Improved limit on neutrinoless double beta decay of $^{76}$Ge from GERDA Phase II (1803.11100v1)

Abstract: The GERDA experiment searches for the lepton number violating neutrinoless double beta decay of $^{76}$Ge ($^{76}$Ge $\rightarrow$ $^{76}$Se + 2e$^-$) operating bare Ge diodes with an enriched $^{76}$Ge fraction in liquid argon. The exposure for BEGe-type detectors is increased threefold with respect to our previous data release. The BEGe detectors feature an excellent background suppression from the analysis of the time profile of the detector signals. In the analysis window a background level of $1.0_{-0.4}^{+0.6}\cdot10^{-3}$ cts/(keV$\cdot$kg$\cdot$yr) has been achieved; if normalized to the energy resolution this is the lowest ever achieved in any 0$\nu\beta\beta$ experiment. No signal is observed and a new 90 \% C.L. lower limit for the half-life of $8.0\cdot10^{25}$ yr is placed when combining with our previous data. The median expected sensitivity assuming no signal is $5.8\cdot10^{25}$ yr.

• The paper presents a threefold increase in BEGe detector exposure, reducing the background index to 1.0×10^-3 counts/keV-kg-year.
• The paper establishes a new lower limit on the half-life of neutrinoless double beta decay in 76Ge at 8.0×10^25 years (90% C.L.).
• The paper highlights a background-free detection strategy that paves the way for future experiments like LEGEND in advancing neutrino physics.

Analysis of GERDA Phase II Results on Neutrinoless Double Beta Decay in $^{76}$Ge

The paper presents significant findings from the GERDA (Germanium Detector Array) experiment, which aims to detect neutrinoless double beta decay (0νββ) in $^{76}$Ge. This research is pivotal for understanding the nature of neutrinos and assessing whether they are Majorana particles, which are their own antiparticles. The GERDA experiment, based in the INFN Gran Sasso Laboratory, uses enriched germanium detectors immersed in liquid argon to reduce background noise, essential for observing such rare decays.

Key Results

1. Increased Exposure and Enhanced Detection: The exposure for BEGe-type detectors increased threefold, enhancing GERDA's sensitivity. This increase led to achieving a background index of $1.0_{-0.4}^{+0.6} \times 10^{-3}$ counts per keV-kg-year (cts/keV-kg-yr), which, adjusted for energy resolution, is the lowest to date in 0νββ decay experiments.
2. Improved Constraints on Half-life: No 0νββ signal was detected, leading to a new lower limit for the half-life of $^{76}$Ge decay at $8.0 \times 10^{25}$ years (90% confidence level). The median expected sensitivity was $5.8 \times 10^{25}$ years.
3. Significantly Low Background: GERDA operates with a "background-free" approach, where expected background events are less than one in the specified energy interval, optimally leveraging the detector's exposure.
4. Comparison with Other Experiments: In comparison, the Majorana Demonstrator, using a similar germanium setup, reports a background rate of 4.0$^{+3.1}_{-2.5}$ cts/ton-yr-FWHM, while both experiments demonstrate highly reduced backgrounds, establishing benchmarks for future research.

Implications and Future Directions

• Beyond the GERDA Experiment: The combination of low backgrounds and high sensitivity in GERDA demonstrates the feasibility and necessity of such technological advancements. These efforts indicate the readiness for future projects like LEGEND, which can exploit combined exposures from GERDA and Majorana Demonstrator to further push the limits of sensitivity in 0νββ decay searches.
• Theoretical Implications: The constraints on 0νββ decay half-lives inform theoretical models and calculations of nuclear matrix elements (NMEs) crucial for understanding neutrino masses and their nature.
• Technological Developments: GERDA’s methodology highlights the efficacy of liquid argon as a shielding and detection medium, coupled with advanced germanium detector technology. Future experiments may refine these techniques for even greater background suppression and energy resolution.

This research solidifies GERDA's status in 0νββ decay searches, marking a substantial contribution to neutrino physics and the continued quest to decipher the neutrino mass hierarchy and its implications for the Standard Model of particle physics.