## Double Beta Decay

The nuclear double beta decay is a rare and second order process involving weak interaction and takes place between two even Z-even isobars differing in charge by two units, where single beta decay is kinematically forbidden

### Two Neutrino DBD

In two neutrino DBD, two neutrons undergo beta decay simultaneously giving two elctrons and two anti-electron neutrinos. This is allowed in the Standard Model as it conserves lepton number and typical half lives are > 10^{17} yrs. Here, the sum energy of the two beta paricles exhibit a continuous beta decay spectrum between 0 and the Q-value of the process. It has already been observed for the 0^{+} → 0^{+} transition in 11 nuclei , namely ^{48}Ca, ^{76}Ge, ^{82}Se, ^{96}Zr, ^{100}Mo, ^{116}Cd, ^{128,130}Te, ^{136}Xe, ^{150}Nd and ^{238}U, out of 35 possible candidates.

### Neutrinoless DBD

In 1939, W.Furry conjectured another decay mode, namely electron emitting neutrionless double beta decay. Its mechanism is based on the emission of an electron anti-neutrino on the first decay vertex and its absorption in the second vertex. This lepton number violating process is possible only if the neutrino is a Majorana particle. Its experimental signature is the sum-energy peak of of the two elctrons at the Q value of the process. It has not been observed so far and the current best lower limits are 1.07 x 10^{26} yrs (90% C.L.) in ^{136}Xe and 2.1 x 10^{25} yrs (90% C.L.) in ^{76}Ge.

Figure : A schematic of the distribution of the sum of electron energies for the two modes of DBD decay. |