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تسجيل مستخدم جديدFinal Results of GERDA on the Search for Neutrinoless Double-$beta$ Decay
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The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-$beta$ ($0 ubetabeta$) decay of $^{76}$Ge, whose discovery would have far-reaching implications in cosmology and particle physics. By operating bare germanium diodes, enriched in $^{76}$Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of $5.2times10^{-4}$ counts/(keV$cdot$kg$cdot$yr) in the signal region and met the design goal to collect an exposure of 100 kg$cdot$yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg$cdot$yr of total exposure. A limit on the half-life of $0 ubetabeta$ decay in $^{76}$Ge is set at $T_{1/2}>1.8times10^{26}$ yr at 90% C.L., which coincides with the sensitivity assuming no signal.
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The Standard Model of particle physics cannot explain the dominance of matter over anti-matter in our Universe. In many model extensions this is a very natural consequence of neutrinos being their own anti-particles (Majorana particles) which implies
that a lepton number violating radioactive decay named neutrinoless double beta ($0 ubetabeta$) decay should exist. The detection of this extremely rare hypothetical process requires utmost suppression of any kind of backgrounds. The GERDA collaboration searches for $0 ubetabeta$ decay of $^{76}$Ge ($^{76}rm{Ge} rightarrow,^{76}rm{Se} + 2e^-$) by operating bare detectors made from germanium with enriched $^{76}$Ge fraction in liquid argon. Here, we report on first data of GERDA Phase II. A background level of $approx10^{-3}$ cts/(keV$cdot$kg$cdot$yr) has been achieved which is the world-best if weighted by the narrow energy-signal region of germanium detectors. Combining Phase I and II data we find no signal and deduce a new lower limit for the half-life of $5.3cdot10^{25}$ yr at 90 % C.L. Our sensitivity of $4.0cdot10^{25}$ yr is competitive with the one of experiments with significantly larger isotope mass. GERDA is the first $0 ubetabeta$ experiment that will be background-free up to its design exposure. This progress relies on a novel active veto system, the superior germanium detector energy resolution and the improved background recognition of our new detectors. The unique discovery potential of an essentially background-free search for $0 ubetabeta$ decay motivates a larger germanium experiment with higher sensitivity.
Neutrinoless double beta decay is a process that violates lepton number conservation. It is predicted to occur in extensions of the Standard Model of particle physics. This Letter reports the results from Phase I of the GERmanium Detector Array (GERD
A) experiment at the Gran Sasso Laboratory (Italy) searching for neutrinoless double beta decay of the isotope 76Ge. Data considered in the present analysis have been collected between November 2011 and May 2013 with a total exposure of 21.6 kgyr. A blind analysis is performed. The background index is about 1.10^{-2} cts/(keV kg yr) after pulse shape discrimination. No signal is observed and a lower limit is derived for the half-life of neutrinoless double beta decay of 76Ge, T_1/2 > 2.1 10^{25} yr (90% C.L.). The combination with the results from the previous experiments with 76Ge yields T_1/2 > 3.0 10^{25} yr (90% C.L.).
We report the results of a search for neutrinoless double-beta decay in a 9.8~kg$cdot$yr exposure of $^{130}$Te using a bolometric detector array, CUORE-0. The characteristic detector energy resolution and background level in the region of interest a
re $5.1pm 0.3{rm~keV}$ FWHM and $0.058 pm 0.004,(mathrm{stat.})pm 0.002,(mathrm{syst.})$~counts/(keV$cdot$kg$cdot$yr), respectively. The median 90%~C.L. lower-limit sensitivity of the experiment is $2.9times 10^{24}~{rm yr}$ and surpasses the sensitivity of previous searches. We find no evidence for neutrinoless double-beta decay of $^{130}$Te and place a Bayesian lower bound on the decay half-life, $T^{0 u}_{1/2}>$~$ 2.7times 10^{24}~{rm yr}$ at 90%~C.L. Combining CUORE-0 data with the 19.75~kg$cdot$yr exposure of $^{130}$Te from the Cuoricino experiment we obtain $T^{0 u}_{1/2} > 4.0times 10^{24}~mathrm{yr}$ at 90%~C.L.~(Bayesian), the most stringent limit to date on this half-life. Using a range of nuclear matrix element estimates we interpret this as a limit on the effective Majorana neutrino mass, $m_{betabeta}< 270$ -- $760~mathrm{meV}$.
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 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 combi
nation 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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