Grant: Subatomic Physics Envelope - Project

Neutrino oscillation experiments have demonstrated that neutrinos are massive but their absolute scale cannot be established from such measurements. Neutrinoless double beta decay (0nbb) experiments are able to probe whether neutrinos are Majorana particles, help determine the neutrino mass spectrum and explore physics beyond the Standard Model. The observation of this lepton number violating process will demonstrate the existence of a new mass generation mechanism unrelated to the Higgs field and shed light on the primordial physics responsible for the matter dominated universe that we are witnessing. The next-generation 0nbb experiments require tonne-scale active mass and extreme low background. The nEXO collaboration is proposing a 5-tonne liquid xenon (LXe) time projection chamber with sufficient sensitivity to the half-life of Xe-136 0nbb decay to be able to explore the inverted mass hierarchy region of the parameter space. To go further and reach the ultimate sensitivity to the normal hierarchy, we propose to explore a sophisticated technique for background rejection which calls for the detection of the single Ba ion produced uniquely by the double beta decay of Xe-136. Previous 0nbb searches have been background limited. With Ba ion tagging, the residual radioactive background can be completely rejected. Events from the high energy tail of the allowed 2nbb decay can only be mitigated by excellent detector energy resolution. The nEXO collaboration conducts an intense R&D program for the development of high-efficiency Ba ion tagging. Significant Canadian efforts have taken place as part of the EXO-Canada project until last year when the collaboration, along with feedback from NSERC, decided that the Ba ion tagging should be pursued as a standalone R&D program. We are proposing a coherent Canadian R&D program to integrate all the components needed for the application of efficient Ba ion tagging to a tonne-scale LXe detector: �demonstrate efficient ion collection from LXe using radioactive ions [at Carleton university] �demonstrate efficient ion extraction to a low-pressure analysis chamber using a RF ion funnel and a multi-reflection time-of-flight mass spectrometer [at McGill university] �design, optimize and construct a high signal-to-noise ratio linear Paul trap with ion cooling and bunching capabilities [at TRIUMF] �demonstrate optical discrimination of the Ba-136 ion against other natural occurring barium isotopes [at Carleton university] The refinement of the Ba ion tagging technique remains very challenging but the method provides a unique opportunity to fully eliminate the residual radioactive backgrounds and it will allow a future tonne-scale experiment to scan all the relevant parameter space associated with the normal mass hierarchy. Given the interdisciplinary nature of the program, we expect to attract many graduate students and provide great opportunities for training technicians and post-doctoral fellows.