Design of the NSF LEGEND-1000 Project

About This Grant

The Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND) uses an isotope of germanium, Ge-76, to search for a postulated rare decay process known as neutrinoless double beta decay (NLDBD). The observation of NLDBD would reveal the quantum nature of the neutrino, demonstrate matter creation, reveal that neutrinos and antineutrinos are indistinguishable, and offer a potential explanation of the mystery of why we see the predominance of matter over antimatter in the universe. LEGEND-1000, an international experiment with participation of over 60 institutions in the U.S. and Europe, aims to answer these high priority questions in fundamental physics. Once constructed, it will achieve world leading discovery sensitivity for NLDBD. LEGEND-1000 will be built with support from the NSF, the U.S. Department of Energy, the Laboratori Nazionali del Gran Sasso, and science agencies in Italy, Germany, Poland, Switzerland, and the United Kingdom. This NSF grant supports the final design of the NSF portion of the LEGEND-1000 project. Potential benefits of this research include fundamentally changing our understanding of the nature and origin of matter, should the decay be observed. Additionally, the technology of large, low-background Ge radiation detectors will enable a new generation of highly-efficient, ultra-low-background gamma spectroscopy measurements. Among the fields that stand to benefit from this technology are: quantum computation and sensors; direct dark matter searches; nuclear structure; nuclear astrophysics; environmental monitoring; atmospheric, ocean, and groundwater environmental transport; methods of radioactive dating; reactor monitoring; bioassay for determining very low occupational exposures to radiation; and biological studies involving radiotracers at very low activities. Likewise many of the same fields will benefit from LEGEND’s production of ultra radio-pure materials, with natural U and Th reduced to ultra-low levels. These technology advances will also likely impact non-low-background applications such as nuclear medicine and Homeland Security. In designing LEGEND-1000, students and postdoctoral fellows will receive training in experimental design, low-background methods, detector technology, cryogenics, nuclear physics and neutrino physics. Neutrinos have been at the forefront of discovery in nuclear and particle physics for decades. The study of their properties drove the conception of the weak interaction and modern quantum field theories. With the realization that neutrinos have small, non-zero masses there is intense interest in understanding their mass generation mechanism and determining the absolute neutrino mass scale and spectrum. Intriguingly there is no fundamental symmetry that would preclude each neutrino mass eigenstate being identical to its anti-particle, that is: a “Majorana” particle. There is also another central question – is lepton number conserved? Experimental evidence of NLDBD decay would demonstrate lepton number violation, definitively establish the Majorana nature of neutrinos, and provide information about the absolute neutrino mass. It would also hint at mechanism for generating the observed matter-antimatter asymmetry in the universe. LEGEND-1000 builds on its predecessor, LEGEND-200, in using novel, large high-purity Germanium radiation detectors with an intrinsic energy resolution of 0.1% that are surrounded by low-Z shielding (water and argon). The instrumentation of the liquid argon provides an active veto through the detection of argon scintillation light. This proposal will complete the final design phase for the NSF portion of the LEGEND-1000 experiment, which includes providing over 400 kg of the planned 1000 kg of detectors. LEGEND-1000 is designed to achieve a discovery potential that covers the inverted-ordering neutrino mass scale region. It will have world leading discovery potential and a half-life sensitivity of > 10^28 yr for a 10-ton yr exposure. 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.