Search for fourth type of neutrino turns up none

A finding of nothing doesn’t often get a run in research reporting, but an almost-certain conclusion that the hypothesised “sterile neutrino” doesn’t exist is an important one in the world of particle physics.

Despite hints over the past few decades that the particle exists, the IceCube Neutrino Observatory buried deep in the Antarctic ice failed to pick up any signs of it – despite two independent analyses of more than a year’s worth of data.

The results, published in Physical Review Letters, means the Standard Model of particle physics – which describes three types of neutrino: muon, electron and tau – remains the same.

“Like Elvis, people see hints of the sterile neutrino everywhere,” says Francis Halzen from the University of Wisconsin-Madison and IceCube principal investigator.

“There was this collection of hints, and theorists were convinced it exists.”

Neutrinos are slippery particles with almost no mass. Even though there are trillion whizzing through your body every second, and they tear along at near the speed of light, they only very, very rarely interact with matter.

It’s no surprise, then, that they’re extremely hard to catch. And a sterile neutrino – which wouldn’t interact with matter at all, only gravity – would be even more difficult.

Essentially, the only way a sterile neutrino could be detected is if it transformed into one of the other three – and we’d have to catch it in the act.

One of the best neutrino nets is the IceCube Neutrino Observatory, a block of clear ice buried more than two kilometres beneath the South Pole.

It’s punctuated by 5,160 sensors hanging from “strings” designed to see the tiniest flash of light produced on the off chance a neutrino smashes into an atom’s nucleus while barrelling through the ice block.

In April, it picked up the highest-energy neutrino yet – and even pinpointed its galaxy of origin.

But, the IceCube physicists realised, a sterile neutrino would have a very specific signature in a particular energy range. And after analysing around 100,000 neutrino events, they turned up … nothing. (Well, they’re 99% sure of this.)

Discovering a new flavour of neutrino would upend the Standard Model, but it might also help solve the mystery of the origin of dark matter and the asymmetry between matter and antimatter in the universe.