IGFAE coordinates the analysis of mysterious events coming from Antarctic ice

Between 2016 and 2018, the ANITA experiment (Antarctic Impulsive Transient Antenna), a network of antennas mounted on a balloon flying over the Antarctic stratosphere, detected two mysterious radio emission signals that sparked great interest in the scientific community. They appeared to come from the Earth underground, as if the particle that produced them had been able to travel thousands of kilometers through the ground without interacting with any other particles along the way.

The characteristics of these electromagnetic signals resembled those produced by particle cascades or showers generated in collisions between highly energetic particles and the atmosphere. However, in this case, the particles came from the opposite direction than usual: instead of arriving from the sky, they came from below the horizon, moving upwards. This led to the proposal that they could be neutrinos (particles with almost no mass that rarely interact with matter), which had traversed the planet and produced a particle cascade upon emerging.

However, this explanation soon encountered problems. At the extremely high energies that the neutrinos would need to have in order to explain the intensity of the signals observed by ANITA, they should be absorbed by the Earth and not be able to simply pass through it. Moreover, if this type of event were common, they should also be detected by other observatories, such as the Pierre Auger Observatory (located in Argentina) or IceCube (located in Antarctica itself).

In this context, the Pierre Auger Collaboration, in which the IGFAE participates, has compiled and analyzed data recorded between 2004 and 2018 by its network of detectors, which covers 3,000 km² in the Argentine province of Mendoza, as well as its 27 fluorescence light telescopes. The results have recently been published in the journal Physical Review Letters, in an analysis carried out by the international Pierre Auger Collaboration and co-led by Enrique Zas, a researcher at IGFAE and professor at the USC. And although some hypotheses about these signals are ruled out, the mystery of their origin has not been completely resolved.

The article rules out that the two anomalous events detected by ANITA can be explained by known particle showers or by interactions predicted by the Standard Model of particle physics. Only one event was found to be compatible with these characteristics, consistent with what would be expected due to reconstruction imperfections, but far fewer than the dozens of events that should have been detected if the phenomena observed by ANITA were common.

“We have shown that these two anomalous events cannot be due to upward-going air showers developing in the atmosphere. This rules out most of the explanations based on new physics beyond the Standard Model that have been suggested. But the mystery of these two events remains unresolved,” summarizes Enrique Zas.

With the most plausible explanations ruled out, the scientific community is now looking toward less-explored phenomena. “One possibility is that the signals recorded by ANITA do not come from particles that traversed the Earth, but from some kind of anomalous reflection of radio waves in the Antarctic ice that is not yet understood,” add Enrique Zas and Jaime Álvarez-Muñiz, also a researcher at IGFAE, who conclude: “This type of interpretation would require a more detailed study of the structure of Antarctic ice and of how signals might be amplified or distorted under certain conditions”.

The role of the Pierre Auger Observatory and the IGFAE

In this analysis, the fluorescence telescopes of the Pierre Auger Observatory played a particularly relevant role. These instruments detect the fluorescent light emitted by atmospheric nitrogen when a particle shower crosses the air. Thanks to this technique, it was possible to rule out that the events detected by ANITA originated in upward-going particle showers.

The Pierre Auger Observatory, currently undergoing an upgrade, will continue to be key in exploring these phenomena involving the study of astroparticles. The Instituto Galego de Física de Altas Enerxías (IGFAE), a joint center of the USC and the Xunta de Galicia, is one of the founding members of this collaboration, which began in the early 21st century. Currently, IGFAE’s staff continue to hold high-responsibility positions within the scientific collaboration, which involves nearly 500 people from almost a hundred institutions around the world.

In addition to Enrique Zas and Jaime Álvarez-Muñiz, the article is co-authored by IGFAE researchers Juan Ammerman Yebra, Sergio Cabana Freire, Lorenzo Cazón Boado, Marvin Gottowik, Yago Lema Capeáns, Miguel Martins, Gonzalo Parente, and Felix Riehn.