The neutrino is one of the many elementary particles which make up the universe. Neutrinos are produced in the fusion reactions inside the sun and other stars, by natural background radiation inside the earth, by supernovae, and by charged particles bombarding Earth’s atmosphere. Despite their abundance, neutrinos remain mysterious because they are difficult to observe. Neutrinos are electrically neutral and only interact via the weak interaction. In the Standard Model of particle physics, neutrinos are assumed to be massless. However, neutrino experiments have allowed us to observe the phenomenon of neutrino oscillations, which can only occur for massive neutrinos. Thus there is compelling evidence that neutrinos do have mass, albeit a very small one.
There are three "flavors" of light neutrinos: electron neutrino, muon neutrino, and tau neutrino, named after their charged particle partners. Based on quantum mechanical principles, if neutrinos have a nonzero mass and the mass eigenstates do not correspond to the flavor eigenstates, then neutrinos can mix. The mixing between the mass and flavor states is governed by the Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrix. (This is analogous to the mixing in the quark sector and the Cabibbo–Kobayashi–Maskawa (CKM) matrix.) Due to this mixing, a neutrino that is created in one flavor state can be observed some time later in a different flavor state. The probability for oscillations depends on the elements of the PMNS matrix, the difference in the squared masses of the neutrinos, the distance the neutrino has traveled, and the energy of the neutrino.
My group is currently participating in the NOvA experiment, a neutrino oscillation experiment that is currently running, and DUNE, a future neutrino oscillation experiment.
More details coming soon!