Nearly two decades ago, an experiment floating high above Antarctica caught a weird signal.
Designed to capture the radio spurts of cosmic rays falling from above, in 2006 the Antarctic Impulsive Transient Antenna (ANITA) recorded a short pulse of radio waves from below – an event that looked like an upside-down shower of cosmic rays, not bouncing off the surface, but emanating from under the ice sheet.
The balloon-borne suite of instruments recorded a similar event in 2014, and scientists have been scratching their heads ever since. No explanation quite fits, suggesting that the culprit could be a particle unknown to science.
“The radio waves that we detected were at really steep angles, like 30 degrees below the surface of the ice,” explains astrophysicist Stephanie Wissel of Pennsylvania State University.
“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.”
The signal itself, a very brief pulse of radio waves, is very similar to what we might see from an elusive tau neutrino, but there are several reasons neutrino interpretation of the signal is difficult to resolve.
Scientists thought that such a neutrino may come from a supernova that then tunnels its way right through Earth and comes out the other side. However, only the 2014 detection coincided with a supernova that could be responsible – no such event was found for the 2006 detection.
In addition, the steep angle of the detection means that the neutrino would have had to tunnel through rock before emerging from the ice. Neutrinos stream through matter all the time; it’s what they do, and why we call them ‘ghost particles’. That’s not the problem.
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“You have a billion neutrinos passing through your thumbnail at any moment, but neutrinos don’t really interact,” Wissel says. “So, this is the double-edged sword problem. If we detect them, it means they have traveled all this way without interacting with anything else. We could be detecting a neutrino coming from the edge of the observable Universe.”
To help solve the mystery of the upward-propagating radio pulses, a large international team of researchers made a careful study of data collected by the Pierre Auger Observatory in Argentina, designed to study high-energy cosmic rays. They conducted simulations to determine how the ANITA events might appear in the Pierre Auger Observatory data, and combed through the observations from 2004 to 2018, looking for similar signals.
They found nothing that explains the ANITA detections. However, in science, finding nothing often means finding something – even if that something is a narrower set of options.
In this case, it means that the scientists could confidently rule out neutrinos as an explanation for the signals. However, that doesn’t automatically mean we’re looking at a new particle, either. We’re going to need further observations, and hopefully new detections, to determine what it is that ANITA saw coming from the Antarctic ice.
ANITA is retired; its last flight took place in 2016. A new Antarctic balloon experiment called the Payload for Ultrahigh Energy Observations (PUEO), the successor to ANITA, is due to commence operations soon.
“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 says.
“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.”
The latest results were published in Physical Review Letters.
