Neutrinoless Double-Beta Decay: a Probe of Physics Beyond the Standard   Model
  In the Standard Model the total lepton number is conserved. Thus, neutrinoless double-beta decay, in which the total lepton number is violated by two units, is a probe of physics beyond the Standard Model. In this review we consider the basic mechanism of neutrinoless double-beta decay induced by light Majorana neutrino masses. After a brief summary of the present status of our knowledge of neutrino masses and mixing and an introduction to the seesaw mechanism for the generation of light Majorana neutrino masses, in this review we discuss the theory and phenomenology of neutrinoless double-beta decay. We present the basic elements of the theory of neutrinoless double-beta decay, our view of the present status of the challenging problem of the calculation of the nuclear matrix element of the process and a summary of the experimental results. 
S.M. Bilenky and C. Giunti
hep-ph hep-ex