Latest Results from the Heidelberg-Moscow Double Beta Decay Experiment

TL;DR

This study analyzes the complete Heidelberg-Moscow $^{76}$Ge double beta decay dataset to search for $2\\nu\\beta\\beta$, Majoron-emitting, and neutrinoless $0\\nu\\beta\\beta$ decays. A Geant3.21-based background model identifies dominant sources and enables robust subtraction, yielding a precise $T_{1/2}^{2\\nu}$ and leading to stringent limits on $0\\nu\\beta\\beta$ and the Majorana mass, with no observed Majoron signal. The results set a world-leading upper bound of $\\langle m\\rangle<0.35$ eV (90% CL) for the Majorana neutrino mass and motivate next-generation approaches like GENIUS to reach ~0.01 eV sensitivity. These findings inform neutrino-mass hierarchies and cosmological models, emphasizing the need for larger, lower-background experiments.

Abstract

New results for the double beta decay of 76Ge are presented. They are extracted from Data obtained with the HEIDELBERG-MOSCOW, which operates five enriched 76Ge detectors in an extreme low-level environment in the GRAN SASSO. The two neutrino accompanied double beta decay is evaluated for the first time for all five detectors with a statistical significance of 47.7 kg y resulting in a half life of (T_(1/2))^(2nu) = [1.55 +- 0.01 (stat) (+0.19) (-0.15) (syst)] x 10^(21) years. The lower limit on the half-life of the 0nu beta-beta decay obtained with pulse shape analysis is (T_(1/2))^(0_nu) > 1.9 x 10^(25) [3.1 x 10^(25)] years with 90% C.L. (68% C.L.) (with 35.5 kg y). This results in an upper limit of the effective Majorana neutrino mass of 0.35 eV (0.27 eV). No evidence for a Majoron emitting decay mode or for the neutrinoless mode is observed.

Figures (6)

• Figure 1: Sum spectrum of all five $^{76}$Ge detectors after 47.4 kg y of measurement. The most prominent identified lines are labeled.
• Figure 2: The simulated background components (shaded areas) compared with the original measured sum spectrum for all five detectors.
• Figure 3: Summed spectra of all five detectors after 47.7 kg y of measurement together with the residual spectrum after subtracting all identified background components. The thick line shows the fitted 2$\nu\beta\beta$-signal.
• Figure 4: Sum spectrum of all five detectors with 53.9 kg y and SSE spectrum with 35.5 kg y in the region of interest for the $0\nu\beta\beta$ -decay. The curves correspond to the excluded signals with ${\rm T}_{1/2}^{0\nu} \geq 1.3 \times 10^{25} {\rm~ y}$ (90% C.L.) and ${\rm T}_{1/2}^{0\nu} \geq 1.9 \times 10^{25} {\rm~ y}$ (90% C.L.), respectively.
• Figure 5: Present situation, and the expectation for the future, of the most promising $\beta\beta$ experiments. Light parts of the bars: present status; dark parts: expectation for running experiments; solid and dashed lines: experiments under construction and proposed experiments, respectively (from nanpino).