“There’s something mysterious coming up from the frozen ground in Antarctica, and it could break physics as we know it.
Physicists don’t know what it is exactly. But they do know it’s some sort of cosmic ray — a high-energy particle that’s blasted its way through space, into the Earth, and back out again. But the particles physicists know about — the collection of particles that make up what scientists call the Standard Model (SM) of particle physics — shouldn’t be able to do that. Sure, there are low-energy neutrinos that can pierce through miles upon miles of rock unaffected. But high-energy neutrinos, as well as other high-energy particles, have “large cross-sections.” That means that they’ll almost always crash into something soon after zipping into the Earth and never make it out the other side.
And yet, since March 2016, researchers have been puzzling over two events in Antarctica where cosmic rays did burst out from the Earth, and were detected by NASA’s Antarctic Impulsive Transient Antenna (ANITA) — a balloon-borne antenna drifting over the southern continent.
ANITA is designed to hunt cosmic rays from outer space, so the high-energy neutrino community was buzzing with excitement when the instrument detected particles that seemed to be blasting up from Earth instead of zooming down from space. Because cosmic rays shouldn’t do that, scientists began to wonder whether these mysterious beams are made of particles never seen before.
Since then, physicists have proposed all sorts of explanations for these “upward going” cosmic rays, from sterile neutrinos (neutrinos that rarely ever bang into matter) to “atypical dark matter distributions inside the Earth,” referencing the mysterious form of matter that doesn’t interact with light [The 18 Biggest Unsolved Mysteries in Physics]
All the explanations were intriguing, and suggested that ANITA might have detected a particle not accounted for in the Standard Model. But none of the explanations demonstrated conclusively that something more ordinary couldn’t have caused the signal at ANITA.
A new paper uploaded today (Sept. 26) to the preprint server arXiv changes that. In it, a team of astrophysicists from Penn State University showed that there have been more upward-going high-energy particles than those detected during the two ANITA events. Three times, they wrote, IceCube (another, larger neutrino observatory in Antarctica) detected similar particles, though no one had yet connected those events to the mystery at ANITA. And, combining the IceCube and ANITA data sets, the Penn State researchers calculated that, whatever particle is bursting up from the Earth, it has much less than a 1-in-3.5 million chance of being part of the Standard Model. (In technical, statistical terms, their results had confidences of 5.8 and 7.0 sigma, depending on which of their calculations you’re looking at.)
Derek Fox, the lead author on the new paper, said that he first came across the ANITA events in May 2018, in one of the earlier papers attempting to explain them.
“I was like, ‘Well this model doesn’t make much sense,’” Fox told Live Science, “but the [ANITA] result is very intriguing, so I started checking up on it. I started talking to my office neighbor Steinn Sigurdsson [the second author on the paper, who is also at Penn State] about whether maybe we could gin up some more plausible explanations than the papers that have been published to date.”
Fox, Sigurdsson and their colleagues started looking for similar events in data collected by other detectors. When they came across possible upward-going events in IceCube data, he said, he realized that he might have come across something really game-changing for physics. [5 Mysterious Particles Lurking Underground]”