The NEXT experiment for neutrinoless double beta decay searches

The Neutrino Experiment with a Xenon TPC (NEXT) seeks to discover the neutrinoless double beta ($\beta\beta 0\nu$) decay of $^{136}$Xe using a high-pressure xenon gas time projection chamber with electroluminescent amplification. The observation of $\beta\beta 0\nu$ decay would prove that neutrinos are Majorana particles and that lepton number is not conserved. The NEXT detector concept offers several features of great value for $\beta\beta 0\nu$-decay searches, including excellent energy resolution, tracking for the active suppression of backgrounds and scalability to large source masses. The initial phase of the NEXT project (2009-2014) was devoted to R&D with two prototypes (DEMO and DBDM) of approximately 1 kg of active xenon mass that demonstrated the performance of the detector concept. During the second phase of the project (2015-2019), the NEXT Collaboration has operated underground at the Laboratorio Subterr\'aneo de Canfranc, in Spain, a radio-pure detector of about 5 kg of xenon mass. The goal of the current phase is the construction, commissioning and operation of the NEXT-100 detector, with a predicted sensitivity to the $\beta\beta0\nu$ decay half-life of $6\times10^{25}$ years (90% CL) after a run of 3 years. The Collaboration is planning as well a future tonne-scale phase to explore $\beta\beta 0\nu$-decay half-lives beyond $10^{26}$ years.