The unusual radio pulses were detected by the Antarctic Impulsive Transient Antenna (ANITA) experiment, a range of instruments flown on NASA balloons high above Antarctica that are designed to detect radio waves from cosmic rays hitting the atmosphere. The goal of the experiment is to gain insight into distant cosmic events by analyzing signals that reach the Earth. Rather than reflecting off the ice, the radio signals appeared to be coming from below the horizon, an orientation that cannot be explained by the current understanding of particle physics and may hint at new types of particles or interactions previously unknown to science.
ANITA was placed in Antarctica because there is little chance of interference from other signals; to capture the emission signals, the balloon-borne radio detector is sent to fly over stretches of ice, capturing what are called ice showers. Image credit: Stephanie Wissel / Penn State.
“The radio waves that we detected were at really steep angles, like 30 degrees below the surface of the ice,” said Dr. Stephanie Wissel, a physicist at Penn State.
“By our calculations, the anomalous signal had to pass through and interact with thousands of km of rock before reaching the detector, which should have left the radio signal undetectable because it would have been absorbed into the rock.”
“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.”
Usually emitted by high-energy sources like the Sun or major cosmic events like supernovas or even the Big Bang, there are neutrino signals everywhere.
The problem with these particles, though, is that they are notoriously difficult to detect.
“You have a billion neutrinos passing through your thumbnail at any moment, but neutrinos don’t really interact,” Dr. Wissel said.
“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.”
“Once detected and traced to their source, these particles can reveal more about cosmic events than even the most high-powered telescopes, as the particles can travel undisturbed and almost as fast as the speed of light, giving clues about cosmic events that happened light-years away.”
“Teams of researchers around the world have been working to design and build special detectors to capture sensitive neutrino signals, even in relatively small amounts.”
“Even one small signal from a neutrino holds a treasure trove of information, so all data has significance.”
“We use radio detectors to try to build really, really large neutrino telescopes so that we can go after a pretty low expected event rate.”
ANITA is one of these detectors, and it was placed in Antarctica because there is little chance of interference from other signals.
To capture the emission signals, the balloon-borne radio detector is sent to fly over stretches of ice, capturing what are called ice showers.
“We have these radio antennas on a balloon that flies 40 km above the ice in Antarctica,” Dr. Wissel said.
“We point our antennas down at the ice and look for neutrinos that interact in the ice, producing radio emissions that we can then sense on our detectors.”
These special ice-interacting neutrinos, called tau neutrinos, produce a secondary particle called a tau lepton that is released out of the ice and decays, the physics term referring to how the particle loses energy as it travels over space and breaks down into its constituents. This produces emissions known as air showers.
“If they were visible to the naked eye, air showers might look like a sparkler waved in one direction, with sparks trailing it,” Dr. Wissel said.
“We can distinguish between the two signals — ice and air showers — to determine attributes about the particle that created the signal.”
“These signals can then be traced back to their origin, similar to how a ball thrown at an angle will predictably bounce back at the same angle.”
The recent anomalous findings, though, cannot be traced back in such a manner as the angle is much sharper than existing models predict.
By analyzing data collected from multiple ANITA flights and comparing it with mathematical models and extensive simulations of both regular cosmic rays and upward-going air showers, the researchers were able to filter out background noise and eliminate the possibility of other known particle-based signals.
The scientists then cross-referenced signals from other independent detectors like the IceCube Experiment and the Pierre Auger Observatory to see if data from upward-going air showers, similar to those found by ANITA, were captured by other experiments.
The analysis revealed the other detectors did not register anything that could have explained what ANITA detected, which led the authors to describe the signal as anomalous, meaning that the particles causing the signal are not neutrinos.
The signals do not fit within the standard picture of particle physics, and while several theories suggest that it may be a hint of dark matter, the lack of follow-up observations with IceCube and Auger really narrow the possibilities.
“Our team is currently designing and building the next big detector,” Dr. Wissel said.
“The new detector, called PUEO, will be larger and better at detecting neutrino signals, and it will hopefully shed light on what exactly the anomalous signal is.”
“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.”
“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.”
The team’s paper was published in the journal Physical Review Letters.
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A. Abdul Halim et al. (Pierre Auger Collaboration). 2025. Search for the Anomalous Events Detected by ANITA Using the Pierre Auger Observatory. Phys. Rev. Lett 134, 121003; doi: 10.1103/PhysRevLett.134.121003
This article is based on a press-release provided by Penn State.
