From Scientific American:
The null result, reported on 8 August in Physical Review Letters, doesn’t spell the end of a decades-long search to find the subatomic particle, which—if found—would upend the standard picture of particle physics. But it is the strongest evidence so far that the sterile neutrino doesn’t exist at the mass range that physicists had hoped, based on anomalies from several experiments over the past three decades.
Neutrinos are everywhere: every second, trillions of the subatomic particles fly through our bodies. But they interact so rarely with matter that to detect them requires large underground experiments equipped to spot neutrino–matter collisions. These experiments have spotted three types: the electron neutrino, muon neutrino and tau neutrino; the three can change type as they travel.
In the mid-1990s, a detector at the Los Alamos National Laboratory in New Mexico saw an anomaly that hinted there might be a fourth kind of neutrino, which would interact even more rarely with matter than would ordinary neutrinos. It was dubbed the ‘sterile’ neutrino, and the Los Alamos experiment suggested it weighed around one-billionth the mass of a hydrogen atom—around 1 electronvolt (1 eV). Other experiments found similar anomalies. More-recent studies that counted neutrinos streaming from nuclear reactors, including last February at Daya Bay in China, have seen strange features in their data that could, in theory, point to a sterile neutrino.
The sterile neutrino can’t be detected directly. “The only way to see it is because it messes up the observation of the three other neutrinos,” says Francis Halzen, a physicist at the University of Wisconsin–Madison. Halzen is the principal investigator at IceCube, an array of more than 5,000 basketball-sized sensors embedded in ice to depths of more than two kilometres. The telescope hunts for neutrinos by detecting the faint flash of light caused whenever such a particle hits an atomic nucleus in the ice.
To search for sterile neutrinos, Halzen’s team looked for the arrival of muon neutrinos that started life on the other side of Earth (see ‘Neutrino observatory‘). These were originally produced by the collision of cosmic rays with air molecules in the atmosphere, and …