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Background-free search for neutrinoless double-β decay of 76Ge with GERDA

Abstract

Many extensions of the Standard Model of particle physics explain the dominance of matter over antimatter in our Universe by neutrinos being their own antiparticles. This would imply the existence of neutrinoless double-β decay, which is an extremely rare lepton-number-violating radioactive decay process whose detection requires the utmost background suppression. Among the programmes that aim to detect this decay, the GERDA Collaboration is searching for neutrinoless double-β decay of 76Ge by operating bare detectors, made of germanium with an enriched 76Ge fraction, in liquid argon. After having completed Phase I of data taking, we have recently launched Phase II. Here we report that in GERDA Phase II we have achieved a background level of approximately 10−3 counts keV−1 kg−1 yr−1. This implies that the experiment is background-free, even when increasing the exposure up to design level. This is achieved by use of an active veto system, superior germanium detector energy resolution and improved background recognition of our new detectors. No signal of neutrinoless double-β decay was found when Phase I and Phase II data were combined, and we deduce a lower-limit half-life of 5.3 × 1025 years at the 90 per cent confidence level. Our half-life sensitivity of 4.0 × 1025 years is competitive with the best experiments that use a substantially larger isotope mass. The potential of an essentially background-free search for neutrinoless double-β decay will facilitate a larger germanium experiment with sensitivity levels that will bring us closer to clarifying whether neutrinos are their own antiparticles.

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Figure 1: Energy scale and resolution.
Figure 2: Energy spectra for the two detector types.
Figure 3: Pulse shape discrimination.
Figure 4: Energy spectra in the analysis window around Qββ.

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Acknowledgements

The GERDA experiment is supported by the German Federal Ministry for Education and Research (BMBF), the German Research Foundation (DFG) via the Excellence Cluster Universe, the Italian Istituto Nazionale di Fisica Nucleare (INFN), the Max Planck Society (MPG), the Polish National Science Centre (NCN), the Russian Foundation for Basic Research (RFBR) and the Swiss National Science Foundation (SNF). These research institutions acknowledge internal financial support. GERDA was constructed and commissioned by the authors of refs 13 and 19. The GERDA Collaboration (https://www.mpi-hd.mpg.de/gerda/) thanks the directors and the staff of the LNGS for their support of the GERDA experiment.

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All authors contributed to the publication, being differently involved in the design and construction of the detector system, in its operation, and in the acquisition and analysis of data. All authors approved the final version of the manuscript. In line with collaboration policy, the authors are listed here alphabetically.

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Reviewer Information Nature thanks P. Barbeau, L. Canonica and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 GERDA Phase II experimental set-up.

a, Overview. 1, water tank with muon veto system PMTs (590 m3, diameter 10 m); 2, LAr cryostat (64 m3, diameter 4 m); 3, floor and roof of clean room; 4, lock; 5, glove box; 6, plastic muon veto system. b, LAr veto system: 1, bottom plate (diameter 49 cm) with 7 3-inch PMTs (R11065-10/20 MOD) with low radioactivity of U and Th (<2 mBq per PMT); 2, fibre curtain (height 100 cm) coated with wavelength shifter; 3, optical couplers and SiPMs; 4, thin-walled (0.1 mm) Cu cylinders (height 60 cm) covered with a Tyvek reflector on the inside; 5, top plate with properties as bottom plate 1 except for 9 3-inch PMTs; 6, calibration source entering slot in top plate; 7, slot for second of three calibration sources. c, Detector array. 1, Ge detectors arranged in 7 strings; 2, flexible bias and readout cables; 3, amplifiers. d, Detector module, view from bottom. 1, BEGe diode; 2, signal cable; 3, high voltage cable. 2 and 3 are attached by 4, bronze clamps to 5, silicon support plate; 6, bond wire connections from diode to signal and high voltage cable; 7, Cu support rods.

Extended Data Figure 2 Detector types.

Cross-section through the germanium detector types (left) and the corresponding photographs of them (right). The p+ electrode is made by a 0.3 μm thin boron implantation. The n+ electrode is a 1 to 2 mm thick lithium diffusion layer and is biased with up to +4,500 V. The electric field drops to zero in the n+ layer, and hence energy depositions in this fraction of the volume do not create a readout signal. The p+ electrode is connected to a charge sensitive amplifier.

Extended Data Figure 3 Germanium detector array.

Photograph of the assembled detector array, with a string of coaxial detectors on the left, and strings of BEGe detectors at middle and right. Each string is enclosed by a cylindrical nylon shroud covered with a wavelength shifter.

Extended Data Figure 4 Liquid argon veto set-up.

Photographs of the LAr veto system: left, fibre curtain with SiPM readout at the top; right, top and bottom arrangement of PMTs.

Extended Data Figure 5 Frequentist hypothesis test.

P value for the hypothesis test as a function of the inverse half-life according to equation (7). The colour bands indicate the spread of the P-value distributions for many Monte Carlo realizations according to the GERDA parameters (with no signal): green and yellow show the central 68% and 90% probability intervals, respectively. The dashed black line represents the median of the distribution; the P value for the GERDA data is shown as a solid black line. The red arrows indicate the results at 90% confidence level, that is, a P value of 0.1: the limit for (full red arrow), and the median sensitivity for (dashed red arrow). For a detailed discussion of their computation, see Methods.

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The GERDA Collaboration. Background-free search for neutrinoless double-β decay of 76Ge with GERDA. Nature 544, 47–52 (2017). https://doi.org/10.1038/nature21717

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