Particles spotted by a giant balloon in Antarctica can’t be explained by our current understanding of physics, a thorough analysis has determined. Now the race is on to figure out what they are.
In 2006 and then again in 2014, NASA researchers using the (ANITA), a balloon equipped with a particle detector, picked up a signal that didn’t make sense. They had spotted evidence of high-energy particles travelling at an angle suggesting they had just whizzed through the planet unimpeded.
A new analysis has now ruled out the best possible explanation for these particles. That means they might be signs of physics beyond the standard model – physicists’ working understanding of particles and forces in the universe.
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The ANITA events registered at ridiculously high energies for a tiny particle, at 0.6 and 0.56 exaelectronvolts (a billion billion electronvolts). “About the same as a professional tennis serve,” says Alex Pizzuto at the University of Wisconsin-Madison, who isn’t involved with ANITA.
One idea was that these particles were neutrinos, which are known to pass through other matter. But at such high energies, these neutrinos should have interacted with particles inside Earth rather than slipping through thousands of kilometres of the planet with ease.
Still, it is possible that a high-energy neutrino could have got through unscathed, in which case it must have come from a point somewhere in space – a distant galaxy, perhaps, accelerating neutrinos and blasting them towards us. Luckily, there is a way to check.
Pizzuto and his colleagues work on the , a simultaneous but separate experiment in Antarctica that is able to detect a wider range of neutrinos, including lower-energy variants. These would have reached Earth around the time of the ANITA events, if a source in space were responsible.
The team has just finished combing through years of data, looking for evidence of any such signals. They came up short, meaning the high-energy ANITA detections are now even more difficult to explain.
“We’re left with the most exciting or most boring possibilities,” says Ibrahim Safa, who also works on IceCube. Either ANITA has found a sign of exotic physics or there is some subtle anomaly with the detector’s readings that everyone has so far overlooked.
at the University of Manchester, UK, says he feels confident that the point source explanation for the weird ANITA detections has now been ruled out.
“Whatever it is, whether it is new physics, or some process we haven’t understood yet, it’s very interesting,” he says.
Physicists are waiting for an update from the ANITA team to be published later this year, in which any anomalous events during the balloon’s fourth and most recent flight in 2016 will be described. That could yield data about additional high-energy detections and help to solve the mystery.
In the meantime, theories abound as to what these enigmatic particles may be, if indeed they do challenge the standard model. Derek Fox at Pennsylvania State University has previously suggested that they could be stau neutrinos, a heavier – or “super” – form of the tau neutrino. This would fit supersymmetry, the theory that all fundamental particles have much heavier counterparts.
The ANITA detections could also be signs of dark matter, or sterile neutrinos, argue some.
Pizzuto isn’t making bets on whether new physics will emerge from the mystery. “I’ll hold off on claiming allegiance to any one model,” he says. “I think it’s still too early to tell if that’s what ANITA has stumbled upon.”
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