nEXO: neutrinoless double beta decay search beyond 10²⁸ year half-life sensitivity

Abstract

The nEXO neutrinoless double beta (0νββ) decay experiment is designed to use a time projection chamber and 5000 kg of isotopically enriched liquid xenon to search for the decay in 136Xe. 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 1028 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 × 1028 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.

Toward the discovery of matter creation with neutrinoless double-beta decay

“…An ex-TABLE VI Specific parameters of Xe-TPC experiments: total mass, enrichment fraction, phase, signal readout, effective background index in units of events per kg of mass in the fiducial volume, and the ratio R between the effective background index used for our sensitivity calculation and the mean background quoted by the experiments, when available. Values are taken from references (Adams et al, 2021a;Adhikari et al, 2022;Agostini et al, 2020a;Akerib et al, 2020;Al Kharusi et al, 2018;Alvarez et al, 2012;Anton et al, 2019;Chen et al, 2017;Martín-Albo et al, 2016) tensive screening campaign (Leonard et al, 2008(Leonard et al, , 2017 and a sophisticated analysis incorporating topological background discrimination (Delaquis et al, 2018) led to an averaged background level within the fiducial volume of 1.8 • 10 −3 events/(keV• kg• yr), corresponding to B =4.7•10 −2 events/(mol• yr), dominated by the 214 Bi gamma line originating from 238 U chain decays outside of the Xe volume. The experiment reported an ultimate limit for 0νββ decay of T 1/2 > 3.5 • 10 25 yr at 90% C.L.…”

“…nEXO (Adhikari et al, 2022;Al Kharusi et al, 2018) is a proposed upgrade of EXO-200 using 5 tons of Xe enriched to 90% in 136 Xe. The TPC design features one monolithic drift volume with ionization read out by silica tiles patterned with metallic electrode strips, and scintillation detection by an array of VUVsensitive silicon photomultipliers on the TPC walls behind the field-shaping grid, yielding an energy resolution of σ = 20 keV.…”

Development of a $^{127}$Xe calibration source for nEXO

“…In this work we start from the reconstruction algorithm used in Ref. [2] and extend it with simplified models of the readout noise to generate the charge and light detection signals.…”

Decomposition of d = 9 short-range 0νββ decay operators at one-loop level

“…The most stringent bound is given by the KamLAND-Zen collaboration with the half-life T 0ν 1/2 > 1.07×10 26 years for 136 Xe at 90% confidence level, this bound implies a limit on the effective Majorana neutrino mass |m ββ | < (61-165) meV [3]. The next generation tonne-scale 0νββ decay experiments such as LEGEND [4] and nEXO [5] aim at the halflife sensitivity of approximately 10 28 years. If all the future results from 0νββ decays, neutrino oscillation experiments and cosmology observation on the light neutrino mass sum are compatible with each other, it would be a strong support to the mass mechanism mentioned above.…”