Something strange is happening in Antarctica. A balloon floating above the icy desolation may have detected a weird particle that can’t be explained by our current understanding of physics.
NASA’s Antarctic Impulsive Transient Antenna (ANITA) detector is an array of radio antennae attached to an enormous balloon that flies 37 kilometres above Antarctica for one month at a time. It looks for radio pulses emitted by objects such as tau particles, a kind of heavy electron, emerging from the ice.
This happens when another kind of particle, neutrinos, decay as they pass through Earth from space. ANITA also looks for cosmic ray showers, a form of high-energy radiation originating in space.
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But in 2006 and 2014, the detector saw two events that didn’t look like either of these things. They reached ANITA’s antennae at steep angles, exiting Earth at 27 and 35 degrees below horizontal.
Normally only a cosmic ray reflected off the ice could approach at these angles, but such rays usually have their polarity, or orientation, flipped. These signals were not flipped, so they couldn’t have come from the ice.
Beyond the standard model
Derek Fox at Pennsylvania State University and his colleagues wanted to investigate whether these signals could be signs of a new particle beyond the standard model of physics, our current best description of how particles and forces interact.
They ran simulations of 1 billion standard model neutrinos passing through the Earth. They ruled out the possibility of ANITA detecting a tau particle from these neutrinos at such steep angles with a confidence of 5.8 sigma.
That means that if there is nothing unusual going on, there is only about a one in 170 million chance that these signals would be observed. Physicists normally treat anything above 5 sigma as an important discovery.
The two events were also both extremely high energy, clocking in at 0.6 and 0.56 exaelectronvolts (a billion billion electronvolts) respectively, and passed a significant distance through the Earth – 5700 and 7200 kilometres. This combination requires a particle capable of blasting through the planet without much slowing down.
“You might think you could start with a really high energy neutrino and it would propagate through the Earth,” says Fox. “But there’s too much stuff in the way.”
Improbable events
A highly energetic tau particle could do this, but only so rarely that it would basically never happen, says Fox. “That’s why looking beyond the standard model is necessary,” he says. “Otherwise, the properties of these events are improbable.”
He and his team suggest that a previously proposed supersymmetric tau, or stau, is behind the mysterious signals. Supersymmetry is a theoretical extension of the standard model that suggests all fundamental particles have a much heavier, or “super”, counterpart.
Supersymmetry models that describe the kind of signals Fox’s team have seen already exist, he says. They include a charged particle that is massive, decays to tau, and has a long enough lifetime that it can travel through about 6000 kilometres of Earth. “Bingo,” he says.
“Our official interpretation of our events is that we’re not sure what they are,” says Stephanie Wissel at California Polytechnic State University, who is a member of the ANITA collaboration. “In my opinion, the simplest explanation for these events is that they’re air showers from cosmic rays.”
She says that if these stau particles exist, other experiments ought to have seen them. The IceCube Neutrino Observatory, also in Antarctica has been detecting neutrinos since 2010, but it is set deep in the ice, so it doesn’t see particles in the air that ANITA sees.
Avi Loeb at Harvard University says we need more evidence to confirm that these potential particles aren’t due to an error in experimental interpretation, but the findings so far are compelling.
“The multi-faceted evidence presented in this work does not allow wiggle room for explaining the two ANITA events with known standard model particles,” he says.
Reference: arXiv,
