Since the 1990s, physicists have pondered the tantalizing possibility of an exotic fourth type of neutrino, called the “sterile” neutrino, which does not interact with ordinary matter at all, except perhaps with its fellow neutrinos. But definitive experimental evidence for the existence of sterile neutrinos has remained elusive. Now it looks like the latest results from the MiniBooNE experiment at Fermilab have completely ruled out the sterile neutrino. paper published in the journal Nature.
How did the possibility of the existence of sterile neutrinos arise in the first place? This is all goes back to the so-called “solar neutrino problem”. Physicists discovered the first solar neutrinos from the Sun in 1966. The only problem was that far fewer solar neutrinos were detected than theory predicted. This mystery became known as the solar neutrino problem. In 1962, physicists discovered second type (“flavor”) of neutrinos, muon neutrino. This was followed by the discovery of a third fragrance, neutrino numberin 2000.
Physicists already suspected that neutrinos could switch from one flavor to another. In 2002scientists from Sudbury Neutrino Observatory (or SNO) announced that they had solved the solar neutrino problem. The missing solar (electron) neutrinos were simply disguised, becoming a different flavor during the long journey between the Sun and Earth. If neutrinos oscillate, they must eventually have tiny masses. This created another difficult problem related to neutrinos. There are three flavors of neutrinos, but none of them have a clearly defined mass. Rather, different kinds of “mass states” mix in different ways to form electron, muon, and tau neutrinos. This is quantum weirdness for you.
And another mystery arose related to the results of the LSND experiment at Los Alamos and MiniBooNE at Fermilab (the predecessor of MicroBooNE). Both found evidence that muon neutrinos are converted into electron neutrinos, which would not be possible if there were only three types of neutrinos. So physicists have suggested that there might be a fourth flavor: the sterile neutrino, so named because, unlike the other three, it does not bond to a charged counterpart through the electroweak force. Its existence will also have big implications for the nature of dark matter. But despite the strangeness teasing hintsterile neutrinos have turned out to be insanely elusive.
