The Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND) (1709.01980v1)

Abstract: The observation of neutrinoless double-beta decay (0${\nu}{\beta}{\beta}$) would show that lepton number is violated, reveal that neutrinos are Majorana particles, and provide information on neutrino mass. A discovery-capable experiment covering the inverted ordering region, with effective Majorana neutrino masses of 15 - 50 meV, will require a tonne-scale experiment with excellent energy resolution and extremely low backgrounds, at the level of $\sim$0.1 count /(FWHM$\cdot$t$\cdot$yr) in the region of the signal. The current generation $^{76}$Ge experiments GERDA and the MAJORANA DEMONSTRATOR 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 0${\nu}{\beta}{\beta}$ signal region of all 0${\nu}{\beta}{\beta}$ experiments. Building on this success, the LEGEND collaboration has been formed to pursue a tonne-scale $^{76}$Ge experiment. The collaboration aims to develop a phased 0${\nu}{\beta}{\beta}$ experimental program with discovery potential at a half-life approaching or at $10^{28}$ years, using existing resources as appropriate to expedite physics results.

• The paper demonstrates innovative use of enriched germanium detectors to search for neutrinoless double beta decay, targeting half-life sensitivities of up to 10^28 years.
• It details the evolution from GERDA and Majorana experiments, employing PPC detectors and advanced shielding methods to achieve unprecedented background suppression.
• LEGEND's phased approach—from LEGEND-200 to LEGEND-1000—offers scalable design improvements and deeper insights into neutrino properties and lepton number violation.

The Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND)

The paper "The Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND)" outlines plans and current efforts to explore the nature of neutrinos through neutrinoless double beta decay ($0\nu\beta\beta$) detection using enriched germanium detectors. Neutrinoless double beta decay, if observed, would signify the violation of lepton number conservation and indicate that neutrinos might be Majorana particles. Understanding this could also provide insights into the absolute neutrino mass scale.

Current Experiments and Background

The current generation of experiments, GERDA (GERmanium Detector Array) and the Majorana Demonstrator (\mjd), leverage high-purity germanium detectors enriched in $^{76}$Ge, offering outstanding energy resolution of approximately 0.12%. These experiments have achieved the lowest background levels in the $0\nu\beta\beta$ decay signal region, largely due to the use of p-type point-contact (PPC) high purity germanium (HPGe) detectors. GERDA and \mjd\ aim to set constraints on the half-life for neutrinoless double-beta decay, currently aiming for sensitivities on the order of $10^{26}$ years.

Progress and Developments

Significant advancements of these experiments include the introduction of a PPC detector design which provides excellent pulse shape discrimination capabilities to differentiate between $0\nu\beta\beta$ events and background noise. For GERDA, a large liquid argon shielding that acts as an active veto for background rejection is implemented, whereas the \mjd\ relies on ultra-clean materials and active vetos from a superconducting shield. Collectively, these experiments have paved the way for the scaled-up LEGEND project by demonstrating exceptionally low background indices and improved detection efficiency.

LEGEND Project Overview

The successor project, LEGEND (Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay), aspires to convert these advancements into a 1000 kg (LEGEND-1000) detector array, expected to increase the sensitivity to the $0\nu\beta\beta$ half-life up to $10^{28}$ years. LEGEND will initially operate a smaller setup, LEGEND-200, which utilizes existing GERDA infrastructure aiming for functionality as a 'background-free' experiment. This phase demands further background reduction strategies, even beyond what current experiments have achieved.

Goals and Strategic Approaches

The LEGEND collaboration strategizes on further decreasing background levels by utilizing low-radioactivity materials, enhancing scintillation light detection, and optimizing detector design with inverted-coaxial configurations. Furthermore, planned moves to sites with significant overburden, potentially at SNOLAB or CJPL, aim to minimize muon-induced backgrounds. LEGEND emphasizes scalable design elements, with the phased addition of detector mass allowing iterative optimization and concurrent data collection.

Implications and Future Outlook

If successful, LEGEND could provide substantial evidence toward characterizing neutrinos as Majorana particles by demonstrating $0\nu\beta\beta$ decay. Such a discovery would impose profound implications on fundamental particle physics and cosmology, contributing to our understanding of the universe's matter-antimatter asymmetry. Upcoming results, especially from LEGEND-200, will guide further detector developments and define experimental refinements needed for LEGEND-1000. With appropriate funding and collaborative support, initial phases of LEGEND might commence as early as 2021.

In summary, the cooperative effort by institutions worldwide aims to push the frontier of neutrino physics, focusing critically on background suppression and detector technology optimization, potentially unlocking new insights into the unexplored characteristics of neutrinos.