Antarctic Ice Detects Cosmic Rays

Scientists at work installing cables and electronic components for the Askaryan Radio Array, a detector for incoming cosmic particles located at the South Pole.
Photo Credit: ARA Collaboration / NSF / Jeffrey Donenfeld

Scientific Frontline: Extended "At a Glance" Summary: Cosmic Ray Detection via Askaryan Radiation

The Core Concept: The Askaryan Radio Array, a grid of sensors buried deep within Antarctic ice, has successfully detected incoming high-energy cosmic rays by capturing the distinct radio wave bursts generated when these particles impact the ice.

Key Distinction/Mechanism: When a cosmic ray strikes an atom in the solid ice, it creates a shower of secondary particles moving near the speed of light. This emits a radio wave burst similar to a sonic boom, known as Askaryan radiation. Unlike electrically neutral neutrinos, cosmic rays carry a charge, which causes their trajectories to scatter within magnetic fields and obscures their exact cosmic origins.

Major Frameworks/Components:

• Askaryan Radio Array (ARA): An international network of ultra-sensitive radio sensors drilled more than 600 feet into the Antarctic ice.
• Askaryan Radiation: The characteristic burst of radio waves produced by high-energy secondary particle showers traveling through a dense, dielectric medium like ice.
• Cosmic Rays: High-energy atomic nuclei (atoms stripped of their electron layers) spawned by extreme cosmic events like supernovae.
• High-Energy Neutrinos: Elusive, rarely interacting cosmic particles that the array was originally designed to capture.

Branch of Science: Particle Physics, Astrophysics, and Astronomy.

Future Application: The analytic techniques developed to identify these cosmic ray signatures provide a crucial baseline for the eventual detection and isolation of high-energy neutrinos.

Why It Matters: This event marks the first natural observation of Askaryan radiation outside of artificial laboratory settings. Furthermore, using ice allows scientists to probe the highly dense inner cores of cosmic ray showers, offering new insights into the universe's elemental abundance and the forces accelerating these particles.

More than six hundred 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 network of sensors deployed deep within 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 thirteen events in which cosmic rays produced particle showers in the ice. This marks the first time scientists have detected these particle showers through the ice, offering 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, which are extremely rare cosmic particles. However, new data analysis techniques may be revealing the array 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 remain 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.

An antenna as it’s lowered into the Askaryan Radio Array boreholes.
Photo Credit: ARA Collaboration / NSF

Visitors from Outer Space

Neutrinos are extremely difficult to detect because they rarely interact with matter. However, they provide a unique window into the wildest phenomena in the universe, such as supermassive black holes and supernovae.

The Askaryan Radio Array, run by an international collaboration of scientists from the United States, Europe, and Asia, led by Professor Amy Connolly at the Ohio State University, was designed to detect these 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 from this new instrument, located near the National Science Foundation’s Amundsen-Scott South Pole Station, they did not observe any signs of neutrinos.

The signals were arriving from the wrong direction. Neutrinos were more likely to be detected as they traveled upward through the two kilometers of solid ice beneath the station.

Then graduate student Kaeli Hughes, PhD '22, noted that there were a few neutrino-like signals, but they were originating from above the array. She hypothesized that some might be cosmic rays, but no one had yet developed the analytic tools to confirm this.

Over the following eight years, science and analytic tools advanced, and a group decided to reexamine the data.

As they combed through the data, Windischhofer and graduate student Nathaniel Alden were surprised to find clear hits for a different kind of visitor from space: cosmic rays.

The same extreme events—such as supernovae—that produce neutrinos also produce cosmic rays, which are atoms with their outer layers stripped away, leaving only 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 recorded in the ice.

A significant difference between cosmic rays and neutrinos is that cosmic rays carry an electric charge. This means their paths are scrambled by magnetic fields on their way to Earth, so scientists cannot trace them back to the specific supernova or black hole that spawned them.

However, 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.

The detectors are installed inside 600-foot-deep holes bored into the Antarctic ice sheet, which allows them to catch showers of particles traveling through the ice. Above, operators drill the holes for the detectors.
Photo Credit: ARA Collaboration / NSF / Jeffrey Donenfeld

Early Findings Help to Improve Hunt for Neutrinos

The array also offers a unique perspective on cosmic rays. For one, it observes higher-energy particles than most other cosmic ray experiments. It is 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 extremely high energy collides with an atom within the ice or similar material on Earth. The collision produces a shower of secondary particles, which travel through the ice at nearly the speed of light. This effect had been created artificially in the laboratory, but it had never before been used to actually detect a particle from the cosmos.

As the team members comb through the remainder of the array’s data, they expect to find more cosmic rays, and 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 Professor Abby Vieregg, David N. Schramm Director of the Kavli Institute for Cosmological Physics and senior author of 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 someday in the detectors we’ve spent years building,” she said.

Funding: National Science Foundation, Taiwan National Science Council, Belgian Fund for Scientific Research, Leverhulme Trust, European Research Council, Belgian American Education Foundation

Published in journal: Physical Review Letters

Title: Observation of In-Ice Askaryan Radiation from High-Energy Cosmic Rays

Authors: N. Alden, S. Ali, P. Allison, S. Archambault, J. J. Beatty, D. Z. Besson, A. Bishop, P. Chen, Y. C. Chen et al. (ARA Collaboration), Y. C. Chen, Y.-C. Chen, S. Chiche, B. A. Clark, A. Connolly, K. Couberly, L. Cremonesi, A. Cummings, P. Dasgupta, R. Debolt, S. de Kockere, K. D. de Vries, C. Deaconu, M. A. DuVernois, J. Flaherty, E. Friedman, R. Gaior, P. Giri, J. Hanson, N. Harty, K. D. Hoffman, M.-H. Huang, K. Hughes, A. Ishihara, A. Karle, J. L. Kelley, K.-C. Kim, M.-C. Kim, I. Kravchenko, R. Krebs, C. Y. Kuo, K. Kurusu, U. A. Latif, C. H. Liu, T. C. Liu, W. Luszczak, A. Machtay, K. Mase, M. S. Muzio, J. Nam, R. J. Nichol, A. Novikov, A. Nozdrina, E. Oberla, C. W. Pai, Y. Pan, C. Pfendner, N. Punsuebsay, J. Roth, A. Salcedo-Gomez, D. Seckel, M. F. H. Seikh, Y.-S. Shiao, J. Stethem, S. C. Su, S. Toscano, J. Torres, J. Touart, N. van Eijndhoven, A. Vieregg, M. Vilarino Fostier, M.-Z. Wang, S.-H. Wang, P. Windischhofer, S. A. Wissel, C. Xie, S. Yoshida, and R. Young
(The ARA Collaboration)

Source/Credit: University of Chicago | Louise Lerner

Reference Number: phy050626_01

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