Grant: Subatomic Physics Envelope - Project

This application seeks continuing support for the search for neutrinoless double beta decay program in xenon with nEXO in Canada. The nEXO Canada group is part of the nEXO international collaboration and consists of 13 PIs at four universities (Carleton, Laurentian, McGill, Sherbrooke) and two national laboratories (SNOLAB, TRIUMF) with an average of 45 HQP being trained per year. Neutrinoless double beta decay (0?ßß) is recognized as one of the most sensitive avenues in the search for physics beyond the Standard Model of particle physics. nEXO is a next-generation detector under development to search for 0?ßß in the isotope Xe-136. The discovery of 0?ßß would be a major breakthrough towards understanding the origin and evolution of the Universe. It would demonstrate that the neutrino is its own antiparticle and may also provide direct access to the, so far elusive, absolute neutrino mass scale. The search for 0?ßß is identified by the Canadian Subatomic Physics Long Range Plan 2022-2026 as a priority. nEXO at SNOLAB is identified in TRIUMF's Five Year Plan 2020-2025 as one of the top priorities. Both SNOLAB and TRIUMF have approved nEXO in the "Gate 0" stage in their project governance, demonstrating the alignment of the project with their respective strategic plans. Using 5 tonnes of enriched xenon in a cryogenic detector, ideally located at the Canadian deep-underground laboratory, SNOLAB, the projected half-life sensitivity of nEXO will reach beyond 1028 years (arXiv:2106.16243), a two orders of magnitude improvement over current limits. This significant increase in sensitivity is obtained by an increase of the Xe-136 target mass along with detector improvements that were achieved by an extensive R&D program. Canadian groups continue to significantly contribute to nEXO's development program in areas aligned with their expertise: the photon detection system, outer detector muon veto and radiation shield, simulations, radioactive background control, calibration system, high voltage R&D, and cryogenics. The proposed research program aims at maintaining Canadian leadership in our areas of expertise and expanding it to deliver critical infrastructure and services towards the construction of nEXO. The nEXO-Canada research program is of great breadth and offers excellent opportunities for training of HQP in cutting-edge technologies. We are confident that the future nEXO detector will be a unique and competitive technology for the successful search of the neutrinoless double beta decay. We are well motivated to see this experiment installed and operating at SNOLAB, the deepest clean-room underground laboratory in North America.