A little over 10 years ago, a scientific balloon recorded something unexpected in the icy silence of Antarctica: radio signals coming from deep within the Earth, defying the known laws of particle physics. These intriguing signals have since sparked years of research and debate among physicists. Now, new data adds another layer of mystery.
This story began in 2006 with the deployment of the Antarctic Impulsive Transient Antenna, or ANITA for short. It is a scientific balloon designed to capture radio signals associated with cosmic rays – high-energy radiation typically linked to extreme astrophysical phenomena such as supernovae or black holes.
A "reverse cosmic radiation shower"
The researchers behind this experiment obviously expected such signals to come from space. But the opposite happened: starting in 2014, ANITA recorded radio pulses coming from... below, as if they had been produced by a phenomenon located beneath the ice. A "reverse cosmic radiation shower," in short.
Researchers initially believed it was an anomaly... until other similar events occurred between 2014 and 2016. This time, there was no longer any doubt: this signal had clearly not been produced by a space phenomenon. Its exact origin, however, continues to perplex specialists.
"The radio waves we detected were at very steep angles, about 30 degrees below the surface of the ice," explains Stephanie Wissel, an astrophysicist at Pennsylvania State University, who has worked extensively on this subject. "It's an interesting problem because we still don't have a clear explanation for these anomalies," she says.
The neutrino trail
Initially, researchers quickly suggested that the key to the enigma could be hidden in the famous neutrinos, particularly because of the angle at which the signal was detected. Since it came from below the horizon, this suggested that the particles that made it up had passed through solid matter before reaching ANITA. Neutrinos were therefore ideal candidates, and for good reason: they are virtually inert, extremely light, electrically neutral, and almost never interact with surrounding matter – hence their nickname “ghost particles”.
Neutrinos: What if “ghost particles” interacted with light?
Furthermore, the angle and energy of the signals were consistent with what one would expect to observe if a particular type of neutrino – a high-energy tau neutrino – were to pass through Earth. If so, the theory suggests that it could have interacted near the surface, producing another sub-particle (a tau lepton) that would then have decayed in the atmosphere.
But here's the thing: the laws of physics, as defined in the Standard Model, suggest that such a phenomenon would be extremely unlikely, statistically speaking. So there was something fishy going on, and the researchers decided to launch a new study to try to get to the bottom of it.
It was in this context that a large international team of researchers began to painstakingly sift through the data collected by the Pierre Auger Observatory in Argentina. The latter is specifically designed to detect high-energy cosmic rays; if this signal was indeed closely or remotely linked to neutrinos, it should be possible to trace them in this data.
But that was easier said than done. Simply sifting through the observatory's data would not be enough to find traces of these signals recorded by ANITA. Because of their rarity and singular nature, they would be drowned in a torrent of other signals that would confuse the issue. If they were indeed present, they would manifest themselves in a discreet and subtle way.
It's a bit like looking for a whisper in the middle of a football stadium: listening intently isn't enough. To find the trace, you first need to have a clear idea of what this signal might sound like in order to look for it specifically. The researchers therefore carried out numerous simulations to determine what these mysterious signals would look like in the Pierre Auger Observatory data, assuming they were present.
A revealing silence in the data
At the end of the process, they came up empty-handed: no pattern, however small, that could correspond to the events recorded by ANITA was detected in the data.
Intuitively, this might seem like a complete failure. But the reality is actually more nuanced. In science, finding nothing can also be very revealing: it often allows us to narrow down the range of hypotheses and get closer to an explanation. And this study is a good example.
Since the anomalies observed by ANITA do not seem to appear in the data from the Pierre Auger Observatory, this strongly suggests that they are ultimately not linked to neutrinos… and, by extension, that they are associated with another phenomenon that the researchers had not considered.
The whole question is to know what it is. Could we be dealing with a completely new particle that has never been documented before? For now, the study's authors don't have the data needed to determine this, and the mystery remains. Although ANITA was retired in 2016, a new balloon, called Payload for Ultrahigh Energy Observations (PUEO), will soon take its place. With more powerful sensors than its predecessor, it may bring back crucial new data that, ideally, will finally help solve this long-standing mystery.
"For now, it remains a long-standing mystery. But we'll have better sensitivity with PUEO, and that's exciting. In principle, we should detect more anomalies, and perhaps we can finally understand their nature. We could also detect neutrinos, which would be, in a way, much more exciting," Wissel enthuses.
So we'll see you in a few years, once PUEO has had time to collect enough data, for the next episode of this fascinating scientific series.
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