After analysing thousands of neutrinos in the IceCube Neutrino Observatory at the South Pole, researchers from the Niels Bohr Institute have now confirmed that there is no evidence for the existence of sterile neutrinos.
Neutrinos, also known as ghost particles, are some of the most abundant in the universe. and potentially hold answers to some of the more mysterious universal phenomena. Able to travel through virtually anything, neutrinos have thus far been identified as falling into three types: muon, electron and tau. The fourth type, the sterile neutrino, was thought to potentially explain phenomena like dark matter.
There have been previous indications of the sterile neutrino in experiments from across the world, prompting great interest from the research community, particular as sterile neutrinos may have different properties, such as interacting only be means of gravity.
“One or more types of sterile neutrinos could help solve a number of mysteries, such as why there is more matter than antimatter in the universe,” explained Jason Koskinen, assistant professor at the Niels Bohr Institute at the University of Copenhagen and group leader in the IceCube research group at the Niels Bohr Institute and team chairman of the IceCube international research team on neutrino oscillations.
“A sterile neutrino could provide an explanation for this imbalance, which currently cannot be explained by the three known neutrinos. A sterile neutrino with gravity could also shed light on the mysterious dark matter.”
When high energy protons hit Earth’s atmosphere they create a shower of particles that includes neutrinos. Due to their unique properties, the neutrinos then pass straight through the Earth while the other particles collide with it.
However, a minute proportion of those forming over the North Pole will end up hitting the ice at the South Pole, where they can be detected by the IceCube detector, a setup consisting of 5,160 light sensors that are frozen in Antarctica’s ice.
The operating theory so far has been that the sterile neutrino could be formed during the fluctuations that cause neutrinos to shift between muon, electron and tau forms. These quantum changes occur during the particles’ travel and, even if there were to be a fourth version, should all be detectable by the IceCube.
“We have analysed hundreds of thousands of neutrinos, which after having passed through the Earth from the Northern Hemisphere have hit the ice on the South Pole, where the collisions have been recorded in the IceCube detector,” explains Koskinen.
“We know of three neutrino types and our international team of researchers has been looking for signals from a fourth neutrino type, the so-called sterile neutrinos. For years, there has been a global mystery about the existence of a sterile neutrino with a mass of about 1 eV. If it existed, it would produce a clear signal at a certain energy interval, but we have not seen a single signal that could come from such a sterile neutrino.”