Phenomenology of neutrinoless double beta decay

Neutrinoless double beta decay (0νββ) violates lepton number by two units, a positive observation therefore necessarily implies physics beyond the standard model. Here, three possible contributions to 0νββ decay are briefly reviewed: (a) The mass mechanism and its connection to neutrino oscillations; (b) Left-right symmetric models and the lower limit on the right-handed W boson mass; and (c) R-parity violating supersymmetry. In addition, the recently published “extended black box” theorem is briefly discussed. Combined with data from oscillation experiments this theorem provides proof that the 0νββ decay amplitude must receive a non-zero contribution from the mass mechanism, if neutrinos are indeed Majorana particles.