Neutrinoless double beta decay

The oscillation experiments in the last decades have demonstrated that neutrinos are not massless particles, as described in the Standard Model, and that the lepton flavor is not conserved. Revealing the nature of neutrino masses is therefore one of the major goals in particle physics. Regardless of the underlying decay mechanism, the observation of the neutrinoless double beta decay (0νββ) has been identified as the most practical way to establish that neutrinos are Majorana particles, that is, fermions equivalent to their anti-particles. When the beta decay is highly suppressed or energetically forbidden, even-even nuclei can undergo double beta decay (ββ), a process in which two bound neutrons are simultaneously transformed into two protons plus two electrons. The mode of this decay emitting two antineutrinos (2νββ) has been directly observed in ten nuclides with half-lives in the range of ∼10¹⁹–10²¹ yr. However, the neutrinoless mode, which would violate lepton number by two units, has not been observed yet and the best limits to the half-life of this process exceed 10²⁶ yr. In the quest towards the discovery of the 0νββ decay, several technological approaches and isotopes are being explored. The NEXT collaboration aims at the competitive search for this decay in ¹³⁶Xe using high-pressure gas electroluminescent time projection chambers (TPCs).