Evrim Yazgin
Cosmos science journalist
A cosmic particle detector in Antarctica has detected radio signals that defy current particle physics theories.
The radio pulses were found by scientists operating the Antarctic Impulsive Transient Antenna (ANITA) experiment which was launched between 2006 and 2016. ANITA includes a range of instruments which are flown on balloons about 37km above the frozen continent.
ANITA was designed to detect radio waves from cosmic rays. The experiment observes the Antarctic ice sheet out to a horizon about 700km away – more than 1 million cubic kilometres of ice. It, like other cosmic particle detectors, was set up in Antarctica to reduce the chance of interference from other signals.
Rather than direct detection of high-energy particles like neutrinos, ANTIA detects the radio waves that result from high-energy particles hitting the ice. The process is known as Askaryan emission.
But new results published in Physical Review Letters don’t make sense according to current theories.
The radio signals appeared to be coming from below the horizon rather than being reflected off the ice. These radio pulses could hint at previously unknown types of particles or interactions.
“The radio waves that we detected were at really steep angles, like 30 degrees below the surface of the ice,” says co-author Stephanie Wissel, an associate professor at Pennsylvania State University in the US.
Calculations show that the signal would have to have passed through and interact with thousands of kilometres of rock before reaching the detector which should have rendered the signal undetectable.
“It’s an interesting problem because we still don’t actually have an explanation for what those anomalies are, but what we do know is that they’re most likely not representing neutrinos,” Wissel says.
“You have a billion neutrinos passing through your thumbnail at any moment, but neutrinos don’t really interact,” she adds. “So, this is the double-edged sword problem. If we detect them, it means they have traveled all this way without interacting with anything else. We could be detecting a neutrino coming from the edge of the observable universe.”
Neutrinos which can interact with ice are called tau neutrinos. When they do interact with ice, they produce a particle called a tau lepton which loses energy as it travels over space and breaks down. This produces radio emissions known as air showers.
But the new signal did not act in this or any other way that matches the mathematical models of how particles interact with the Antarctic ice below ANITA. Some have even suggested the signals could hint at dark matter, but this is unlikely as there were no correlating signals at IceCube and Auger detectors also stationed in Antarctica.
“My guess is that some interesting radio propagation effect occurs near ice and also near the horizon that I don’t fully understand, but we certainly explored several of those, and we haven’t been able to find any of those yet either,” Wissel said.
Wissel is hopeful that a new airborne detector called PUEO (Payload for Ultrahigh Energy Observations) will be sensitive and powerful enough to help physicists determine the origin of the strange signal. PUEO was originally set for launch in December 2024, but is now due to launch this year or in 2026.
“In principle, we should pick up more anomalies, and maybe we’ll actually understand what they are. We also might detect neutrinos, which would in some ways be a lot more exciting.”
