nEXO: a Search for Neutrino-less Double Beta Decay with a Tonne-Scale Enriched Xenon Time Projection Chamber

A fundamental question in contemporary science is why the Universe contains more matter than anti-matter. This award supports investigations into one possible explanation for the origin of this imbalance through the study of whether the mass of the lightest fundamental massive particle in Nature, the neutrino, is matter or anti-matter, or perhaps is a combination of both at the same time.

A neutrino could simultaneously be both, enabling a nuclear process known as neutrino-less double-beta decay in which a nucleus transforms into another by emitting two electrons and no neutrinos. If observed, this decay would indicate that neutrinos and anti-neutrinos are the same particle. The NSF/DOE Nuclear Science Advisory Committee (NSAC) has recommended a US-led, ton-scale neutrino-less double-beta decay experiment as the top priority for the field.

The UMass group, along with scientists from universities and national laboratories in the US, Canada, China, France, Germany, Russia, South Africa, South Korea, and Switzerland have proposed the nEXO experiment to search for neutrino-less double-beta decay of xenon-136. Using five tons of isotopically enriched liquid xenon nEXO will achieve a 100-fold improved sensitivity with respect to today’s experiments.

The work at UMass focuses on R&D for the design and optimization of nEXO and the extraction of science from the dataset previously recorded by EXO-200, the nEXO predecessor. The nEXO detector will be built around a Time Projection Chamber (TPC) that allows scientists to identify and suppress background signals which can mimic neutrino-less double-beta decay.

The detector measures the position and energy of every ionizing event inside its volume. Combined with a detector design that minimizes the radioactivity in its constituents, the feeble neutrino-less double beta decay signal can emerge above environmental noise.

The UMass group will work on the development of key elements for the nEXO detector. The PI is the Level-2 scientist for the nEXO TPC Support (TPCS) team and the UMass group will develop cryogenics, refrigeration infrastructure, Liquid Xenon storage capabilities, xenon handling, and purification systems. The PI will also study the long-term stability of VUV-sensitive Silicon PhotoMultipliers (SiPMs) for nEXO using a cryostat where xenon can be condensed into a test cell.

The group will develop a calibration source for nEXO which can be injected into the xenon flow stream. And the group will also search for sub-dominant neutrino-less double beta decay of xenon-136 and xenon-134 in the previously recorded EXO-200 dataset. The activities covered by this award will train students in STEM fields, open doors for students from diverse backgrounds, and develop technologies that align with strategic sectors of our economy such as data science, artificial intelligence, nuclear medicine, and national security.

The Amherst Center for Fundamental Interactions (ACFI) contributes to promoting the physics related to this proposal to the broader physics community.

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.