# Understanding the properties of neutrino mass: the nEXO experiment for neutrinoless double beta decay

## Andrea Pocar

### (University of Massachusetts, Amherst)

nEXO will search for neutrinoless double beta (0νββ) decay in 5 tonnes of xenon enriched to 90% in the ββ-decaying isotope xenon-136 [1]. The observation of 0νββ decay would imply lepton number non-conservation, an otherwise conserved quantity in nature. At the same time, it would require neutrinos and anti-neutrinos to be the same particle, i.e., a Majorana fermion, a unique property among fundamental particles of the Standard Model. The observation of 0νββ decay could occur via different underlying microphysics. A unique mass mechanism exists for Majorana particles that could explain why neutrinos are so light and link the neutrino mass with the 0νββ decay rate.

nEXO operates with 5 tonnes of liquid xenon (LXe) in single-phase in a cylindrical time-projection chamber (TPC) with diameter and height of about 1.3m [1]. The nEXO TPC measures the energy, position, and topological multiplicity of each event. By the simultaneous event-by-event detection of ionization and scintillation nEXO will have an anticipated energy resolution of better than 1% at the ββ endpoint with a projected 0νββ decay half-life sensitivity of 1.35 x 1028 years (90%CL) after 10 years of lifetime [2].

The ionization electrons are recorded by a segmented anode plane at one end of the TPC [3]. The prompt VUV scintillation photons (175 nm) are recorded by a ~4.5m2 array of VUV-sensitive silicon photomultipliers (SiPMs) mounted on the cylinder barrel surface [4]. Both detection systems will be immersed in the LXe, along with the ASICs-based front-end readout electronics. Integration of SiPMs into larger modules is currently being detailed.

I will present the nEXO program in the context of the global search for 0νββ decay, link it to its predecessor experiment, EXO-200, discuss the nEXO detector design and illustrate aspects of the ongoing related R&D.

References
[1] nEXO Collaboration (S. Al Kharusi, et al.), arXiv:1805.11142v2 (2018)
[2] nEXO Collaboration (G Adhikari et al.), J. Phys. G: Nucl. Part. Phys. 49, 015104 (2022).
[3] nEXO Collaboration (Z. Li, et al.), JINST 14, P09020 (2019)
[4] nEXO Collaboration (G. Gallina, et al.), NIMA 940, 371 (2019)

INDICO