Radio Blips in the Ice Are Promising Sign for Neutrino Hunt
Detection of high-energy neutrinos, elusive particles produced in supernovae and other astrophysical events, is opening up a new window on the Universe. One way to spot them is to search for signals of neutrino collisions with molecules in large sheets of polar ice. An international collaboration working in Antarctica has now reported the detection of ice-traversing radio waves that originate from cosmic-ray-induced particle showers [1]. Even though the radio waves were generated by cosmic rays rather than neutrinos, the result establishes a proof of principle that the technique should work for neutrinos too.
Ice-sheet-based detection of cosmic neutrinos has been reported previously from the IceCube Neutrino Observatory at the South Pole [2]. In that experiment, neutrino collisions with water molecules produce flashes of visible light, called Cherenkov radiation, generated by fast-moving collision by-products. This method becomes challenging for neutrinos of extremely high energy (around 1018 electron volts, or 1 exa-electron-volt) because these neutrinos are expected to be exceedingly rare. Researchers would need detectors spread over hundreds of cubic kilometers of ice to have a chance of seeing Cherenkov radiation from such a rare exa-electron-volt event.
An alternative ice-based detection method that could reach the exa-electron-volt range seeks to exploit an effect first predicted in 1962 by particle physicist Gurgen Askaryan. The effect occurs when a high-energy particle strikes an atom, creating a cascade of secondary particles that draws in electrons from the surrounding material. As a result, the front of the shower develops a negative charge that in turn produces a pulse of radio waves. Such “Askaryan” radiation has been detected in air, initiated by cosmic-ray particles [3]. The effect is predicted to occur in ice as well [4] but hasn’t been seen previously.
The Askaryan Radio Array (ARA) is an experiment near the South Pole that aims to detect high-energy cosmic neutrinos [5]. A prototype was set up in 2011, and the detector system now comprises five stations, spaced about 2 km apart. Each station consists of several radio antennae sunk to depths of 150–200 m in channels drilled into the ice. During a 208-day observation campaign in 2019 using only one station, the ARA team recorded 13 detection events from unknown sources [6]. The team has now reanalyzed those signals and concluded that they come from Askaryan radiation produced by high-energy cosmic rays (typically particles such as protons and atomic nuclei).
“It was already hypothesized that some of those events could be due to cosmic rays impacting the ice sheet,” explains team member Philipp Windischhofer of the University of Chicago. But that hypothesis went unverified until now, as the team required advanced computer simulation techniques that only became available recently.
The key challenge was eliminating the many sources of background noise, including mobile radio communication signals from the nearby Amundsen-Scott South Pole Station, spark-plug discharges from vehicles in the vicinity, airplane radar, and friction-induced electrical discharges from snow blown over the ice. Unlike these background sources, cosmic-ray-induced particle showers are expected to originate from within the ice, to span a wide frequency range, and to be emitted in a cone-shaped pattern. By carefully analyzing these and other characteristics, the researchers say that the chance that the 13 events would be created in such a time window from background sources rather than cosmic rays is less than 1 in 3.5 million (corresponding to a statistical significance of 5.1 sigma).
Olaf Scholten of the University of Groningen in the Netherlands, who works on cosmic-ray detection, calls the work a breakthrough. “It is a true achievement that the [ARA researchers] have shown they can operate these in-ice antennae at a level where they can separate—beyond doubt—signal events from the background.”
“I think this is a pretty firm result,” agrees astroparticle physicist Anna Nelles of the University of Erlangen-Nuremberg in Germany. “Those signals should absolutely be there, and they look like what we think they should look like.”
The main goal of the instrument, however, is to look for neutrinos, not cosmic rays. “The radio signals expected for neutrinos are very similar to those now observed for cosmic rays,” Windischhofer says. The team can, however, discriminate neutrinos from cosmic rays by looking at the arrival angle of the radio pulses at the antennae: Cosmic-ray showers would be induced at the ice surface, because these particles can’t penetrate very far, whereas neutrino showers should mostly be generated from deep within the ice. A new data release—which includes observations made by all five stations over many years—should be made public soon, and the researchers anticipate finding between zero and about seven candidate neutrino events in that data set, Windischhofer says.
–Philip Ball
Philip Ball is a freelance science writer in London. His next book, The Man Who Broke Reality, a biography of Niels Bohr, will be published in December.
References
- N. Alden et al. (ARA Collaboration), “Observation of in-ice Askaryan radiation from high-energy cosmic rays,” Phys. Rev. Lett. 136, 151003 (2026).
- M. G. Aartsen et al., “The IceCube Neutrino Observatory: Instrumentation and online systems,” J. Instrum. 12, P03012 (2017).
- A. Aab et al. (Pierre Auger Collaboration), “Probing the radio emission from air showers with polarization measurements,” Phys. Rev. D 89, 052002 (2014).
- S. De Kockere et al., “Simulation of in-ice cosmic ray air shower induced particle cascades,” Phys. Rev. D 106, 043023 (2022).
- P. Allison et al., “Design and initial performance of the Askaryan Radio Array prototype EeV neutrino detector at the South Pole,” Astropart. Phys. 35, 457 (2012).
- P. Allison et al. (ARA Collaboration), “Low-threshold ultrahigh-energy neutrino search with the Askaryan Radio Array,” Phys. Rev. D 105, 122006 (2022).