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Search for neutrinoless double beta decay with enriched 76Ge in Gran Sasso 1990-2003

H. V. Klapdor-Kleingrothaus, I. V. Krivosheina, A. Dietz, O. Chkvorets

TL;DR

The HEIDELBERG-MOSCOW experiment used about 11 kg of enriched $^{76}$Ge detectors at Gran Sasso to search for neutrinoless double beta decay over 1990–2003, achieving an exceptionally low background and high sensitivity. A peak near the $\beta\beta$ decay $Q$ value ($Q_{\beta\beta}=2039.006\pm0.050$ keV) yields a $\sim4.2\sigma$ signal for $0\nu\beta\beta$, enabling a half-life estimate of $T_{1/2}^{0\nu}=(0.69-4.18)\times10^{25}$ y and an effective Majorana neutrino mass in the range $\langle m\rangle\approx0.24-0.58$ eV (subject to nuclear matrix-element uncertainties). The result implies lepton-number violation and a nonzero Majorana neutrino mass, with significant implications for neutrino mass hierarchies and beyond-Standard-Model physics; the work also highlights the importance of high energy resolution and pulse-shape analysis for distinguishing signal from background in next-generation experiments.

Abstract

The results of the HEIDELBERG-MOSCOW experiment which searches with 11 kg of enriched 76Ge for double beta decay in the GRAN Sasso underground laboratory are presented for the full running period August 1990 - May 2003. The duty cycle of the experiment was ~80%, the collected statistics is 71.7 kg y. The background achieved in the energy region of the Q value for double beta decay is 0.11 events/ kg y keV. The two-neutrino accompanied half-life is determined on the basis of more than 100 000 events. The confidence level for the neutrinoless signal has been improved to 4.2 sigma.

Search for neutrinoless double beta decay with enriched 76Ge in Gran Sasso 1990-2003

TL;DR

The HEIDELBERG-MOSCOW experiment used about 11 kg of enriched Ge detectors at Gran Sasso to search for neutrinoless double beta decay over 1990–2003, achieving an exceptionally low background and high sensitivity. A peak near the decay value ( keV) yields a signal for , enabling a half-life estimate of y and an effective Majorana neutrino mass in the range eV (subject to nuclear matrix-element uncertainties). The result implies lepton-number violation and a nonzero Majorana neutrino mass, with significant implications for neutrino mass hierarchies and beyond-Standard-Model physics; the work also highlights the importance of high energy resolution and pulse-shape analysis for distinguishing signal from background in next-generation experiments.

Abstract

The results of the HEIDELBERG-MOSCOW experiment which searches with 11 kg of enriched 76Ge for double beta decay in the GRAN Sasso underground laboratory are presented for the full running period August 1990 - May 2003. The duty cycle of the experiment was ~80%, the collected statistics is 71.7 kg y. The background achieved in the energy region of the Q value for double beta decay is 0.11 events/ kg y keV. The two-neutrino accompanied half-life is determined on the basis of more than 100 000 events. The confidence level for the neutrinoless signal has been improved to 4.2 sigma.

Paper Structure

This paper contains 14 sections, 8 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Experimental Parameters
  3. Background of the experiment
  4. Control of stability of the experiment and data taking
  5. $Q$ Value
  6. Development of the experiment
  7. Reliability of data acquisition and data
  8. Data and Analysis
  9. Results
  10. Time structure of events
  11. Half-Life of Neutrinoless Double Beta Decay of $^{76}{Ge}$
  12. Consequences
  13. Conclusion - Perspectives
  14. Acknowledgement

Figures (8)

  • Figure 1: Left: Threshold ranges for detectors 4 and 5 in the spectrum measured during 1995 - 2003. Right: Sum of the ($\sim$360) weekly calibration spectra for the 2614.5 keV line from $^{228}{Th}$ over the measuring time 1995 - 2003.
  • Figure 2: Left: Sum of the sum of the weekly calibration spectra of all five detectors shown in Fig. \ref{['fig:Thres-Range5det']}(right) ($\sim$1800 spectra). Right: Sum of calibration spectra for each detector (absolute intensities) for the 2103.5 keV line for the period 1995 - 2003 (from top to bottom detectors: 2,3,5,1,4).
  • Figure 3: The total sum spectrum measured over the full energy range with all five detectors (in total 10.96 kg enriched in $^{76}{Ge}$ to 86%) - for the period August 1990 to May 2003.
  • Figure 4: The total sum spectrum of all five detectors (in total 10.96 kg enriched in $^{76}{Ge}$), in the range 2000 - 2060 keV and its fit, for the periods: Top: left - August 1990 to May 2000 (50.57 kg y); right - August 1990 to May 2003 (71.7 kg y). Bottom: left - November 1995 to May 2003 (56.66 kg y); right - scan for lines in the spectrum shown on the left, with the MLM method (see text). The Bi lines at 2010.7, 2016.7, 2021.8 and 2052.9 keV are seen, and in addition a signal at $\sim$ 2039 keV.
  • Figure 5: The spectra of the individual five detectors in the range 2000-2060 keV for the period November 1995 to May 2003. The sum corresponds to the spectrum shown in Fig. \ref{['fig:Sum90-95-03-Scan']}, bottom left.
  • ...and 3 more figures