Search for the Neutrinoless Double Beta Decay with the LEGEND Experiment

Neutrinos, elementary particles that have no charge and were long believed to have zero mass, permeate the universe. They are notoriously difficult to detect, as they interact extremely weakly with any other known elementary particles. Three types of neutrino have been identified, presumably independent of each other.

Yet a remarkable discovery of neutrino oscillations between these types in 1998 demonstrated that neutrinos have mass. This discovery motivated further investigations of properties of neutrinos more than ever before. Unraveling the nature of neutrinos can have far-reaching implications for better understanding of elementary particles and their fundamental interactions.

Some natural conjectures suggest that deeper understanding of neutrinos may provide a glimpse onto phenomena at extremely high energies that existed in the early Universe, could broaden our knowledge of the fundamental interactions, may shed light onto the dominance of matter over antimatter, and can help to better understand the early evolution of the Universe. This award supports the PI and his group in conducting research in experimental neutrino physics and participating in the LEGEND experiment, which is designed to search for an extremely rare nuclear transition - neutrinoless double beta decay.

The LEGEND Experiment will not only advance our knowledge about some of the rarest phenomena in Nature but it will also develop a unique low background counting technique that may be applied outside of physics and be of general benefit to society. Applications may include measurements of anthropogenic radiation in the environment or ultra-sensitive detection of radioactivity.

These activities educate and train young researchers in building a state-of-the-art science project.

The LEGEND Experiment will employ high-purity germanium crystals grown out of enriched germanium-76. There will be about 300 detectors of a total mass of one ton. In a five-year exposure the experiment will reach the half-life sensitivity of about 10^{28} years.

To achieve this goal many sources of natural and cosmogenic radioactivity that may mimic a signal must be eliminated. LEGEND has adopted a multi-layered experimental setup composed of a water tank that houses a cryogenic dewar with both underground-mined argon, which is depleted in argon-39, and atmospheric liquid argon surrounding the crystals. Detector strings will be further actively shielded by light-collecting fibers or thin plates to tag and suppress background radioactivity in the vicinity of the crystals.

Ultra-high radio-purity and ultra-low radon emanation of all detector components are necessary to reach experimental objectives. The experiment is currently testing critical strategies by collecting data with about 200 kg of crystals and using a setup developed previously. This preliminary phase will clear a way for the new and better optimized detector configuration currently being designed.

The PI and his team will collaborate with institutions to simulate and construct prototypes of various configurations involving radio-pure plastics for detecting light-yielding events in liquid argon around germanium crystals. Achieving the objectives requires engaging industrial partners with which the group has had a long-standing cooperation.

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.