Particles observed by Antarctic Impulsive Transient Antenna (ANITA) over Antarctica may not be explained by our current knowledge of physics, according to an in-depth analysis. Now the challenge begins to find out what these particles are.
Researchers from NASA picked-up a rather strange signal twice, first in 2006 and later in 2014. The signal came from ANITA , a large balloon equipped with a particle detector. They found evidence of high-energy particles arriving at an angle that suggested they had just flown undisturbed through the planet.
A recent analysis has now ruled out the best possible explanation for these particles. This means that the particles may be a sign of outside the standard model physics, our current description of the particles and forces in the universe.
Beyond standard model physics
The two particles found by ANITA have a ridiculously high energy for their small size, namely 0.6 and 0.56 exaelectron volt. One of the theories were that the particles were neutrinos. It is known that neutrinos can pass through matter. But with so much energy, these neutrinos should have interacted with the particles inside Earth. Instead, they've traveled through thousands of miles of planet with ease.
Now suppose that a neutrino with an extremely high energy would somehow have passed through the earth unharmed. In that case the particle must have come from some point in space. This could be, for example, a distant galaxy that accelerates neutrinos and sends them in this direction. Fortunately, there is no way to confirm this theory.
IceCube Neutrino Observatory
The IceCube Neutrino Observatory is a simultaneous but separate experiment in Antarctica that can detect a larger variety of neutrinos - including those with less energy. These variants should have reached Earth around the time of ANITA's unexplained measurements, if a source in space was indeed responsible for the particles found.
The team has just finished reviewing years of data, looking for evidence of such signals. They haven't found anything, which means that ANITA's high-energy observations are now even more difficult to explain.
We now only have the most exciting and boring possibilities left, says Ibrahim Safa, who also works at IceCube. ANITA has either found a sign of exotic physics, or something has gone wrong with the detector's measurements that everyone has overlooked so far.
Particle physicist Stefan Söldner-Rembold of the University of Manchester, is reasonably sure that the particle did not come from another galaxy. Whether it's new physics or a process scientists haven't yet understood, this are very interesting events, he says.
Physicists are waiting for an update from the ANITA team, which will be published later this year. It will describe every anomalous measurement during the fourth and most recent flight (2016) of the balloon. This overview may contain data on other high-energy observations that may help to unravel the mystery.
What could explain the strange events ANITA detected?
Meanwhile enough theories are going around about what the enigmatic particles could be, if they are indeed outside the standard model physics. Astronomer Derek Fox of Pennsylvania State University previously suggested that they could be tau neutrinos. This would fit within the theory of supersymmetry, a theory which says that all fundamental particles have a much heavier counterpart.
Other researchers suppose that the ANITA observations can also be a sign of dark matter or sterile neutrinos - neutrinos that interact with matter even less than 'ordinary' neutrinos.
A search for IceCube events in the direction of ANITA neutrino candidates
Submitted 6 January, 2020
Authors: IceCube Collaboration, M. G. Aartsen, M. Ackermann, J. Adams, J. A. Aguilar, M. Ahlers, M. Ahrens, C. Alispach, K. Andeen, T. Anderson, I. Ansseau, G. Anton, C. Argüelles, J. Auffenberg, S. Axani, P. Backes, H. Bagherpour, X. Bai, A. Balagopal V., A. Barbano, S. W. Barwick, B. Bastian, V. Baum, S. Baur, R. Bay , et al. (336 additional authors not shown)
Abstract: During the first three flights of the Antarctic Impulsive Transient Antenna (ANITA) experiment, the collaboration detected several neutrino candidates. Two of these candidate events were consistent with an ultra-high-energy up-going air shower and compatible with a tau neutrino interpretation. A third neutrino candidate event was detected in a search for Askaryan radiation in the Antarctic ice, al… read more
The IceCube Neutrino Observatory - Contributions to ICRC 2017 Part IV: Searches for Beyond the Standard Model Physics
Submitted 3 October, 2017
Authors: IceCube Collaboration, M. G. Aartsen, M. Ackermann, J. Adams, J. A. Aguilar, M. Ahlers, M. Ahrens, I. Al Samarai, D. Altmann, K. Andeen, T. Anderson, I. Ansseau, G. Anton, C. Argüelles, J. Auffenberg, S. Axani, H. Bagherpour, X. Bai, J. P. Barron, S. W. Barwick, V. Baum, R. Bay, J. J. Beatty, J. Becker Tjus, K. -H. Becker , et al. (291 additional authors not shown)
Abstract: Papers on searches for beyond the standard model physics, submitted to the 35th International Cosmic Ray Conference (ICRC 2017, Busan, South Korea) by the IceCube Neutrino Observatory Collaboration. Read more