Neutrino
A fundamental particle produced by a decay caused by the weak interaction (e.g. beta radioactivity of unstable atomic nuclei). There are three different types of neutrinos, each associated with the electron, the muon, and the tau. Neutrinos were long thought to have zero mass. Recent experiments (based on the study of neutrinos' oscillations) have shown that their masses are very small compared to other particles, but not null.