If you like discoveries of new fundamental particles, this is not a good month. Last week CERN announced that a rumored new particle was nothing but a fluke, and today comes the news that there is no fourth type of neutrino, a hypothetical particle that only interacts with gravity, known as a sterile neutrino.
The study, published in Physical Review Letters, comes from IceCube, a neutrino observatory in Antarctica. Scientists analyzed about 100,000 neutrino events observed in 2015 without finding a single indication of the existence of sterile neutrinos. The team is 99 percent certain that it does not exist.
The observatory is located in the South Pole, and the instruments occupy a volume of about 1 cubic kilometer (0.24 cubic miles) of clear Antarctic ice, hence the name IceCube. The observatory has 5,160 digital optical modules located at depths between 1,450 and 2,450 meters (4,760 and 8,040 feet). When a neutrino interacts with the ice it emits a cascade of particles, and the detectors record the energy and direction of the new particles. Researchers can then work out what the original neutrino looked like.
Sterile neutrinos were introduced to solve one of the problems with the Standard Model of particle physics. According to the model, there are three types of neutrinos (electron, muon, and tau) and they are massless. Neutrinos are only left-handed (their spin always points in the opposite way of their momentum), with this exclusivity only being allowed as long as they are massless.
In 1998, experimental evidence indicated that neutrinos were able to oscillate from one type to another, a discovery that earned the researchers the 2015 Nobel Prize in Physics, but which also meant that neutrinos could have mass. This led scientists to introduce the concept of a right-handed neutrino, called a sterile neutrino, to explain the limitations of the Standard Model.
The IceCube Neutrino Observatory instruments occupy a volume of roughly 1 cubic kilometer of clear Antarctic ice at the South Pole. IceCube collaboration.
“If you throw in a fourth neutrino, it changes everything,” explained Francis Halzen, a University of Wisconsin-Madison professor of physics and principal investigator for IceCube, in a statement.
“Sterile means it doesn’t interact with matter itself, although it can dramatically interfere with the way conventional neutrinos do.”
These neutrinos would only interact through gravity and none of the other forces, similar to dark matter, another mysterious and yet to be experimentally confirmed physical phenomenon. Although there has never been a discovery, several experiments had previously hinted at potential detections of sterile neutrinos.
“Like Elvis, people see hints of the sterile neutrino everywhere,” added Halzen. “There was this collection of hints, and theorists were convinced it exists.”
Although it is likely that the sterile neutrino doesn’t exist, this is not yet a scientific certainty. That will only be reached when they have reached a 99.99994 percent certainty. This might not be the end of sterile neutrino, but it’s a very strong indication that we need to start looking for different explanations.