Neutrinoless double beta decay is a very rare nuclear transition, whose existence can provide essential information about neutrino properties like the hierarchy and the absolute mass scale.
T2K (Tokai to Kamioka) is the world leading experiment dedicated to the study of neutrino oscillations over a long distance using µ-neutrino and µ-anti-neutrino beams.
Neutrinos exist in three different types (called 'flavours'): νe, νμ and ντ.