The unusual radio pulses were detected by the Antarctic Impulsive Transient Antenna (ANITA) experiment, a range of instruments flown on balloons high above Antarctica that are designed to detect radio waves from cosmic rays hitting the atmosphere. (Credit: Stephanie Wissel / Penn State)

In a nutshell ANITA’s mysterious radio signals from Antarctica remain unexplained, but they’re likely not caused by exotic new particles

The world’s largest cosmic ray detector found zero evidence for the exotic particles that some theories predicted

Scientists expect future missions like PUEO will help solve this lingering cosmic puzzle

ANTARCTICA — Strange radio signals detected by a balloon-borne antenna over Antarctica sparked wild theories about physics-defying particles. Now, the world’s largest cosmic ray detector has delivered a reality check that could deflate those exotic explanations.

Between 2016 and 2018, NASA’s Antarctic Impulsive Transient Antenna (ANITA) detected two mysterious radio pulses that seemed to defy the laws of physics. These signals appeared to come from deep beneath the Earth’s surface, something that should be impossible under our current understanding of how particles behave. The discovery sent ripples through the scientific community, with some researchers proposing radical new physics involving undiscovered particles that could somehow travel through thousands of miles of solid rock.

But a massive new study using the Pierre Auger Observatory in Argentina has thrown cold water on the most exotic explanations. After scanning the skies for 15 years and analyzing over 7.6 million cosmic events, researchers found exactly what standard physics would predict: virtually nothing. The study’s results are published in Physical Review Letters.

The Physics Mystery That Broke All the Rules

ANITA’s anomalous events were like finding footprints that led directly through a brick wall. The radio pulses showed all the characteristics of cosmic ray showers: massive cascades of particles that rain down when high-energy cosmic rays smash into Earth’s atmosphere. But these particular signals seemed to be traveling upward through the planet.

“The radio waves that we detected were at really steep angles, like 30 degrees below the surface of the ice,” says Stephanie Wissel, associate professor of physics, astronomy and astrophysics at Penn State who worked on the ANITA team, in a statement. By their calculations, she explained, 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.

Standard physics says that high-energy particles get absorbed when traveling through dense material like rock. For the ANITA signals to make sense as upward-traveling showers, particles would need to tunnel through roughly 6,000 to 7,000 miles of solid Earth. The paper describes this as “about 8–10 interaction lengths at the required neutrino energy,” which essentially means the particles would face nearly impossible odds of survival.

Some scientists initially proposed that exotic particles called tau neutrinos might explain the mystery. These ghostly particles barely interact with matter and could theoretically travel through Earth, then decay near the surface to create upward-moving showers. But as Wissel noted, “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.”

Argentina’s Reality Check Detector

Enter the Pierre Auger Observatory, a cosmic ray detector sprawling across 3,000 square kilometers of Argentine countryside, an area larger than Rhode Island. Unlike ANITA’s balloon-borne antenna, Auger uses a network of ground-based telescopes that can spot the faint fluorescent glow produced when cosmic ray showers pass through the atmosphere.

Researchers specifically designed their search to look for the type of upward-moving air showers that could explain ANITA’s anomalous signals. Using sophisticated computer simulations, they calculated exactly how many such events they should see if the exotic explanations were correct.

Stephanie 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. (Credit: Stephanie Wissel / Penn State)

The team simulated millions of cosmic ray events and upward-going showers, developing a complex selection process to separate genuine upward-moving events from background noise. They even had to account for laser pulses routinely fired across the array for calibration, which naturally travel upward and could mimic the signals they were hunting.

After applying their rigorous selection criteria to 15 years of data, the researchers found just one candidate event that might represent an upward-going shower. But this lone signal was entirely consistent with what they expected from ordinary cosmic ray backgrounds — essentially statistical noise.

When the Numbers Don’t Add Up

Based on their observations, the team calculated how many upward-going showers should exist if the ANITA anomalies were real. Their results show that if exotic particles were causing these events, the Auger Observatory should have detected between 8 and 59 similar signals, depending on the energy distribution of the particles.

Instead, they found effectively zero.

The researchers cross-referenced their findings with other independent detectors like the IceCube Experiment to see if similar upward-going air showers had been captured elsewhere. As Wissel explained, “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.’”

The stark difference between ANITA’s mysterious detections and Auger’s null results comes down to their complementary strengths. While ANITA III flew for just seven days, Auger has been operating continuously for over two decades. The Argentine detector’s exposure to upward-going showers exceeds ANITA’s by factors ranging from 2 to 2,000, depending on the energy range.

Back to the Drawing Board

The Auger results don’t necessarily prove that ANITA’s anomalous events were measurement errors or equipment malfunctions. But they do severely constrain explanations involving exotic physics that would produce a steady stream of upward-traveling particles.

Wissel offers her own take on what might be happening: “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.”

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. (Credit: Stephanie Wissel / Penn State)

The signals do not fit within the standard picture of particle physics, and while several theories suggest dark matter involvement, the lack of follow-up observations with IceCube and Auger really narrow the possibilities. Scientists are now designing new detectors like PUEO, which will be larger and better at detecting neutrino signals, hoping to solve the mystery.

“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,” Wissel said. “In principle, we should pick up more anomalies, and maybe we’ll actually understand what they are.”

The ANITA mystery illustrates a fundamental principle of science: extraordinary claims require extraordinary evidence. Auger’s massive scale and long observation period make it an ideal complement to ANITA’s specialized capabilities. The fact that the world’s largest cosmic ray detector found essentially nothing where exotic theories predicted dozens of events means those physics-breaking particles traveling through Earth’s core likely don’t exist.