Billion-tonne IceCube: Sterile neutrino does not exist

We're, um, Straight Outta 'fourth' 'trons... 'trinos... you know what we mean – scientists

Photo credit: Erik Beiser, IceCube/NSF

Physicists are almost certain that the sterile neutrino does not exist after failing to find any sign of the ghostly particle at the IceCube Neutrino Observatory in the South Pole.

Results published today in Physical Review Letters show that scientists have concluded sterile neutrinos do not exist with 99 per cent certainty.

The idea of a new type of neutrino was sparked 20 years ago. Scientists working at the Liquid Scintillator Neutrino Detector at the Los Alamos National Laboratory, New Mexico, found strange bumps in their data which could be explained if there was a fourth type of neutrino.

Neutrinos come in three "flavours": electron, muon and tau neutrinos. A fourth neutrino was never predicted from the Standard Model of particle physics, but the strange bump began to crop up in other neutrino experiments. Scientists began to abandon theory and speculate about the presence of "sterile" neutrinos.

The latest search, however, has proved fruitless. Scientists found no hint of sterile neutrinos streaming through IceCube, located in the icy depths of Antarctica.

Sterile neutrinos are different to the other flavours of neutrinos as they don’t directly interact with matter. The only way to find them is to catch them in the act of transforming into one of the other flavours of neutrinos.

The detector is a massive block of ice weighing a billion tonnes sitting more than a mile beneath the South Pole. It has 5,160 light-detecting sensors frozen in the ice that detect the flashes of light emitted when neutrinos collide into the ice block, creating secondary particles and a telltale streak of blue light.

Neutrinos stream through Earth in high numbers and the detectors managed to detect one every six minutes. Scientists analysed a year’s worth of data to look for oscillations of energy produced by sterile neutrinos, Professor Francis Halzen, a theoretician working at IceCube and the University Wisconsin-Madison, explained.

“Traveling through the Earth, the neutrinos not only oscillate, the electron-neutrino interacts with atomic electrons and this modifies the oscillations in the presence of matter. If the electron neutrino oscillates into a sterile neutrino some of the time, then its interactions with matter are modified,” Professor Halzen told The Register.

The interactions caused by sterile neutrinos produce a characteristic spike in energy. Interestingly, in the presence of sterile neutrinos, most muon neutrinos should disappear from the atmospheric beam, but scientists did not observe this either, Halzen said.

Finding a fourth type of neutrino breaks the rules in the Standard Model, but could have helped solve the mystery of why there is a mismatch between the amount of matter and antimatter in the universe and the origin of dark matter, as well as why neutrinos have mass. ®


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