“It’s an interesting problem because we still don’t actually have an explanation for what those anomalies are,” explained Stephanie Wissel, a physicist at Penn State, after a dozen or more years of close scrutiny of the steep-angle, upward-pointing radio bursts seen by NASA’s Antarctic Impulsive Transient Antenna, or ANITA. The enigma only grew as subsequent studies, such as those conducted by the Pierre Auger Observatory and the IceCube Neutrino Observatory, have not been able to discover corroborative evidence, leaving the physics community both baffled and invigorated.
ANITA’s shape is as innovative as the signals it discovered. Dangling from a high-flying balloon, its 24-antenna array scans the Antarctic ice cap, hunting for the characteristic radio pulses produced when ultra-high-energy neutrinos particles so rare they’re called “ghostly” zap atoms in the ice. Born in the universe’s most extreme environments, these neutrinos are thought to travel enormous distances, sometimes interacting with matter to create detectable secondary showers of particles. But the signals that have focused international attention didn’t fit this model.
ANITA picked up radio pulses twice coming from directions about 30 degrees under the surface of Antarctica, a path that, based on the Standard Model of particle physics, should not be possible. As Wissel explained, “The radio waves that we detected nearly a decade ago were at really steep angles, like 30 degrees below the surface of the ice.” Such a signal would have to be produced by a neutrino that traversed thousands of miles of rock, a highly unlikely trip, since even these “ghost particles” ought to be absorbed by so much matter at such high energies. They are expected to arrive from slightly below the horizon, where there is not much Earth for them to be absorbed, University of Wisconsin, Madison’s Justin Vandenbroucke said. “The ANITA anomalous events are intriguing because they appear to come from well below the horizon, so the neutrinos would have to travel through much of the Earth. This is not possible according to the Standard Model of particle physics”.
Steepness and source of the signals have prompted scientists to use up traditional explanations. The Pierre Auger Observatory, a hybrid detector in Argentina, employs both atmospheric fluorescence and surface water tanks to monitor cosmic rays. In a new paper in Physical Review Letters, the Auger collaboration recast their analysis to look for upward-going air showers, relying on computer simulations to mimic what an ANITA-like event would appear in their data. Even with a vast collecting area bigger than ANITA’s they saw no hint of such events. “If the ANITA anomalous events are produced by any particle traveling through the Earth and then producing upward-going showers, then Auger should have detected many of them, and it did not,” Vandenbroucke said (source).
The IceCube Neutrino Observatory, which has more than 5,000 light sensors buried deep within the Antarctic ice, also hunted for these anomalies. IceCube’s special sensitivity, particularly to tau neutrinos a type of the three “flavors” of neutrinos ought to have enabled it to see an avalanche of events in the case of signals from the ANITA if they were caused by such particles. But according to Alex Pizzuto of the University of Wisconsin–Madison, “for each anomalous event that ANITA detects, IceCube should have detected many, many more which, in these cases, we didn’t”. This finding, which was seconded by coauthor Anastasia Barbano of the University of Geneva, essentially eliminates the final remaining Standard Model explanation for the ANITA anomalies.
Others have suggested that the signals could be caused by tau neutrino regeneration, a mechanism in which a tau neutrino will interact, create a tau lepton, which decays and regenerates a tau neutrino, potentially enabling it to come out of the Earth. But as Wissel correctly observed, “You expect all these tau neutrinos to be very, very close to the horizon, like maybe one to five degrees below the horizon. These are 30 degrees below the horizon. There’s just too much material. They really would actually lose quite a bit of energy and not be detectable”.
With the Standard Model creaking, eyes have turned to more esoteric possibilities. Hypotheses now vary from sterile neutrino interactions neutrinos that communicate only through gravity to supersymmetric particles and even dark matter. Some propose the solution can be found in an unknown effect to radio wave propagation in Antarctic ice. As Wissel explained, “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.”
The search for answers is now being transferred to a new generation of instruments. The Payload for Ultra-High Energy Observations (PUEO), which will make its first Antarctic flight this December, is a game-changer. PUEO’s design accommodates ten times higher sensitivity than ANITA, utilizing sophisticated radio detection arrays and data analysis to search for even weaker signals. As Wissel explained, I’m excited that when we fly PUEO, we’ll have better sensitivity. In principle, we should be able to better understand these anomalies which will go a long way to understanding our backgrounds and ultimately detecting neutrinos in the future. The mission of the instrument: to shed some light on whether the ANITA events were statistical flukes, unknown systematic errors, or real indications of new physics.
The scientific world is holding its breath. As University of Delaware physicist Benjamin Flaggs said, There are theorists proposing some beyond-standard-model interactions from different types of particles. But, as Wissel and others point out, the most probable explanation may still be a prosaic but neglected component of particle or radio physics. “Sometimes you just have to go back to the drawing board and really figure out what these things are,” Wissel said.
The story of the ANITA anomalies is far from over. As PUEO prepares for launch, the promise of deeper insight and perhaps a fundamental shift in the understanding of particle physics looms on the Antarctic horizon.