A groundbreaking balloon-borne experiment conducted over the icy expanse of Antarctica has unexpectedly captured strange signals that seem to originate from deep within the ice. These signals are raising questions that challenge our current understanding of particle physics, according to scientists involved in the research.
The Antarctic Impulsive Transient Antenna (ANITA) experiment employs a series of radio antennas suspended from NASA balloons flying at altitudes between 19 to 24 miles (30 to 39 kilometers) above the Antarctic surface. In recent observations, ANITA has detected radio pulses that appear to be emanating from beneath the Earth's surface, specifically at “really steep angles, like 30 degrees below the surface of the ice,” explained co-author Stephanie Wissel, an associate professor of physics at Penn State.
These findings suggest that the detected radio pulses have traveled through an astonishing 6,000 to 7,000 kilometers (3,700 to 4,300 miles) of solid rock before reaching the detector, a phenomenon that current particle physics models deem implausible. Wissel remarked, “It’s an interesting problem because we still don’t actually have an explanation for what those anomalies are.” The team's findings were published in the journal Physical Review Letters in March.
ANITA’s primary objective is to collect data on cosmic events by analyzing signals that reach Earth. This endeavor is crucial in the search for neutrinos, which are elusive particles with no electric charge and the smallest mass among subatomic particles. Neutrinos are prevalent throughout the universe, continuously moving through matter, and typically emanate from high-energy sources like the Sun or supernovae. However, detecting their signals is notoriously challenging, as Wissel pointed out.
To tackle this issue, ANITA aims to detect the radio emissions produced when neutrinos interact with Antarctic ice. As the balloon-borne detector traverses vast ice sheets, it seeks out “ice showers,” which are cascades of particles triggered by neutrinos impacting the surface ice. This interaction generates radio signals detectable by ANITA.
Additionally, ice-interacting neutrinos create a secondary particle known as a tau lepton. This particle decays and loses energy, producing what is referred to as an “air shower.” Researchers utilize these distinct signals to identify the nature of the originating particle and trace it back to its source. However, the sharply angled anomalous signals detected by ANITA eliminate the likelihood that they are linked to ice-interacting neutrinos or their resultant tau leptons.
Wissel and her team meticulously analyzed data from various ANITA flights, comparing it against mathematical models and simulations of cosmic rays and air showers. This comprehensive analysis enabled them to rule out the possibility of ANITA detecting any other known particle-based signals. They also cross-referenced their findings with other major neutrino detectors, including the IceCube Experiment and the Pierre Auger Observatory, to ascertain if similar anomalies had been recorded elsewhere. Unfortunately, these searches yielded no explanations for ANITA’s unusual signals.
The only definitive conclusion drawn by Wissel and her colleagues is that the particles responsible for these bizarre signals are not neutrinos. Anticipation builds as the next significant detector is set to shed light on these perplexing anomalies.
At Penn State, Wissel’s research team is in the process of designing and constructing the Payload for Ultrahigh Energy Observation (PUEO) mission. This new detector is expected to be larger and more proficient at detecting neutrino signals. Wissel has already begun forming an early hypothesis regarding the nature of the anomalies. “My guess is that some interesting radio propagation effect occurs near ice and also near the horizon that I don’t fully understand,” she stated. “We certainly explored several of those, and we haven’t been able to find any of those yet either.”
Wissel concluded with excitement about the future: “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 be even more thrilling.”
