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Scientists are trying to solve a decade-long mystery by determining the identity of abnormal signals detected under Antarctic ice.
Strange radio waves emerged in search of another anomalous phenomenon, high-energy cosmic particles known as neutrinos. When you arrive on Earth from the far reach of the Cosmos, neutrinos are often referred to as “ghosts.” Because they are very unstable, steamy and can experience all kinds of problems without changing.
Over the past decade, researchers have conducted multiple experiments using vast water and ice, designed to search for neutrinos that can shed light on the mysterious cosmic rays, the most energetic particles in the universe. One of these projects was the experiment of NASA’s Antarctic impulsive temporary antenna, or Anita, that flew a balloon carrying musical instruments over the Antarctic between 2006 and 2016.
It was during this hunt that Anita picked up some extraordinary radio waves that were thought to be non-neutrinos.
The signal comes from under the horizon and suggests that it has passed thousands of miles of rock before reaching the detector. However, the radio waves should have been absorbed by the rocks. The Anita team believed that these anomalous signals could not be explained by current understanding of particle physics.
Follow-up observations and analysis on other instruments, including those recently conducted by the Pierre Auger Observatory in Argentina, did not find the same signal. The results of the Pierre Auger collaboration were published in the Journal Physical Review Letters in March.
The origin of the unusual signal remains unknown, according to Stephanie Wissel of Study Coauthor, an associate professor of physics, astronomy and astrophysics at Pennsylvania State University.
“Our new research shows that such (signals) are not seen in the experiment… like the Pierre Auger Observatory,” Wissel said. “So it’s not that there’s new physics, it’s indicating that there’s more information to add to the story.”
Larger, more sensitive detectors may be able to solve the mystery and prove whether the abnormal signal is fluorescent in the end.
By detecting neutrinos on Earth, researchers can track them to their sources. Scientists believe it is primarily cosmic rays that strike the atmosphere of our planet.
Cosmic rays, the most highly energy-type particles in the universe, are composed primarily of protons or nuclei, and whatever produces them warns the power of a large hadron colider. Neutrinos help astronomers to better understand cosmic rays and what fire them in space.
However, neutrinos are difficult to find because they have little mass and cannot pass through the most extreme environments, such as stars and the entire galaxy. But they interact with water and ice.
Anita said Justin Vandenbrooke, an associate professor of physics at the University of Wisconsin in Madison, was designed to search for the best energy neutrinos in the universe with higher energy than is still detected. The experimental radio antenna searches for short pulses of radio waves that are generated when neutrinos collide with atoms in Antarctica ice, leading to a shower of low-energy particles, he said.
During that flight, Anita discovered a high-energy fountain of particles coming from the ice. Detectors are also sensitive to ultra-high energy cosmic rays that create radio bursts that rain on Earth and act like a flashlight beam of radio waves.
When Anita sees cosmic rays, the flashlight beam is a burst of one-millionths of a radio wave that can be mapped like a wave to show how it really reflects outside of the ice.
Twice with Anita Flights data, the original team from the experiment discovered signals appearing in the ice at a much sharper angle than any model would predict, making it impossible to track the signal to its original source.
“The radio waves we detected nearly 10 years ago were at a very steep angle, like 30 degrees below the surface of the ice,” Wissel said.
Neutrinos can overcome many problems, but they don’t go through the Earth, Vandenbrooke said.
“They are expected to arrive from just below the horizon, where there is not much earth for them to be absorbed,” he wrote in an email. “It’s interesting that Anita’s extraordinary events appear to come from far below the horizon, so neutrinos have to travel through most of the Earth, which is not possible according to standard models in particle physics.”
The Pierre Auger Collaboration, which includes hundreds of scientists from around the world, has analyzed over a decade of data in an attempt to understand the anomalous signals detected by Anita.
The team also tried to use the observation deck to find the same signal. The Auger Observatory is a hybrid detector that uses two methods to find and study cosmic rays. One method relies on finding high-energy particles when interacting with water in tanks on the surface of the Earth, while the other tracks high interactions with ultraviolet rays in the planet’s atmosphere.
“The Auger Observatory uses very different techniques to use the secondary brilliance of charged particles to determine the direction of the cosmic rays that started it across the atmosphere. “By using computer simulations of what a shower of such particles would look if they acted like an anita’s extraordinary event, they can generate a kind of template for similar events and see if something like that would appear.”
Gorham, who was not involved in the new study, designed the Anita experiment and conducted other studies to further understand the anomalous signals.
The Auger Observatory was designed to measure downward particle showers produced in the atmosphere by ultra-high energy cosmic rays, but the team redesigned the data analysis to search for upward air showers. Vandenbroucke was not working on any new research, but he peer reviewed it before publication.
“Auger has a huge collection area for such events, which is bigger than Anita,” he said. “If Anita’s extraordinary events were produced by particles that move around the Earth and then produce an upward shower, the auger had to detect many of them, but that wasn’t the case.”
Another follow-up using IceCube experiments with sensors embedded deep in Antarctica ice also searched for abnormal signals.
“IceCube is so sensitive that if the anita’s unusual events were neutrinos, they would have detected them,” writes Vandenbroucke, who served as a collegiate for the IceCube Neutrino Sources Working Group between 2019 and 2022.
“It’s an interesting question because we don’t really explain what those anomalies are, but what we know is that they are more likely to not represent neutrinos,” Wissel said.
Oddly, another type of neutrino, known as tau neutrinos, is one of the hypotheses that some scientists have presented them as the cause of abnormal signals.
Tau neutrinos can be regenerated. When collapsed at high energy, another tau neutrino and particles called tau leptons are produced. It’s similar to electrons, but much heavier.
But what makes the tau neutrino scenario very unlikely is the steep angle connected to the signal, Wissel said.
“We expect all of these tau neutrinos to be very close to the horizon, like 1-5 degrees below the horizon,” Wissel said. “These are 30 degrees below the horizon. There’s too much material. They actually lose a lot of energy and can’t be detected.”
After all, Gorham and other scientists don’t know what the origins of the unusual Anita events are. So far, no interpretations are consistent with the signal. This is what attempts to bring back the scientists and solve the mystery. However, you may be able to see the answer.
Wissel is also working on new detectors, which are ultra-high energy observations or payloads of Pueo, flying across Antarctica for a month starting in December. Pueo, which is bigger and 10 times more sensitive than Anita, was able to reveal more information about the cause of the abnormal signals detected by Anita, Wissel said.
“Now, that’s one of these long-standing mysteries,” Wissel said. “When we fly Pueo, we are excited to get better sensitivity. As a rule, we should be able to better understand these anomalies.
Gorham said Pueo, an acronym referring to Hawaiian owls, should have the sensitivity to capture many unusual signals and help scientists find the answer.
“Sometimes, you need to go back to the drawings and really get a sense of what these things are,” Wissel said. “The most likely scenario is that what you can explain is common physics, but you’re knocking on every door to try and figure out what they are.”
