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Neutrinoless double-$beta$ decay ($0 ubetabeta$ decay) is a hypothetical process that can occur if the neutrino is its own antiparticle. The COBRA collaboration operates a demonstrator to search for these decays at the Laboratori Nazionali del Gran Sasso in Italy using CdZnTe semiconductor detectors. The exposure of $234.7,$kg,d considered in this analysis was collected between September 2011 and February 2015. The analysis focuses on the decay of the nuclides $^{114}$Cd, $^{128}$Te, $^{70}$Zn, $^{130}$Te and $^{116}$Cd. A Bayesian analysis is performed to estimate the signal strength of $0 ubetabeta$ decay. No signal is observed for any of these nuclides. Therefore, the following half-life limits at 90% credibility are set: $T_{1/2}^{0 u}>1.6cdot10^{21},$yr ($^{114}$Cd), $T_{1/2}^{0 u}>1.9cdot10^{21},$yr ($^{128}Te$), $T_{1/2}^{0 u}>6.8cdot10^{18},$yr ($^{70}$Zn), $T_{1/2}^{0 u}>6.1cdot10^{21},$yr ($^{130}$Te), and $T_{1/2}^{0 u}>1.1cdot10^{21},$yr ($^{116}$Cd).
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Neutrinoless double-beta decay is a hypothesized process where in some even-even nuclei it might be possible for two neutrons to simultaneously decay into two protons and two electrons without emitting neutrinos. This is possible only if neutrinos are Majorana particles, i.e. fermions that are their own antiparticles. Neutrinos being Majorana particles would explicitly violate lepton number conservation, and might play a role in the matter-antimatter asymmetry in the universe. The observation of neutrinoless double-beta decay would also provide complementary information related to neutrino masses. The Majorana Collaboration is constructing the Majorana Demonstrator, a 40-kg modular germanium detector array, to search for the Neutrinoless double-beta decay of 76Ge and to demonstrate a background rate at or below 3 counts/(ROI-t-y) in the 4 keV region of interest (ROI) around the 2039 keV Q-value for 76Ge Neutrinoless double-beta decay. In this paper, we discuss the physics of neutrinoless double beta decay and then focus on the Majorana Demonstrator, including its design and approach to achieve ultra-low backgrounds and the status of the experiment.
The Gerda experiment designed to search for the neutrinoless double beta decay in 76Ge has successfully completed the first data collection. No signal excess is found, and a lower limit on the half life of the process is set, with T1/2 > 2.1x10^25 yr (90% CL). After a review of the experimental setup and of the main Phase I results, the hardware upgrade for Gerda Phase II is described, and the physics reach of the new data collection is reported.
The observation of neutrinoless double-beta decay would determine whether the neutrino is a Majorana particle and provide information on the absolute scale of neutrino mass. The MAJORANA Collaboration is constructing the DEMONSTRATOR, an array of germanium detectors, to search for neutrinoless double-beta decay of 76-Ge. The DEMONSTRATOR will contain 40 kg of germanium; up to 30 kg will be enriched to 86% in 76-Ge. The DEMONSTRATOR will be deployed deep underground in an ultra-low-background shielded environment. Operation of the DEMONSTRATOR aims to determine whether a future tonne-scale germanium experiment can achieve a background goal of one count per tonne-year in a 4-keV region of interest around the 76-Ge neutrinoless double-beta decay Q-value of 2039 keV.
The MJ Collaboration is operating an array of high purity Ge detectors to search for neutrinoless double-beta decay in $^{76}$Ge. The MJ DEM comprises 44.1~kg of Ge detectors (29.7 kg enriched in $^{76}$Ge) split between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. Here we present results from data taken during construction, commissioning, and the start of full operations. We achieve unprecedented energy resolution of 2.5 keV FWHM at qval and a very low background with no observed candidate events in 10 kg yr of enriched Ge exposure, resulting in a lower limit on the half-life of $1.9times10^{25}$ yr (90% CL). This result constrains the effective Majorana neutrino mass to below 240 to 520 meV, depending on the matrix elements used. In our experimental configuration with the lowest background, the background is $4.0_{-2.5}^{+3.1}$ counts/(FWHM t yr).
The GERmanium Detector Array (GERDA) experiment located at the INFN Gran Sasso Laboratory (Italy), is looking for the neutrinoless double beta decay of Ge76, by using high-purity germanium detectors made from isotopically enriched material. The combination of the novel experimental design, the careful material selection for radio-purity and the active/passive shielding techniques result in a very low residual background at the Q-value of the decay, about 1e-3 counts/(keV kg yr). This makes GERDA the first experiment in the field to be background-free for the complete design exposure of 100 kg yr. A search for neutrinoless double beta decay was performed with a total exposure of 47.7 kg yr: 23.2 kg yr come from the second phase (Phase II) of the experiment, in which the background is reduced by about a factor of ten with respect to the previous phase. The analysis presented in this paper includes 12.4 kg yr of new Phase II data. No evidence for a possible signal is found: the lower limit for the half-life of Ge76 is 8.0e25 yr at 90% CL. The experimental median sensitivity is 5.8e25 yr. The experiment is currently taking data. As it is running in a background-free regime, its sensitivity grows linearly with exposure and it is expected to surpass 1e26 yr within 2018.
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