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The Antarctic Impulsive Transient Antenna balloon experiment has flown four times over Antarctica and has seen two hard-to-explain particle events.


Oddball particles tunneling through Earth could point to new physics

Twice in the past 13 years, 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 argues in a new study.

“So… uh, folks? I think @steinly0 [Steinn Sigurdsson], some colleagues and I just broke the Standard Model,” tweeted Derek Fox, an observational astrophysicist at Pennsylvania State University in University Park. 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.”

The data come from the Antarctic Impulsive Transient Antenna (ANITA), a NASA-funded experiment that has floated around the South Pole four 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.

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.

However, twice, 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.

The events are more easily explained by new physics, the researchers say. 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, they say. A theoretical framework called supersymmetry offers candidates that would do the trick, Fox says. He adds that the interpretation also gets some support from IceCube, a gigantic array of particle detectors sunk into the Antarctic ice, which sees weaker evidence of unusual upward-going events.

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 conceivable 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.”

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.

The fate of ANITA may hinge on the debate, too. NASA has yet to fund a fifth flight, Besson says, and if it does the flight will probably focus on such weird events.

As for his titillating tweet, Fox says he’s not worried that it will be overblown by the public or the scientific press. “We’re just out here on Twitter talking and having fun.”