First results on 76Ge neutrinoless double beta decay from CDEX-1 experiment
Li WangQian YueKejun KangJ. ChengYuanjing LiTsz-King Henry WongShin-Ted LinJ. P. ChangC. H. ChengQinghao ChenYunhua ChenZhi DengQ. DuHui GongLi HeQing-Ju HeJinWei HuHan‐Xiong HuangTeng-Rui HuangLiping JiaHao JiangHau-Bin LiHong LiJianmin LiJin LiJun LiLi XiaXue-Qian LiYulan LiF. K. LinShukui LiuHao MaJ.L. MaXingyu PanJie RenXichao RuanManbin ShenV. SharmaL. SinghM. K. SinghM. K. SinghA. K. SomaChangjian TangWeiyou TangC. H. TsengJimin WangQing WangShiyong WuYucheng WuHaoyang XingYin XuTao XueL. T. YangS. W. YangYi NanChunxu YuHaiJun YuWeihe ZengX. H. ZengZhi ZengLan ZhangYunhua ZhangM. G. ZhaoWei ZhaoJifang ZhouZuying ZhouJingjun ZhuWeibin ZhuZhong-Hua Zhu
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The predictions for the effective Majorana mass || in neutrinoless double-beta decay in the case of 3-neutrino mixing and massive Majorana neutrinos are reviewed. The physics potential of the experiments, searching for neutrinoless double-beta decay and having sensitivity to $|| \gtap 0.01$ eV, for providing information on the type of the neutrino mass spectrum, on the absolute scale of neutrino masses and on the Majorana CP-violation phases in the PMNS neutrino mixing matrix is also discussed.
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The most plausible see-saw explanation of the smallness of the neutrino masses is based on the assumption that total lepton number is violated at a large scale and neutrinos with definite masses are Majorana particles. In this review we consider in details difference between Dirac and Majorana neutrino mixing and possibilities of revealing Majorana nature of neutrinos with definite masses.
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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.
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A brief overview is given of theoretical analyses with neutrinoless double beta decay experiments.Theoretical bounds on the "observable", m ββ , are presented.By using experimental bounds on m ββ , allowed regions are obtained on the m l -cos 2θ 12 plane, where m l stands for the lightest neutrino mass.It is shown that Majorana neutrinos can be excluded by combining possible results of future neutrinoless double beta decay and 3 H beta decay experiments.A possibility to constrain one of two Majorana phases is discussed also.
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The observation of neutrinoless double-beta decay would resolve the Majorana nature of the neutrino and could provide information on the absolute scale of the neutrino mass. The initial phase of the Majorana experiment, known as the Demonstrator, will house 40 kg of Ge in an ultra-low background shielded environment at the 4850' level of the Sanford Underground Laboratory in Lead, SD. The objective of the Demonstrator is to determine whether a future 1-tonne experiment can achieve a background goal of one count per tonne-year in a narrow region of interest around the 76Ge neutrinoless double-beta decay peak.
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The search for neutrinoless double beta decay is the only practical way to test whether neutrinos are Majorana or Dirac particles. The next generation of experiments aim to probe the effective Majorana neutrino mass down to few 10 meV, as predicted by oscillation experiments in case of the inverse mass hierarchy. According to recent nuclear matrix calculations, the predicted decay rates per mass of double beta isotope are varying within a factor of few when comparing them within the same theoretical model framework. The sensitivity of the upcoming experiments depend therefore primarily on the available mass of double beta isotopes and the experimental conditions. In particular, the achievable background suppression and the detection efficiency will be decisive for their success.
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The research objective of the supported work was to search for neutrinoless double-beta decay in 76Ge and investigate and identify backgrounds to a double-beta decay search. Understanding the neutrino mass generation mechanism, the absolute neutrino mass scale, the neutrino mass spectrum, and the possible Majorana nature of neutrinos are some of the future discoveries of the next generation neutrino experiments. The Majorana collaboration operated the Demonstrator experiment comprising 44 kg (30 kg enriched in 76Ge) of Ge detectors in total split between two modules contained in a low background shield at the Sanford Underground Research Facility in Lead, South Dakota. Observation of 0νββ would have profound implications for the Standard Model of particle physics by demonstrating that neutrinos are Majorana particles, that lepton number is not conserved, and it would provide a measure of the effective Majorana neutrino mass. The decay rate of 0νββ is incredibly small and may only produce a few signature events in a detector per year. Therefore, observation of this rare process demands a careful study and implementation of signal processing, detector design, and new levels of background suppression.
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The problem of the nature of the neutrino, namely i f it is a massless Dirac particle different from its antineutrino or a Majorana particle with finite mass, is discussed. The question is related to the recent results showing the presence of neutrino oscillations clearly indicating that the difference between the squared mass of neutrinos of different flavours is different from zero. Neutrinoless double beta decay (DBD) is at present the most powerful tool to determine the effective value of the mass of a Majorana neutrino. The results already obtained in this lepton violating process will be reported and the two pres ently running DBD experiments briefly discussed. The future second generation experiments will be reviewed with special emphasis to those already partially approved. In conclusion the peculiar and interdisciplinary nature of these searches will be stressed in their exciting aim to discover if neutrino is indeed a Majorana particle
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The study of the neutrinoless double beta (0νββ) decay is of great interest because of its potential to provide us with information about the lepton number conservation and neutrino properties as the neutrinos character (are they Dirac or Majorana particles?) and their absolute mass. Since the 0νββ decay has not yet been discovered experimentally, one can only extract limits of the absolute neutrino mass. For that, one needs accurate calculations of both nuclear matrix elements (NMEs) and phase space factors (PSFs) which appear in the theoretical lifetime expressions, corroborated with experimental lifetime limits. In this paper I first present recent shell model (ShM) calculations of the NMEs and PSFs for 0νββ decay performed by our group, in the hypothesis that the mechanism of its occurrence is the exchange of light Majorana neutrinos between two nucleons inside the nucleus. Also, the consensus on the use of different nuclear structure ingredients/parameters in the computation of the NMEs is discussed. Then, I present new limits of the Majorana neutrino mass parameter derived from the analysis of 0νββ decay of nine isotopes.
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