South Pole Experiment's Hunt For Exotic Neutrino Comes Up Empty

It's been a rough few weeks of null results for physicists. First, a promising underground experiment failed to find evidence of dark matter. Then news broke that the Large Hadron Collider hadn't discovered an exciting new particle after all. And now the IceCube collaboration in Antarctica has announced yet another null result. The experiment failed to find evidence for an exotic fourth type of neutrino. A new paper in Physical Review Letters reports that, after analysing about 100,000 neutrino events, the collaboration is now 99 per cent sure that this so-called "sterile neutrino" does not exist.

Quick refresher for those whose neutrino knowledge might be a bit rusty: Neutrinos are ultra-light subatomic particles, often dubbed "ghost particles". There are billions coursing through us every second, but they don't affect us, because neutrinos only interact with other particles via the weak nuclear force. That means the neutrinos have to be so close to other particles as to nearly be touching their nuclei. Only then does the weak force kick in. It also makes neutrinos devilishly hard to detect, which is why physicists have built their neutrino observatories underground -- the better to avoid interference from things like cosmic rays hitting the Earth's atmospheres.

There are three known types, or flavours, of neutrinos -- muon, electron and tau -- and they have just the tiniest bit of mass, which is how they can change from one type to another.

More recently, some odd experimental results prompted physicists to contemplate the existence of a fourth type, the sterile neutrino -- one that doesn't interact with regular matter at all, although it could interfere with other neutrinos. This would be a very big deal, because the Standard Model of particle physics only allows for three types of neutrinos. It could also shed some light on why neutrinos have mass, the nature of dark matter, and why there was more matter than antimatter in the early universe.

"Like Elvis, people see hints of the sterile neutrino everywhere," Francis Halzen, a physicist at the University of Madison, Wisconsin, who heads up the IceCube collaboration, said in a statement. "There was this collection of hints, and theorists were convinced it exists."

Alas, it's seeming far less likely in light of these new IceCube results.

IceCube features an array of 5160 detectors buried in the ice beneath the South Pole. They're ideal for detecting atmospheric neutrinos -- those created by cosmic ray collisions in the Earth's upper atmosphere. The Earth itself filters out all the noise from cosmic rays and other particles; only neutrinos can pass right through. Every now and then, a neutrino will collide into a nuclei to create a muon, and when that happens it will emit a flash of blue light, akin to a sonic boom.

Based on those earlier experiments, theoretical predictions called for a distinctive sterile neutrino signal to show up within a certain energy range (between 320 GeV to 20 TeV, where eV stands for electron volts). It would constitute solid evidence of a sterile neutrino changing into one of the three known types. But there was no sign of such a signature in all those recorded events, representing a full year's worth of data.

Physics had such a promising start to 2016, too, with the discovery of gravitational waves. But remember, that breakthrough took over 40 years. Most experiments yield null results, at least initially. It's how all scientific progress is made. Nature no doubt has plenty of surprises in store for us -- once we figure out where and how to look.

[Physical Review Letters]

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