The view of Antarctica from an altitude of 23 miles, captured by a balloon loaded with sophisticated instruments, may challenge assumptions about the physics that govern the universe.
That’s the finding of a study co-authored by literally hundreds of scientists who have spent years puzzling over weird observations from the Antarctic Impulsive Transient Antenna (ANITA), an aerial experiment that searched for signatures left by particles known as neutrinos.
The anomalies detected by ANITA could hint at new physics beyond the so-called Standard Model, which is a well-corroborated explanation of how matter interacts with the four known fundamental forces of nature. That said, the new study—which has been submitted to The Astrophysical Journal and is not yet peer-reviewed—is focused on ruling out explanations that are consistent with the Standard Model.
“Our paper was less about exotic Beyond the Standard Model scenarios as it was investigating one of the few remaining Standard Model explanations of these odd events ANITA detected,” explained Alex Pizzuto, a graduate student in physics at the University of Wisconsin-Madison and one of the study’s leads, in an email.
“As tantalizing as explaining anomalous results (like the ANITA events) with exotic new physics can be, it's our job to make sure that there's no way of explaining these events within our current understanding of physics,” he added.
So what exactly did this Antarctic balloon observe, and why is it so weird?
ANITA is one of many ingenious detectors designed to capture neutrinos, which are about 500,000 times less massive than electrons, making them the most lightweight elementary particles known to scientists.
Neutrinos are electrically neutral, hence the name. This combination of low mass and no charge means that most neutrinos tread lightly on the universe, passing through planets and other celestial matter without slowing down or leaving easily detectable tracks. They are also extremely abundant—some 100 trillion neutrinos pass through your body every second.
Scientists know that neutrinos are produced by natural sources such as stars or galactic cores, as well as human-made technologies like particle accelerators and nuclear reactors. But ANITA was calibrated to detect a more mysterious class of neutrinos that scientists think get juiced up during interactions with ultra-high-energy cosmic rays (UHECRs).
UHECRs are the universe’s most energetic particles, and they zoom around space at near light speeds. Nobody knows exactly what forces are accelerating UCECRs to such overwhelming energies, but high-energy neutrinos could help unlock that mystery.
“If you could detect even a few of those neutrinos, you have the potential to make a real breakthrough,” said Derek Fox, an astrophysicist at Penn State University and co-author of the new study, in a call.
“The neutrino would point you back to the source that is making those ultra-high-energy cosmic rays,” he added. “Then we could resolve this mystery of, basically, what are the highest energy particle accelerators in the universe?”
Since 2006, ANITA has been launched on three flights over the Antarctic continent that lasted several weeks each. It carried sensors designed to pick up the radio signatures of high-energy neutrinos as they skimmed the ice below, an effect known as Askaryan radiation.
Because it was located at such a high altitude, ANITA was able to monitor an enormous patch of this frigid continent, which raised its odds of catching a rare glimpse of a high-energy neutrino. “The reason you fly above Antarctica is because you’re using close to about a million square kilometers of Antarctic ice as your detector medium,” Fox explained.
The mission detected one neutrino candidate that produced this Askaryan signature, but they also stumbled across something completely unexpected: Two anomalous signals that seemed to shoot up from the ground, suggesting that high-energy neutrinos may have traveled through thousands of miles of solid rock and mantle within Earth.
While zipping through Earth is no big deal for low-energy neutrinos, the Standard Model predicts that high-energy neutrinos would be more interactive with solid matter, preventing them from penetrating deeper than a few hundred miles of our planet.
To figure out how the anomalies might have punched through Earth, against all odds, the researchers analyzed more than a decade of data. The team hoped to retrace the trajectories of those two signals back to an obvious place in the sky, such as a star, a supernova, or a blasar, which is a type of energetic galactic core—or some other unknown source.
“To me, the most energizing reason to do the analysis is because you hope you will find a source,” Fox said. “Then, it’s sort of an open-and-shut case and then we’d know that ANITA did see an actual neutrino and there’s a source there that we can study.”
However, the team’s comprehensive analysis did not reveal an obvious source for the weird anomalies detected by ANITA. Indeed, the two high-energy signals should have been accompanied by a shower of neutrinos at lower energies detectable by IceCube, an enormous observatory buried underneath the ice at the South Pole. But the team found no such corresponding signals in IceCube’s observational history.
“The new question that this work raises is: Can this signal be explained with a weird artifact in the ANITA detector or the Antarctic ice, or this a hint of new physics?” Pizzuto said.
“We cannot claim any new physics before first ruling out *any* other possibility,” he emphasized.
One anticlimactic scenario is that ANITA simply suffered some kind of systemic or instrumental error that produced unusual results. The team is currently investigating that possibility.
But assuming technical difficulties are ruled out, the findings may genuinely point to new types of particles and physics that shape the high-energy universe. The team outlines potential avenues to explore those possibilities in the future.
“When you test a model that includes exotic physics, you hypothesize that our current picture is incomplete,” Pizzuto said. “In the case of the ANITA events, that might mean new interactions or new particles involving neutrinos that would allow them to traverse Earth and make it to the detector before they smash into something.”
For instance, perhaps the signals were accelerated inside Earth by a type of hypothetical substance called heavy dark matter. The anomalies could also be linked to speculative objects called sterile neutrinos, supersymmetric particles, or axions.
Ultimately, testing out those theories will require a lot more data from otherworldly neutrino detectors like IceCube, as well as more theoretical work establishing the potential parameters and properties of exotic physics Beyond the Standard Model.
“One needs to be patient when doing neutrino physics,” Pizzuto said. “But if the history of neutrinos tells us anything, it’s that surprises are the norm in this field.”