NEXO: Neutrinoless double beta decay search beyond $10^{28}$ year half-life sensitivity

TL;DR

The nEXO experiment aims to detect neutrinoless double beta decay in xenon-136 with a sensitivity surpassing 10^28 years, utilizing advanced detector design, simulation, and background reduction techniques to explore neutrino mass properties.

Contribution

This work presents an improved detector design, enhanced simulation accuracy, and refined background mitigation strategies, enabling nEXO to achieve unprecedented half-life sensitivity in neutrinoless double beta decay searches.

Findings

Projected half-life sensitivity of 1.35×10^28 years at 90% CL in 10 years

Significant background reduction through use of electroformed copper

Robustness of the experiment against background deviations

Abstract

The nEXO neutrinoless double beta decay experiment is designed to use a time projection chamber and 5000 kg of isotopically enriched liquid xenon to search for the decay in $^{136}$Xe. Progress in the detector design, paired with higher fidelity in its simulation and an advanced data analysis, based on the one used for the final results of EXO-200, produce a sensitivity prediction that exceeds the half-life of $10^{28}$ years. Specifically, improvements have been made in the understanding of production of scintillation photons and charge as well as of their transport and reconstruction in the detector. The more detailed knowledge of the detector construction has been paired with more assays for trace radioactivity in different materials. In particular, the use of custom electroformed copper is now incorporated in the design, leading to a substantial reduction in backgrounds from the intrinsic radioactivity of detector materials. Furthermore, a number of assumptions from previous sensitivity projections have gained further support from interim work validating the nEXO experiment concept. Together these improvements and updates suggest that the nEXO experiment will reach a half-life sensitivity of $1.35\times 10^{28}$ yr at 90% confidence level in 10 years of data taking, covering the parameter space associated with the inverted neutrino mass ordering, along with a significant portion of the parameter space for the normal ordering scenario, for almost all nuclear matrix elements. The effects of backgrounds deviating from the nominal values used for the projections are also illustrated, concluding that the nEXO design is robust against a number of imperfections of the model.