Strange radio pulses detected coming from ice in Antarctica

UNIVERSITY PARK, Pa. — Several years ago, a cosmic particle detector in Antarctica observed a series of unusual radio signals, according to an international research group that includes scientists from Penn State. The strange radio pulses were detected between 2016 and 2018 by NASA’s Antarctic Impulsive Transient Antenna (ANITA), a range of instruments flown on balloons high above Antarctica that are designed to detect radio waves from cosmic rays hitting the atmosphere, and a new study provides additional context to the nearly decade-old results. 

The goal of the ANITA experiment was to gain insight into distant cosmic events by analyzing signals that reach the Earth. Rather than reflecting off the ice, the signals — a form of radio waves — appeared to be coming from below the horizon, an orientation that could not be explained by the current understanding of particle physics and may have hinted at new types of particles or interactions previously unknown to science, the team said at the time.

A new study using the Pierre Auger Observatory in Argentina analyzed 15 years of cosmic data to try to make sense of those signals. The team of international scientists, including Penn State researchers, recently published their results in the journal Physical Review Letters.

“The radio waves that we detected nearly a decade ago were at really steep angles, like 30 degrees below the surface of the ice,” said Stephanie Wissel, associate professor of physics, astronomy and astrophysics who worked on the ANITA team searching for signals from elusive particles called neutrinos. "While the origin of these events is still unclear, our new study indicates that such events have not been seen by an experiment with a long exposure like the Pierre Auger Observatory. So, it does not indicate that there is new physics, but rather more information to add to the story."

She explained that by their calculations, the anomalous signal had to pass through and interact with thousands of kilometers 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,” Wissel said.

Neutrinos, a type of particle with no charge and the smallest mass of all subatomic particles, are abundant in the universe. 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, Wissel explained.

“You have a billion neutrinos passing through your thumbnail at any moment, but neutrinos don't really interact,” she 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, Wissel added, as the particles can travel undisturbed and almost as fast as the speed of light, giving clues about cosmic events that happened lightyears away. 

Wissel and 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, she said.

“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,” said Wissel, who has designed experiments to spot neutrinos in Antarctica and South America.

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 kilometers above the ice in Antarctica,” 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, Wissel explained. The researchers 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, Wissel said. The 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 researchers 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.

Analysis revealed the other detectors did not register anything that could have explained what ANITA detected, which led the researchers to describe the signal as “anomalous,” meaning that the particles causing the signal are not neutrinos, Wissel explained. Back when they were first detected, theories swirled that signals did not fit within the standard picture of particle physics, and others suggested that it may be a hint of dark matter, but the recent lack of observations with IceCube and Auger really narrow the possibilities, Wissel said.

The teams have been working on balloon projects for over a decade, Wissel explained, and added that her team is currently working on designing and building the next big detector. The new detector, called PUEO, will be larger and better at detecting neutrino signals, Wissel said, 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,” 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 be able to better understand these anomalies which will go a long way to understanding our backgrounds and ultimately detecting neutrinos in the future.”

Peter Gorham at the University of Hawaii led the original ANITA flights and the original 2016 and 2018 studies. Abigail Vieregg is leading the PUEO project, which includes collaborators from the University of Chicago, the Ohio State University, the University of Hawaii, the University of Delaware, the University of Kansas, and Washington University in St. Louis in addition to Penn State. The other Penn State co-author on the new study is Andrew Zeolla, a doctoral candidate in physics. The research conducted by scientists from Penn State was funded by the U.S. Department of Energy and the U.S. National Science Foundation. The paper contains the full list of collaborators and authors.

At Penn State, researchers are solving real problems that impact the health, safety and quality of life of people across the commonwealth, the nation and around the world.

For decades, federal support for research has fueled innovation that makes our country safer, our industries more competitive and our economy stronger. Recent federal funding cuts threaten this progress.

Learn more about the implications of federal funding cuts to our future at Research or Regress.