Final Results of GERDA on the Search for Neutrinoless Double-$β$ Decay (2009.06079v1)

Abstract: The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-$\beta$ ($0\nu\beta\beta$) decay of $^{76}$Ge, whose discovery would have far-reaching implications in cosmology and particle physics. By operating bare germanium diodes, enriched in $^{76}$Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of $5.2\times10^{-4}$ counts/(keV$\cdot$kg$\cdot$yr) in the signal region and met the design goal to collect an exposure of 100 kg$\cdot$yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg$\cdot$yr of total exposure. A limit on the half-life of $0\nu\beta\beta$ decay in $^{76}$Ge is set at $T_{1/2}>1.8\times10^{26}$ yr at 90% C.L., which coincides with the sensitivity assuming no signal.

• The paper presents comprehensive final results on the search for neutrinoless double-beta decay in 76Ge, establishing a half-life limit >1.8×10^26 years at 90% C.L.
• It employs high-purity germanium detectors in a liquid argon environment with enhanced background suppression techniques, improving sensitivity by an order of magnitude.
• Robust statistical analysis using both frequentist and Bayesian methods provides a benchmark for future large-scale experiments like the upcoming LEGEND project.

Insights on GERDA's Search for Neutrinoless Double-Beta Decay

The paper presents the final results from the GERmanium Detector Array (GERDA) experiment, which focused on the search for neutrinoless double-beta ($0\nu\beta\beta$) decay in $^{76}$Ge. This nuclear process is significant because its observation would confirm lepton-number violation and suggest that neutrinos are Majorana particles, potentially addressing fundamental questions about the asymmetry between matter and antimatter in the universe.

Methodological Overview

GERDA utilized high-purity germanium detectors enriched in $^{76}$Ge, providing high detection efficiency as the source and detector coincide. These detectors were operated within a liquid argon (LAr) environment, offering both shielding and cooling, which facilitated the reduction of background events around the decay's Q-value of 2039.06 keV. The experiment underwent two phases, with enhanced sensitivity and background suppression techniques introduced in Phase II. A critical improvement in Phase II was the installation of a LAr veto system alongside the original germanium detectors, effectively reducing the background to a low index of $5.2\times10^{-4}$ counts/(keV kg yr).

Experimental Data and Analysis

The GERDA experiment achieved a total exposure of 127.2 kg yr, combining data from Phase I and Phase II. This exposure did not reveal any evidence of the $0\nu\beta\beta$ decay, leading to the establishment of a lower limit on the half-life of $T_{1/2} > 1.8\times10^{26}$ years at 90% confidence level (C.L.). The significance of this result is underscored by the background-free achievement during Phase II, including a background index of around 0.3 counts in the signal region.

The analysis employed both frequentist and Bayesian statistical frameworks to derive the limits on the half-life and effective Majorana neutrino mass. The rigorous likelihood analysis model, which considered several detector characteristics and partition-dependent parameters, confirmed the robustness of the results.

Implications and Prospective Developments

GERDA significantly enhanced the background suppression and increased the half-life sensitivity by an order of magnitude compared to previous experiments with $^{76}$Ge. The sensitivity achieved positions GERDA at the forefront of neutrinoless double-beta decay investigations. The insights from this experiment set an essential benchmark for future studies exploring the fundamental nature of neutrinos.

Furthermore, GERDA's success supports the feasibility of a large-scale, background-free experiment based on $^{76}$Ge, paving the way for its successor, the LEGEND experiment. LEGEND aims to further extend the sensitivity of $0\nu\beta\beta$ decay searches to half-lives up to $10^{28}$ years, thereby enhancing the potential for revolutionary discoveries in particle physics and cosmology.

In summary, the results of the GERDA experiment represent a critical step in advancing our understanding of neutrino properties and the overarching puzzle of the universe's matter-antimatter asymmetry. The methodologies refined in this paper will inform future experimental designs, emphasizing high sensitivity and minimal background interference in the ongoing search for $0\nu\beta\beta$ decay.