctica, ultrasensitive detectors picked up the tracks of cosmic rays crashing down from outer space.
The Askaryan Radio Array is a group of sensors drilled deep into the ice. For years, the array has been patiently listening for faint radio signals near the South Pole.
In a study published last month, University of Chicago researchers announced that new analysis techniques revealed 13 events where cosmic rays produce particle showers in the ice. This marks the first time that scientists have detected these particle showers through ice, which offers a chance to study never-before-seen aspects of the phenomenon.
“When these particles strike the ice, they produce a burst of radio waves that is a bit like a sonic boom,” said UChicago postdoctoral fellow Philipp Windischhofer, one of the study’s two main authors.
The Askaryan Radio Array was designed to hunt for high-energy neutrinos, an extremely rare cosmic particle. But new data analysis techniques may be revealing it to be an unexpected boon for the wider study of particles from space. Even after the researchers’ landmark findings, more than seven years of data remains to be studied; scientists hope to learn more about elusive cosmic rays—and perhaps finally catch the signal of a neutrino.
“We’re looking forward to analyzing the rest of the data and hopefully, gaining new insights into the highest-energy phenomena in our universe,” Windischhofer said.
The paper is published in Physical Review Letters.
Visitors from outer space
Neutrinos are extremely difficult to catch, as they very rarely interact with matter. But they are a unique window into the wildest phenomena in the universe, like supermassive black holes and exploding stars.
The Askaryan Radio Array, run by an international collaboration of scientists from the U.S., Europe, and Asia led by Prof. Amy Connolly at The Ohio State University, was designed to pick up such very high-energy neutrinos from space. The first prototype was installed in 2011; UChicago scientists Eric Oberla and Cosmin Deaconu developed and built the newest section, deployed in 2018.
However, as scientists initially analyzed the data that came in from this new instrument, located near the National Science Foundation’s Amundsen-Scott South Pole Station, they didn’t see any signs of neutrinos.
The signals were coming from the wrong direction. Neutrinos were more likely to be picked up as they traveled upward through the two kilometers of solid ice below the station.
Then-graduate student Kaeli Hughes, PhD’22, noted there were a few neutrino-like signals, but they were coming from above the array. She hypothesized that some might be cosmic rays, but no one had yet developed the analytic tools to confirm it.
Over the following eight years, however, science and analytic tools advanced. And a group decided to take another look at the data.
As they combed through, Windischhofer and graduate student Nathaniel Alden were surprised to see clear hits for a different kind of visitor from space: cosmic rays.
The same extreme events—like supernovae—that make neutrinos also produce cosmic rays, which are actually atoms with their outer layers stripped away to leave just a nucleus. When an incoming cosmic ray strikes an atom on Earth, the collision creates a characteristic shower of secondary particles, which the array’s detectors picked up in the ice.
A big difference between cosmic rays and neutrinos is that cosmic rays have a charge. This means their paths get scrambled by magnetic fields on the way to Earth, so scientists can’t trace them back to whatever supernova or black hole might have spawned them.
But each visitor from outer space has something to tell us.
“By studying what elements these cosmic rays are, you can learn about the abundance of elements in the universe and what is accelerating them to such high energies,” explained Alden.
Early findings help to improve hunt for neutrinos
The array also offers a unique perspective on cosmic rays. For one, it looks at higher energy particles than most other cosmic ray experiments. It’s also the only detector that measures how the signals travel through ice.
“This means the Askaryan Radio Array can probe the very dense inner core of the cosmic ray shower, which is very hard to do with other setups,” said Windischhofer.
Finally, the detection marks the first time that a phenomenon known as Askaryan radiation has been observed by itself “in the wild.”
This type of radiation, first predicted in 1962 by Armenian physicist Gurgen Askaryan, occurs when a particle traveling with very high energy smacks into an unsuspecting atom inside ice or similar material on Earth. The collision produces a shower of secondary particles, which travel through the ice at very nearly the speed of light. This effect had been created artificially in the lab, but it had never before been used to actually detect a particle from the cosmos.
As the team members comb through the rest of the array’s data, they expect to find more cosmic rays, but possibly neutrinos as well.
“In that sense, it’s a nice stepping stone because you haven’t seen a neutrino yet, but now you have seen the same signature that you would expect for a neutrino in your detector, and so you’re able to try out ideas that people have had about how to look for both these particles,” said Alden.
“It is not often that a graduate student walks into your office and shows you a plot that says something truly new about nature that you’ve not seen before,” said Prof. Abby Vieregg, David N. Schramm Director of the Kavli Institute for Cosmological Physics and senior author on the paper.
“Seeing high energy cosmic rays for the first time through their radio emission in ice is important not just for characterizing cosmic rays in a new way, but also for allowing us a glimpse of what the highest energy neutrino signals could look like some day in the detectors we’ve spent years building,” she said.
Citation: “Observation of in-ice Askaryan radiation from high-energy cosmic rays.” Alden and Windischhofer et al, Physical Review Letters, April 17, 2026.
Funding: National Science Foundation, Taiwan National Science Council, Belgian Fund for Scientific Research, Leverhulme Trust, European Research Council, Belgian American Education Foundation.
