NEXT-White results and roadmap towards a ton-scale detector

The goal of the NEXT collaboration is the sensitive search of the neutrinoless double beta decay of ¹³⁶Xe at the Laboratorio Subterráneo de Canfranc. A first large-scale prototype of a high-pressure gas-Xenon electroluminescent TPC, NEXT-White, was operated from 2016 to 2021. This 4–kg radiopure detector has proven the outstanding performance of the NEXT technology in terms of the energy resolution (<1% FWHM at 2.6 MeV) and the topology-based background rejection. After measuring the relevant backgrounds and their spatial origin, the NEXT-White data collected with ¹³⁶Xe-enriched and ¹³⁶Xe-depleted gas have been used to perform a measurement of the two-neutrino mode of the double beta decay. A novel technique based on a direct background subtraction yields a half-life of ${2.34}_{-0.46}^{+0.80}{(\mathrm{stat})}_{-0.17}^{+0.30}\mathrm{(sys)}\times {10}^{21}\,\mathrm{yr}$. This result is consistent with the half-life derived from a background-model-dependent approach. Scaling the technological solutions of NEXT-White, the NEXT-100 detector is being constructed during 2022 and will start the physics data taking in 2023. Holding 100 kg of ¹³⁶Xe and with a background index below 5×10⁻⁴ counts/keV/kg/year, NEXT-100 will perform the first competitive search for the neutrinoless double beta decay within the NEXT roadmap. After 3 years of data taking, a sensitivity to the half-life of 6×10²⁵ yr at 90% C.L. will be achieved. The design for a future ton-scale detector, NEXT-HD, is being carried out in paralel. Operating with low-difussion gas mixtures and a reduced background budget, NEXT-HD will reach sensitivities above 10²⁷ yr. A promising R&D line aims at eventually equiping the NEXT techonlogy with ¹³⁶Ba-tagging capabilities, opening the doors for a truly background-free experiment.