A Midscale U.S. Proposal for the LEGEND 200 Experiment

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Neutrinos are the most prolific particle in the Universe and they have an important role in understanding why the Universe has more matter than anti-matter. Neutrinos have very small mass and zero electric charge so they could be their own anti-particle. If that is the case, this might explain why there is more matter than anti-matter. The only experimentally feasible way to answer this question is to look for neutrinoless double beta decay (NLDBD), a very rare form of nuclear decay. This award will support a project that adds to and brings together the best U.S. (Majorana Demonstrator) and European (GERDA) experiments using germanium to look for neutrinoless double beta decay into a new experiment called LEGEND-200. The experiment will use 200 kg of special detectors made out of enriched germanium and will take place at the Laboratori Nazionali del Gran Sasso in Italy. Among the fields that may benefit from the new technology developed as part of the experiment are: 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. Students and postdocs will be trained in underground science related disciplines, such as low-background techniques, detector technology, nuclear physics and neutrino physics. The current generation Ge-76 experiments, GERDA and the Majorana Demonstrator (MJD), utilizing high purity Germanium detectors with an intrinsic energy resolution of 0.12%, have achieved the lowest backgrounds by over an order of magnitude in the NLDBD signal region compared to other NLDBD experiments. Building on this success, the LEGEND Collaboration is developing a next-generation NLDBD experiment with discovery potential at a half-life of 1028 years. This award provides mid-scale support for U.S. involvement in building and commissioning an almost-next-generation NLDBD experiment using up to 200 kg of Ge-76 diodes ie LEGEND-200 (L-200) that has potential to deliver physics and serve as a forerunner to a future ton-scale experiment. L-200 is based on combining the techniques developed by MJD to produce ultra-clean components and GERDA's background suppression from shielding the detectors in a liquid argon bath, in addition to physically combining the existing MJD and GERDA detectors, resources and infrastructure with new detectors. L-200 will be based at the Laboratori Nazionali del Gran Sasso in Italy, and deploy the detectors in the existing GERDA cryostat. L-200 would have world leading discovery potential and a half-life sensitivity of 10^27 yr for a 1 t*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.