An international team of scientists, including researchers from Penn State, discovered unusual radio impulses originating from beneath the ice of Antarctica that cannot be explained. During the Antarctic Impulsive Transient Antenna (ANITA) experiment, the researchers analyzed signals traveling to Earth using a variety of instruments, including balloon-borne devices designed to detect cosmic radio signals.
The ANITA experiment is designed to register radio waves arising from the interaction of cosmic rays with the Earth's atmosphere. The goal of using balloons was to gain new understandings of cosmic events throughout the universe, with the instruments sent up 40 kilometers (29 miles) above the ice to catch emission signals.
Instead of the expected signals reflected from the ice, the ANITA instrument recorded radio pulses coming from below the horizon at an angle of about 30 degrees below the surface. Stephanie Wissel, associate professor of physics, astronomy, and astrophysics at Penn State and one of the researchers, revealed that the anomalies detected are "very likely not neutrinos," which were the target of the search. “It's an interesting problem because we still have no explanation for these anomalies, but we know they most likely do not represent neutrinos,” said Wissel.
According to Wissel, the anomalous signal would have had to traverse thousands of kilometers of rock and ice beneath Antarctica and interact with it before reaching the detector, which should have made the radio waves undetectable as they would have been absorbed.
Initially, researchers thought the signals might be related to neutrinos, but analysis showed that the recorded pulses do not correspond to the properties of neutrinos, and the unusual signal does not fit into the picture of particle physics. Neutrinos are abundant in the universe, often produced by cosmic events like supernovae or the Big Bang, and they are emitted by high-energy sources like the Sun.
“Neutrinos are important to the understanding of the universe because they are emitted by high-energy sources and are typically hard to detect. You could have a billion neutrinos passing through you at any moment, but they don't interact with you,” said Wissel.
The team compared the signals with those from other detectors like the IceCube experiment and the Pierre Auger Observatory and found nothing, indicating that what they found was not neutrinos but something else entirely. “You have a billion neutrinos passing through your thumb at any moment, but neutrinos don't actually interact. So, that's the two-edged sword problem. If we detect them, it means they have traveled all this way without interacting with anything else. We could detect a neutrino coming from the edge of the observable universe,” said Wissel.
Several theories suggest that the signal could be an indication of dark matter, but the fact that other observatories have not seen it limits this possibility, according to Wissel. Some scientists speculate that the signals may represent a new and previously unidentified type of particle or interaction. The ANITA team's observation in Antarctica contradicts current models of particle physics, and this unexplained discovery has puzzled physicists, who are working to understand the cause of the anomaly.
To study the phenomenon more deeply, the team is developing a new detector called PUEO, which will have greater sensitivity. “I'm looking forward to us having better sensitivity with PUEO. In principle, we should detect more anomalies, and maybe then we'll understand what they are,” said Wissel.
The research findings were published in the journal Physical Review Letters.
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