Double beta decay is a rare kind of radioactive decay in which two neutrons decay into two protons, two electrons and two antineutrinos. This process can occur through two known mechanisms, namely the neutrinoless double beta decay and the two-neutrino double beta decay. Neutrinoless double beta decay occurs only if neutrinos are Majorana particles, and so far, no evidence has been found for this phenomenon. Two-neutrino double beta decay, on the other hand, is a standard model process which has been observed in several isotopes. Other synonyms for double beta decay are beta-beta decay and double beta emission, and they are used interchangeably in the field of nuclear physics.