Discovery of Atmospheric Neutrino Oscillations

Neutrinos are a type of fundamental particles, like electrons and quarks. They have no electric charge and they come in three types (flavors), namely the electron neutrino (νe), the muon neutrino (νμ) and the tau neutrino (ντ). They are produced in various places such as in the Earth’s atmosphere and in the center of the Sun. The interactions of neutrinos with matter are so weak that they easily penetrate through the Earth and even the Sun. They do interact with matter, however, but only very rarely. Because a charged current (CC) νe (νμ,ντ) interaction produces an electron (muon, tau), physicists can determine a neutrino’s flavor by observing the lepton produced when it interacts. The Standard Model of elementary particle interactions was established in the early 1970s and it describes the strong, weak and electromagnetic interactions very well. However, in the Standard Model these interactions are not unified and are treated as independent entities. Neutrinos are assumed to be massless within the Standard Model. However, physicists have wondered whether neutrinos really have no mass. If, on the other hand, neutrinos do have mass, they can change their type from one flavor to the other(s). This phenomenon is called “neutrino oscillations,” and was theoretically predicted by Maki, Nakagawa and Sakata [1] and also by Pontecorvo [2]. For example, imagine that neutrino oscillations occur between νμ and ντ. Assuming that only these two