Research and Development for a Next Generation Ge-76 Double Beta-Decay Experiment

Neutrinos are amongst the most abundant fundamental particles in the universe, yet understanding their exact nature has proven to be a most difficult experimental challenge. Determining these properties provides insight into our basic understanding of fundamental interactions and their role in the universe. Recent revolutionary measurements of neutrinos from the sun and from neutrinos created by cosmic rays in the atmosphere found the surprising result that neutrinos have non-zero masses, in disagreement with the long-held existing 'standard model' of fundamental interactions. (Two of these pioneering experiments were co-awarded the 2015 Nobel Prize in Physics for this discovery.)The fact that neutrinos are not massless, has opened the possibility to search for a long postulated rare decay process known as 'neutrinoless double beta decay'. If observed, this would prove that neutrinos are their own anti-particles (unlike all spin 1/2 electrically charged fundamental particles where the particle and anti-particles are distinct particles). Such a discovery would have profound implications, not only showing neutrinos are Majorana particles, but demonstrating that a fundamental symmetry of nature known as lepton number is violated, while also allowing one to precisely 'weigh' (determine the mass of) neutrinos.Neutrinoless double beta decay experiments use the atomic nucleus as an exquisitely sensitive laboratory to search for this postulated extremely rare process. Beta decay is the process where within an unstable atomic nucleus a neutron changes to a proton while emitting an electron and a neutrino. Likewise, there are a few unstable nuclei with the special property that they cannot beta-decay, but where it is possible to have what is known as two neutrino double beta decay, where two neutrons change to two protons while emitting two electrons and two neutrinos. This rare process has been observed in about a dozen or more different nuclei. The postulated even more elusive decay mode known as neutrinoless double beta decay where two neutrons change to two protons while emitting two electrons but zero neutrinos has not yet been observed.The Large Scale Ge double beta-decay consortium proposes R&D research activities to demonstrate the required down-selection criteria for a next-generation, large-scale neutrinoless double beta decay (0νββ) experiment using 76Ge in significant quantity to cover the inverted ordering mass region. A future large-scale 0νββ experiment has been identified as the top priority for new construction in the U.S. 2015 Long Range Plan for Nuclear Science. The sensitivity goal of such a future experiment requires the ability to probe 0νββ decay half-lives exceeding 1027 years.This proposal is founded on information and knowledge derived from the construction and operation of both the MAJORANA DEMONSTRATOR and the GERDA Phase I and II 76Ge-based 0nbb experiments. Initial background results recently reported by both GERDA Phase II and the DEMONSTRATOR experiments are the lowest achieved to date by any 0νββ experiments in the 0νββ region-of-interest. These low backgrounds coupled with the intrinsic superior energy resolution of Ge detectors offer a promising path to a next generation experiment. Based on these successes, a viable ton-scale 76Ge-based future experiment can be realized by deploying an array with a GERDA-like active veto while incorporating DEMONSTRATOR-like low activity materials that are in close proximity to detectors. However, even utilizing the strengths of both of these current experiments, demonstrating that one can attain sufficiently low backgrounds requires additional R&D efforts. We propose three R&D activities to demonstrate that a future 76Ge-based experiment can reach the necessary background levels: further development of robust signal and high voltage cables and connectors, additional studies of ultra-clean materials, and improvements to the active veto system. The outcomes from this proposed R&D will demonstrate that a future ton scale 76Ge 0νββ experiments can attain the requisite backgrounds while also benefiting other future experiments searching for rare phenomena, including 0νββ experiments and searches for dark matter