“Unsolved Mystery ” -Strange Particle from Deep Space Detected at Antarctica’s South Pole

Particles from Outer Space


“They say great discoveries often start with someone saying “Huh, that’s weird,” observes Tyce DeYoung, a member of the IceCube and HAWC collaborations, two large international groups of physicists who are building and operating experiments to detect the highest energy particles in the Universe in an email to The Daily Galaxy. “But unfortunately, most of the time we say “That’s weird” it really is just some everyday weirdness, not a great discovery.”

Strange events at Antarctica’s South Pole in 2018 appear to fall into DeYong’s first category. Events that are more easily explained by new physics, said Derek Fox, an observational astrophysicist at Pennsylvania State University Center for Theoretical and Observational Cosmology. Like the opening scene of a Hugo-winning science fiction novel, scientists have detected a mystery particle shooting into space from deep beneath the ancient South Polar ice cap. It appears to be a high-energy particle that’s traveled through space for billions of years, crashed into the Earth, and escaped back out again on the opposite side, defying what physicists call the Standard Model (SM) of particle physics. But cosmic rays shouldn’t do that; scientists began to wonder whether these mysterious beams are made of particles never seen before.

New Type of Massive Particle?

“An ultrahigh-energy cosmic ray on the far side of Earth could have spawned a new type of particle, about 500 times as massive as the proton, that pierced the planet before decaying to produce the upward air shower. A theoretical framework called supersymmetry offers candidates that would do the trick”, Fox added. 

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The Elusive Superpartner Particle

Supersymmetry predicts that every type of particle in the standard model of physics has some elusive superpartner particle, which is difficult to detect because it interacts so weakly with the standard model particles. 

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 hovering over the southern continent, leading physicists to proposed several theories for these “upward going” cosmic rays, from sterile neutrinos (neutrinos that rarely ever bang into matter) to “atypical dark matter distributions inside the Earth.”

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Points to the existence of new particles?

Twice in the past 17 years, reported Adrian Cho in Science, particles from outer space tunneled through Earth and up into the atmosphere above Antarctica, triggering faint pulses of radio waves that were picked up by a balloon-borne detector 35 kilometers above the ice cap. Those two events poke a hole in physicists’ standard model of fundamental particles and forces, and point to the existence of new particles, a team of astrophysicists argued.

But Dave Besson, a physicist at the University of Kansas in Lawrence and a member of the team that originally observed the events with the balloon experiment, says that within that collaboration, “I don’t think there’s anybody who’s up for saying we broke the standard model.”

ANITA, a NASA-funded experiment, has floated around the South Pole five times since 2006. It primarily looks for evidence of elusive particles called neutrinos crashing into the ice below and triggering a spray of particles that then emits telltale radio waves. ANITA has yet to spot such signals. However, the instrument has found radio signals produced by other types of particles from space, known generically as cosmic rays, as they crash into Earth.

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When a cosmic ray such as a proton hits the atmosphere, it sets off an avalanche of high-energy charged particles called an air shower. The trajectory of the shower bends in Earth’s magnetic field, which causes it to produce radio waves that beam ahead of the shower like a headlight. Typically, ANITA sees radio waves from downward moving air showers after they bounce off the ice and reflect up to the balloon. Occasionally it spots radio waves coming directly from air showers traveling sideways in the atmosphere from the horizon.

The two signals differ in a key way. Radio waves are polarized in a way determined by the direction of Earth’s magnetic field. But that polarization flips when the radio waves reflect off the ice, whereas a signal from a sideways shower keeps its original polarization.



Odd Radio Waves Detected

However, twice, continues Cho in Science, during its first flight 2006 and its third flight in 2014, ANITA detected odd radio waves with unflipped polarizations coming up from the surface below instead of the horizon. That suggests the signals were produced by upward-zooming air showers triggered by particles that tunneled through Earth. At first blush, that’s not a problem for the standard model. Neutrinos barely interact with matter, so a couple of cosmic neutrinos might have barreled through the planet before smacking an atomic nucleus in the ice and setting off an upward air shower.

However, when examined in detail, that explanation falls apart, argue Fox and his colleagues. Given the showers’ directions, the particles that made them must have traveled through more than 5700 kilometers of Earth, the researchers estimate. However, the showers’ large sizes show that the particles must have had energies in excess of 0.5 exa-electron volts—70,000 times higher than the energy achieved with the most powerful particle accelerator. Such extreme energy increases the probability that neutrinos will interact with other matter, so there’s no chance that such a high-energy neutrino could make it through that much rock, the researchers argue in the new paper posted to the arXiv preprint server and submitted to Physical Review D.

Did ANITA detect a particle not accounted for in the Standard Model? 

The new paper uploaded on Sept. 26 to the preprint server arXiv showed that there have been more upward-going high-energy particles than those detected during the two ANITA events. Three times, they wrote, IceCube (the other, larger neutrino observatory in Antarctica shown below) detected similar particles, though no one had yet connected those events to the mystery at ANITA.

The ANITA team itself noticed the two strange events, describing them in previous papers, but Besson cautions against jumping to conclusions. The entire analysis assumes that the two signals’ unflipped polarizations prove they come from particles coming up through Earth. “That’s not a slam dunk,” Besson says. Surface effects and other factors could conceivably unflip the polarization of a signal from a downward going particle shower, he says. “My personal take is that we’re making too much of these events that completely depend on a polarization argument.” 


Does Data Rule Out a Standard Model?

Fox says his goal is to get the community to take seriously the possibility that the ANITA results are pointing to new physics. He emphasizes that he’s not claiming the discovery of a new particle. But he sticks by the claim in the paper that the data rule out a standard model explanation of the events at the level of statistical confidence particle physicists require to claim a definite discovery.

NASA funded a fifth ANITA flight, Besson says, and if it does the flight will probably focus on such weird events. ANITA-2 is a compact gas analyzer which can analyze and quantify 33 trace contaminants in the atmosphere aboard the International Space Station (ISS) automatically. ANITA-2 can also detect the presence of unknown substances which can be evaluated later on the ground.

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1-in-3.5 Million Chance of Being Part of the Standard Model

By combining the IceCube and ANITA data sets, the Penn State researchers calculated that, whatever particle is shooting up from the depths of the South Pole, 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.)

Fox and colleagues started hunting 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. 

“That’s what really got me going, and looking at the ANITA events with the utmost seriousness,” Fox said, adding, “This is what physicists live for. Breaking models, setting new constraints [on reality], learning things about the universe we didn’t know.”

The Last Word –ANITA Events Remain a Mystery

“The anomalous ANITA events remain a mystery,” wrote Tyce DeYoung, Professor of Physics and Astronomy at Michigan State University in his email to The Daily Galaxy. “They’re clearly not Standard Model neutrinos.,” he notes. “The question is whether they are a fantastic new discovery, or just cosmic rays that for some reason didn’t look like they were expected to. The ANITA team have seen two of these events, in data sets collected two years apart with different experimental configurations, so I don’t believe there was a problem with the experiment.  The ANITA team are careful scientists and they’ve done a great job of ruling out the known backgrounds, but I worry that there may be unexpected ways for cosmic rays to appear strangely and be mistaken for something more exotic.”  

“For example,” DeYoung explains in his email, “Shoemaker et al. have suggested there may be unexpected effects when radio waves reflect from the Antarctic ice cap, which could cause cosmic rays interacting in the atmosphere to look like they’re coming through the Earth — kind of like a mirage. There’s a high burden of proof for a major discovery, and I don’t think we’re there yet.”

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Tyce DeYoung, Science and Live Science

Image top of page: With thanks to the NSF

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