Nuclear Double‐Beta Decay and Derivation of Neutrino Masses

The standard model (SM) of weak interactions incorporates neutrinos as massless particles which are left‐handed partners of the charged bosons. However, in the recent past, convincing evidence of massive neutrinos has been obtained in experiments that observe neutrino oscillations. The observation of the neutrino oscillations determines only mass square differences of the neutrino types and thereby sets a lower bound on the largest of the neutrino masses. So, the absolute scale of the neutrino masses remains still unknown. Neutrinoless double beta decay experiments are at present the most sensitive ones for measuring the absolute mass of the electron neutrino. Moreover, these are the only experiments that could answer the fundamental question of whether neutrinos are their own anti‐particles, that is whether they are Majorana or Dirac particles. That is why the double‐beta decay nuclear process is studied intensively both theoretically and experimentally. In this lecture I give an up‐to‐date review on this process. After a general introduction in which I stress the importance of the ββ decay study for searching beyond SM physics, I refer to the calculation of nuclear matrix elements involved in ββ decay half‐lives and to the to derivation of neutrino mass in connection to other constraints coming from neutrino oscillation experiments and cosmological data.