A Search for Neutrino-less Double Beta Decay with nEXO

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One fundamental question in science today is why the Universe and everything in it is made almost exclusively of matter and essentially no anti-matter when the equations describing Nature lead us to expect a nearly equal amount of matter and anti-matter. This award supports investigations at the University of Massachusetts, Amherst into one possible origin of this imbalance through the study of whether the lightest fundamental particle in Nature, the neutrino, is made of both matter and anti-matter. If this is the case, a very rare nuclear process known as neutrino-less double beta decay is possible. This decay, in which a nucleus transforms into another by emitting two electrons and nothing else, would unambiguously determine that neutrinos and anti-neutrinos are the same particle, i.e. that they are Majorana particles. The work at UMass Amherst focuses on key R&D aspects for the design of the detector for the nEXO experiment, with research carried out in the lab with postdocs and students (graduate and undergraduate), in an academic, research-intensive environment. The PI and his group are also involved in operating and analyzing the data of the EXO-200 experiment, a smaller scale nEXO predecessor running in New Mexico and holding one of the best sensitivities for neutrino-less double beta decay to date. The activities supported by this award train human resources of diverse backgrounds and develop technologies that align with strategic sectors for the US, such as data science and artificial intelligence, nuclear medicine and non-proliferation, and national security programs at national laboratories. The UMass Amherst particle astrophysics group has a record of excellence and diversity for undergraduate involvement in research, and the Amherst Center for Fundamental Interactions (ACFI) actively contributes to promoting the physics related to this proposal to the broader physics community.The core of the nEXO detector is a Time Projection Chamber (TPC), a technology that can identify and suppress with high efficiency most background signals mimicking neutrinoless-double beta decays. The detector will measure the position and energy of each ionizing event occurring inside its volume, as well as finer information about the spatial distribution and sharing of event energy between two detection channels, ionization and scintillation light. Combined with a detector design that minimizes the residual radioactivity in its constituents, this information provides a powerful environment for a neutrino-less double beta decay signal to emerge once all other interactions are properly identified. The UMass Amherst group works on the development of key elements of nEXO. The PI coordinates the design of the nEXO TPC. With his group, he studies novel silicon-based detectors (specifically Silicon PhotoMultipliers, SiPMs) sensitive to the xenon scintillation light (178 nm) that combine good light collection efficiency, minimal radioactive contamination, uniformity of response, and are practically available to cover several square meters of surface in nEXO. An integral part of the program is a continued participation in the EXO-200 experiment, and the analysis of its data. Of particular interest are refined searches for neutrino-less double beta decay of xenon-136 and xenon-134.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.