A balloon-borne experiment over Antarctica, designed to detect cosmic radio waves, has instead picked up bizarre signals that appear to be coming from deep within the ice. These signals challenge our current understanding of particle physics, scientists say. 

The Antarctic Impulsive Transient Antenna (ANITA) experiment consists of radio antennas flown on NASA balloons 19 to 24 miles (30 to 39 kilometers) over the surface of Antarctica. In recent years, the detector has recorded radio pulses that seemed to rise up through the Earth. ANITA detected these signals at “really steep angles, like 30 degrees below the surface of the ice,” co-author Stephanie Wissel, an associate professor of physics at Penn State, said in a university statement. This suggests that the radio pulses traveled up through 6,000 to 7,000 kilometers (3,700 to 4,300 miles) of solid rock to reach the detector—which shouldn’t be possible.

According to current models of particle physics, these radio pulses should have been completely absorbed by the rock, making detection impossible. “It’s an interesting problem because we still don’t actually have an explanation for what those anomalies are,” Wissel said. She and her colleagues published their findings in the journal Physical Review Letters in March.

ANITA’s overarching goal is to gather information about deep space events by analyzing signals that reach Earth. This experiment plays a pivotal role in the search for neutrinos—elusive particles with no charge and the smallest mass of all subatomic particles. 

Neutrinos are abundant throughout the universe—they’re constantly passing through us—and they usually come from high-energy sources like the Sun or supernovae. The problem is that their signals are very difficult to detect, according to Wissel. ANITA aims to overcome this challenge by sniffing out the radio emissions neutrinos emit when they interact with Antarctic ice. 

As the balloon-borne detector flies over stretches of ice, it looks for “ice showers,” cascades of particles triggered by neutrinos hitting surface ice. These particle showers produce radio signals that ANITA can detect. Ice-interacting neutrinos also produce a secondary particle called a tau lepton that gradually breaks down and loses energy. This decay triggers another type of emission known as an “air shower.” Researchers can distinguish between ice and air showers to characterize the particle that created the signal, then trace the signal back to its origin. 

But the unusually sharp angle of the anomalous signals ruled out the possibility that they were coming from ice-interacting neutrinos or the tau leptons they produce. Wissel and her colleagues analyzed data from multiple ANITA flights and compared it to mathematical models and simulations of both cosmic rays and air showers. This allowed them to eliminate the possibility of ANITA detecting other known particle-based signals. 

Next, the researchers compared the ANITA data to findings from other major neutrino detectors such as the IceCube Experiment and the Pierre Auger Observatory to see if they had captured similar anomalies. They still didn’t find an answer. The other detectors did not register anything that could explain ANITA’s anomalies. The only thing Wissel and her colleagues can say for certain is that the particles causing the strange signals are not neutrinos. 

Hopefully, the next big detector will reveal more information about these anomalies. At Penn State, Wissel’s team is designing and building the Payload for Ultrahigh Energy Observation (PUEO) mission. This new detector will be larger and better at detecting neutrino signals, according to Wissel. 

She’s already forming an early hypothesis about the nature of these anomalies. “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.

“So, right now, it’s one of these long-standing mysteries, and I’m excited that when we fly PUEO, we’ll have better sensitivity. 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.”