Location: SNOLAB
Participating Canadian institutions: Carleton, Laurentian, McGill, Sherbrooke, TRIUMF, British Columbia
International partners: China, Germany, Russia, South Africa, Switzerland, USA
A major open question is the nature of neutrinos and how they influence the evolution of the universe. The discovery that neutrinos are not massless has been transformative in that a neutrino could have an astonishing property of being its own anti-particle. An extremely rare nuclear decay mode known as neutrinoless double beta (\(0\nu\beta\beta\)) decay offers the most sensitive experimental method to test for this possibility. The observation of this most exotic decay mode would provide irrefutable evidence that neutrinos are their own anti-particle and correspondingly that the symmetry of lepton number conservation is violated. Its observation would also provide strong experimental guidance for theories that go beyond the Standard Model, yielding insights into the origin of neutrino mass. In particular, if neutrinos are indeed their own antiparticles, they could not gain their mass through the interactions with Higgs particles in the same way as all other elementary particles in the Standard Model. Neutrinos with this property could also be key players in generating the observed excess of matter over anti-matter in our universe.
The nEXO experiment will search for \(0\nu\beta\beta\) decay in the isotope \(^{136}\)Xe. It is the successor to EXO-200, which over the past decade observed the \(2\nu\beta\beta\) in this isotope and carried out several searches for the neutrino-less mode. The goal of nEXO is to push the sensitivity by a factor of 100 or more, reaching half-lives of \(10^{28}\) years 1. The optimum location for nEXO is the SNOLAB underground laboratory in Sudbury, Ontario, shown as an artist’s conception in Figure 1. It has advantages of depth (equal very low cosmic background), extensive clean room facilities, and existing capabilities and expertise for the design, construction and operation of the experiment. With a final approval by DOE expected soon, the collaboration, which has a very substantial Canadian component with 6 involved institutions, has embarked on extensive R&D for key detector technologies. Current Canadian contributions include the development of novel photon sensors, assembly and testing of the light-collection system, radioactive background control, an external calibration source deployment system, a water shield with active muon veto, a water purification and assaying system, and SNOLAB infrastructure, and development of low-background techniques for a future upgrade.
Commissioning is expected around 2027/28, contingent on the DOE selection process; nEXO will then take data for at least one decade to reach the \(10^{28}\) year half-life goal.
Canadian groups constitute about 20-25% of the EXO-200 and nEXO Collaborations, and take substantial responsibilities, including chair of the EXO-200 collaboration board for the period of the last 5-year plan, serving as one of two EXO-200 analysis coordinators, and contributions to operations as part of the EXO-200 management team. Within nEXO, two Canadian PIs are Level-2 sub-systems physicists out of 11 subsystems (with the two subsystems as full Canadian responsibility), and three Canadian PIs hold L3 leadership responsibilities for subsystems in other WBSes. In parallel to the deployment of the nEXO apparatus, the Canadian team is pursuing techniques that would greatly suppress background, such as Barium tagging. In this approach, a small volume surrounding a \(0\nu\beta\beta\) event is extracted from the detector and probed for the presence of a Ba-ion, an unambiguous tag for a true decay event.