Neutrinoless double beta decay (NLDBD) offers a window to new physics. It provides a sensitive probe into the mysteries of the neutrino, including their Dirac or Majorana nature, with implications for unsolved questions in physics, such as the observed excess of matter in the universe. This was highlighted in the 2015 Nuclear Science Advisory Committee Long-Range Plan, which recommended the timely development of a U.S.- led ton-scale experiment.

Neutrinos are point-like fundamental particles similar to electrons, yet they do not have an electric charge. These traits would allow them to be Majorana fermions, blurring the distinction between particle and anti-particle and allowing oscillation between matter and anti-matter versions of themselves. The magnitude of the oscillation rate could be linked to the currently unknown absolute mass of neutrinos. Understanding neutrino nature may shed light on their unnaturally light mass and the matter/antimatter asymmetry in the universe.

Argonne is making strategic investments in this area by working with the Neutrino Experiment with Xenon Time Projection Chamber (NEXT) collaboration, which uses pressurized xenon gas for its excellent energy resolution, access to event topology, and continuous purification of fluid source material. Argonne’s xenon work is focused on designing and prototyping key systems for the upcoming 100-kg NEXT-100 experiment (Figure 4.4-1), including the field cage and high-voltage feedthrough, as well as developing a one-of-a-kind local test facility and R&D program for new detector systems suited for high- pressure xenon time projection chambers (TPCs). We will design, develop, and prototype a range of components needed in next-generation pressurized gas xenon experiments. The expertise leveraged in a high voltage, high-pressure design will put Argonne at the center of planning and executing a next-generation 1-ton- scale experiment in the next decade.

The key advantage of high-pressure xenon TPCs for NLDBD searches is the combination of topological information with precision calorimetry. Techniques for calorimetry are conceptually simple and well established, but Argonne scientists have been leading the development of AI methods for topological selection of events in NLDBD searches. By leveraging supercomputing facilities at Argonne and Oak Ridge, we have achieved state-of-the-art results in mere minutes of AI training, which are scalable to all future xenon TPC detectors. Further development of AI techniques in NLDBD is a key element of our NLDBD program.