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Search for Majorana neutrinos with the first two years of EXO-200 data


Many extensions of the standard model of particle physics suggest that neutrinos should be Majorana-type fermions—that is, that neutrinos are their own anti-particles—but this assumption is difficult to confirm. Observation of neutrinoless double-β decay (0νββ), a spontaneous transition that may occur in several candidate nuclei, would verify the Majorana nature of the neutrino and constrain the absolute scale of the neutrino mass spectrum. Recent searches carried out with 76Ge (the GERDA experiment) and 136Xe (the KamLAND-Zen and EXO (Enriched Xenon Observatory)-200 experiments) have established the lifetime of this decay to be longer than 1025 years, corresponding to a limit on the neutrino mass of 0.2–0.4 electronvolts. Here we report new results from EXO-200 based on a large 136Xe exposure that represents an almost fourfold increase from our earlier published data sets. We have improved the detector resolution and revised the data analysis. The half-life sensitivity we obtain is 1.9 × 1025 years, an improvement by a factor of 2.7 on previous EXO-200 results. We find no statistically significant evidence for 0νββ decay and set a half-life limit of 1.1 × 1025 years at the 90 per cent confidence level. The high sensitivity holds promise for further running of the EXO-200 detector and future 0νββ decay searches with an improved Xe-based experiment, nEXO.

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Figure 1: Effect of de-noising on the energy resolution, σ/E.
Figure 2: Comparison of energy and standoff distance distributions of a 226Ra calibration source for SS events in simulation and data.
Figure 3: Event multiplicity in data and simulation.
Figure 4: Fit results projected in energy.
Figure 5: Profile likelihood, λ, for 0νββ counts.
Figure 6: Comparison with recent results from 136Xe and 76Ge 0νββ experiments.

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EXO-200 is supported by the DOE and NSF in the United States, NSERC in Canada, SNF in Switzerland, NRF in Korea, RFBR (12-02-12145) in Russia and the DFG Cluster of Excellence ‘Universe’ in Germany. EXO-200 data analysis and simulation used resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the US DOE under contract no. DE-AC02-05CH11231. The EXO-200 collaboration acknowledges the WIPP for their hospitality.

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Each of the authors participated in the collection and analysis of the data reported here, with the following exceptions: D.B. contributed to the slow controls system; G.F.C. performed energy resolution simulations; X.S.J. and Y.B.Z. provided electronics expertise; M. Danilov, A.D., T.K. and P.V. contributed to the initial conception and design of the experiment; M. Danilov and A.D. also contributed to the acquisition of the xenon, while P.V. also advised on nuclear and particle theory; J.D., R.N. and A.R. provided engineering, operations and technical support at the WIPP facility; A.J., J.J.R. and A.W. supported data acquisition, data processing and software. In line with collaboration policy, the authors are listed here alphabetically. EXO-200 was constructed and commissioned by the authors of refs 15 and 31.

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The EXO-200 Collaboration. Search for Majorana neutrinos with the first two years of EXO-200 data. Nature 510, 229–234 (2014).

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