Last week we reported that strange events at the South Pole may be explained by supersymmetry physics, according to Derek Fox, an observational astrophysicist at Pennsylvania State University. The team of researchers has found new evidence that suggests some particles detected in Antarctica do not fit the Standard Model.
They posted a paper outlining their argument,on the arXiv preprint server, exploring whether “Beyond the Standard Model” (BSM) particles are required to explain events detected by NASA’s Antarctic Impulsive Transient Antenna (ANITA)—a balloon-borne antenna hovering over the southern continent. The team concluded that if correctly interpreted, they are.
Their analysis just shows that unless there’s an error in data collection or analysis, it is very unlikely (less than 1 in 3 million) that the observations can be explained by the standard model. In the paper they do claim that the observations could be consistent with a particle hypothesized in a certain supersymmetric extension of the standard model consistent with string theory, but that is only one of many possible explanations.
Confirmation of the phenomenon of sub-EeV Earth-emergent cosmic rays in data from other facilities support the reality of the ANITA events, reports Phys.org. “Three events from the Extremely High Energy Northern Track neutrinos of the IceCube Neutrino Observatory provide new evidence in the form of sensor data from the IceCube experiment in which sensors buried in the Antarctic ice continually detect particle events. Data from the sensors showed that three events with unexplained properties had occurred. The researchers suggest the two unconnected sources of data indicate that it is time to start asking whether the anomalies hint at the possibility of particles beyond the Standard Model.”
The Penn State team detected the 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 back out again 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.
“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.
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 propose 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.”
Twice in the past 13 years, reports Adrain 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 argues in a new study.
“So… uh, folks? I think @steinly0 [Steinn Sigurdsson], some colleagues and I just broke the Standard Model,” tweeted Fox. 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 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, 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.
So, 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 at the top of the page) 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 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.
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.)
“I was like, ’Well this model doesn’t make much sense,’” Derek Fox reported to Rafi Letzter at Live Science. 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. “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 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.”
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