A massive detector at the South Pole has found no evidence of a 'sterile' neutrino: a near-massless particle that is thought to interact only through gravity.

Hints of this possible fourth type of neutrino first emerged in the 1990s, and were rekindled early this year by an experiment in China. In the latest work, researchers at the IceCube Neutrino Observatory in Antarctica, led by Francis Halzen at the University of Wisconsin–Madison, counted neutrinos of a known type that hit the detector from below. A dearth of these neutrinos at particular energies would have revealed that some of the particles had temporarily mutated into sterile neutrinos during their trip through Earth, but the researchers found no such feature in their data.

The experiment did not rule out the existence of heavier sterile neutrinos. A fourth kind of neutrino would challenge the standard model of particle physics, which allows for only three neutrino types.

Phys. Rev. Lett. 117, 071801 (2016)