Neutrinos, meaning ‘little neutral ones’, are everywhere, all around us. These tiny elementary particles travel through space at almost the speed of light and have no electric charge. They were once thought to have no mass either, but scientists recently proved that they do. The neutrino mass is estimated to be less than a billionth of the mass of a hydrogen atom. Research continues to find out how big this mass actually is.
There are three different kinds, or "flavours", of neutrinos: the electron neutrino, the muon neutrino and the tau neutrino, named after their partner lepton in the Standard Model.
Neutrinos interact only via the weak force, which is one of the four fundamental forces of nature. Their existence has been confirmed experimentally. A fourth, ‘sterile’ type has been proposed, which would be immune to the weak force of the Standard Model. Were sterile neutrinos to be found, a new realm of physics beyond the Standard Model would open up.
But even the three confirmed types of neutrinos are special: they "oscillate" – electron, muon and tau neutrinos change from one flavour to another.