Sterile Neutrino Search Comes Up Empty At IceCube Lab

After conducting a diligent search for a hypothetical subatomic particle — the “sterile neutrino” — that would have filled in another blank of the Standard Model of particle physics, scientists at a particle detector in the South Pole are now almost certain that such a particle does not exist.

Three types, or flavors, of neutrinos are already known to science and are part of the Standard Model; they are called muon, electron and tau. Neutrinos have almost no mass and rarely interact with matter, making them difficult to detect. For instance, billions of neutrinos pass through Earth and even your body every minute but no one feels a thing. The three known flavors have been detected because they are charged with weak nuclear interaction — one of the four fundamental forces of the Standard Model.

Experiments have hinted at a fourth flavor, nicknamed the sterile neutrino — a hypothetical particle that would not interact with matter at all, except maybe through gravity. Discovery of the particle could help not just solve the mystery behind the origin of dark matter, which makes up more than a quarter of the mass and energy in the observable universe, it could also explain the asymmetry between matter and antimatter in the Universe, and would have also thoroughly upset the Standard Model.

The only way to observe a sterile neutrino would be to catch it in the process of transformation into one of the other three flavors. In a study published Monday in the journal Physical Review Letters, researchers said they found no trace of the particle “in either of two independently developed analyses, each using one year of atmospheric neutrino data.”

The data was collected at the IceCube Neutrino Observatory, an Antarctic laboratory half under ice, by “using so-called atmospheric neutrinos, neutrinos created when cosmic rays crash into particles in the upper atmosphere of the Earth.” Scientists concluded with 99 percent certainty that sterile neutrinos do not exist.

Francis Halzen, a University of Wisconsin-Madison professor of physics and principal investigator for the IceCube Neutrino Observatory, said in a statement: “Like Elvis, people see hints of the sterile neutrino everywhere. There was this collection of hints, and theorists were convinced it exists.”

With the new study, IceCube scientists have placed “strong limits on the possible existence of a sterile neutrino,” according to David W. Schmitz from the University of Chicago, who was reacting to the study in the same journal.

Three months ago, in May, IceCube scientists had precluded another chunk of the “parameter space” for finding neutrinos. With the new paper, scientists are still no closer to finding out why neutrinos have mass, a key to understanding many mysteries of the universe.