Simplest Explanation For "Anomalous" Signals Coming From Underneath Antarctica Ruled Out

A particle detector flying above Antarctica has recently caused a stir in the particle physics world after it detected highly unusual radio pulses coming from beneath the ice. 

The detection has prompted much speculation about new types of neutrinos, perhaps another particle altogether, and even (pretty out there) claims of evidence for parallel universes. While already a big mystery, a paper published by a team from the Pierre Auger Observatory in Argentina appears to rule out the most plausible explanation within the standard model. 

The Antarctic Impulsive Transient Antenna (ANITA) experiment floats a range of instruments above Antarctica using a stratospheric balloon. The experiment is aimed at detecting cosmic neutrinos, tiny particles that only interact via gravity and the weak force, originating from distant astrophysical sources as they interact with the ice.

“We have these radio antennas on a balloon that flies 40 kilometers above the ice in Antarctica,” Stephanie Wissel, associate professor of physics, astronomy, and astrophysics at Penn State University, explained in a statement on the discovery. “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.”

While detecting neutrinos is odd enough, the team was met with particularly unusual radio signals in 2016 and 2018, which appear to defy our current understanding of particle physics. When they were first reported, the team suggested plausible hypotheses for how they could have been created.

"These events could be produced by the atmospheric decay of an upward-propagating 𝜏 lepton produced by a 𝜈_𝜏 interaction," the team wrote in their 2018 paper, "although their relatively steep arrival angles create tension with the standard model neutrino cross section."

Particularly puzzling was that the two "anomalous" radio pulses came from below the horizon, underneath the ice.

“The radio waves that we detected were at really steep angles, like 30 degrees below the surface of the ice,” Wissel added. According to Wissel, the team's calculations showed that for the radio pulse to be detected, it must have had to pass through 6,000-7000 kilometers (3,700-4300 miles) of rock. But after traveling through the rock, it should have interacted and been absorbed, leaving it undetectable.

“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.

As tau neutrinos hit the ice, they interact and produce radio emissions, known as "ice showers". But they also produce a secondary particle, known as a tau lepton, which goes on to decay as it travels, with the resulting emission known as "air showers". 

By analyzing the signals, researchers can determine properties of the particle that created it and attempt to trace it back to its source. But in this case, there is a problem in that the angle of the radio pulse is much sharper than models predict.

Looking at data from the IceCube Experiment and the Pierre Auger Observatory, the team found that they had not seen similar upward-traveling air showers, nor anything that could explain what ANITA had detected. 

"These events show strong horizontal polarization, but without the polarity inversion expected for reflected pulses from UHECR showers," the team explained in their recent paper. 

"They could be induced by air showers developing in the upward direction, as could be expected from tau lepton decays produced in ultrahigh-energy tau-neutrino interactions below the surface. However, the direction of the observed pulses implies that the neutrinos would need to travel about 6000–7000 km through the Earth before interacting below the ice surface. This corresponds to about 8–10 interaction lengths at the required neutrino energy 𝐸𝜈 ≳0.2  EeV, causing severe attenuation and requiring a 𝜈𝜏 flux that should have been observed with IceCube and the Pierre Auger Observatory, the latter being particularly sensitive to Earth-skimming tau neutrinos."

The team suspects that they are not caused by neutrinos, but other particles have also been ruled out. 

"The results of this search do not support the interpretation that the anomalous pulses detected during the ANITA I and III flights were caused by air showers sourced from particle interactions or decays," the team added, "such as by decays of upward-going taus, the latter being the basis of proposed explanations based on physics beyond the [standard model]."

To give you a flavor of the ideas being posited, one pre-print paper from 2018 suggested that the signal was evidence for the existence of a parallel universe running backwards in time. 

According to that paper, the signal could be produced by a candidate dark matter particle within the Earth, specifically "a 480 PeV right-handed neutrino that decays into a Higgs boson and a light Majorana neutrino. The latter interacts in the Earth’s crust to produce a τ lepton that in turn initiates an atmospheric upgoing shower".

According to the team, this would be evidence for a CPT symmetric universe, which requires that there be a parallel universe running backwards in time created at the same time as the Big Bang. However, after several publications ran stories on the suggestion, NASA and the University of Hawaiʻi at Mānoa put out statements distancing themselves quite widely from the idea.

“NASA and Gorham’s team do not believe the gathered data is proof of a parallel universe,” a statement from the agency said, per CNN. “NASA relies on the scientific community peer-review process through research journals and publications. Tabloids have misleadingly connected NASA and Gorham’s experimental work, which identified some anomalies in the data, to a theory proposed by outside physicists not connected to the work. Gorham believes there are more plausible, easier explanations to the anomalies.”

For now, we don't really know what the signal is, and it is far too early to leap to conclusions about a CPT symmetric universe. It is possible that the detections could lead to new physics, though as yet we can't really say what that would be either.

“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 added. “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 pick up more anomalies, and maybe we'll actually understand what they are. We also might detect neutrinos, which would in some ways be a lot more exciting.”