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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.


  1. Majorana, E. Theory of the symmetry of electrons and positrons. Nuovo Cimento 14, 171–184 (1937)

    CAS  Article  Google Scholar 

  2. Racah, G. On the symmetry of particles and antiparticles. Nuovo Cimento 14, 322–328 (1937)

    Article  Google Scholar 

  3. Mourik, V. et al. Signatures of majorana fermions in hybrid superconductor-semiconductor nanowire devices. Science 336, 1003–1007 (2012)

    ADS  CAS  Article  Google Scholar 

  4. Camilleri, L., Lisi, E. & Wilkerson, J. F. Neutrino masses and mixings: status and prospects. Annu. Rev. Nucl. Part. Sci. 58, 343–369 (2008)

    ADS  CAS  Article  Google Scholar 

  5. Mohapatra, R. N. & Senjanovic, G. Neutrino mass and spontaneous parity violation. Phys. Rev. Lett. 44, 912–915 (1980)

    ADS  CAS  Article  Google Scholar 

  6. Gell-Mann, M., Ramond, P. & Slansky, R. in Supergravity (eds van Nieuwenhuizen, P. & Freedman, D. ) 315–321 (North Holland, 1979)

    Google Scholar 

  7. Schechter, J. & Valle, J. Neutrinoless double-β decay in SU(2) x U(1) theories. Phys. Rev. D 25, 2951–2954 (1982)

    ADS  CAS  Article  Google Scholar 

  8. Barabash, A. Precise half-life values for two-neutrino double-β decay. Phys. Rev. C 81, 035501 (2010)

    ADS  Article  Google Scholar 

  9. Albert, J. B. et al. (EXO-200 Collaboration). An improved measurement of the 2νββ half-life of Xe-136 with EXO-200. Phys. Rev. C 89, 015502 (2014)

    ADS  Article  Google Scholar 

  10. Vogel, P. Nuclear structure and double beta decay. J. Phys. G 39, 124002 (2012)

    ADS  Article  Google Scholar 

  11. Agostini, M. et al. (GERDA Collaboration). Results on neutrinoless double-β decay of 76Ge from Phase I of the GERDA experiment. Phys. Rev. Lett. 111, 122503 (2013)

    ADS  CAS  Article  Google Scholar 

  12. Gando, A. et al. (KamLAND-Zen Collaboration). Limit on neutrinoless ββ decay of 136Xe from the first phase of KamLAND-Zen and comparison with the positive claim in 76Ge. Phys. Rev. Lett. 110, 062502 (2013)

    ADS  CAS  Article  Google Scholar 

  13. Auger, M. et al. (EXO-200 Collaboration). Search for neutrinoless double-beta decay in 136Xe with EXO-200. Phys. Rev. Lett. 109, 032505 (2012)

    ADS  CAS  Article  Google Scholar 

  14. Klapdor-Kleingrothaus, H. & Krivosheina, I. The evidence for the observation of 0νββ decay: the identification of 0νββ events from the full spectra. Mod. Phys. Lett. A 21, 1547–1566 (2006)

    ADS  CAS  Article  Google Scholar 

  15. Auger, M. et al. The EXO-200 detector, part I: Detector design and construction. J. Instrum. 7, P05010 (2012)

    Article  Google Scholar 

  16. Redshaw, M., Wingfield, E., McDaniel, J. & Myers, E. G. Mass and double-beta-decay Q value of 136Xe. Phys. Rev. Lett. 98, 053003 (2007)

    ADS  Article  Google Scholar 

  17. 3M HFE-7000 (2014)

  18. Esch, E.-I. et al. The cosmic ray muon flux at WIPP. Nucl. Instrum. Meth. A 538, 516–525 (2005)

    ADS  CAS  Article  Google Scholar 

  19. Leonard, D. et al. Systematic study of trace radioactive impurities in candidate construction materials for EXO-200. Nucl. Instrum. Meth. A 591, 490–509 (2008)

    ADS  CAS  Article  Google Scholar 

  20. Allison, J. et al. Geant4 developments and applications. IEEE Trans. Nucl. Sci. 53, 270–278 (2006)

    ADS  Article  Google Scholar 

  21. Kotila, J. & Iachello, F. Phase-space factors for double-β decay. Phys. Rev. C 85, 034316 (2012)

    ADS  Article  Google Scholar 

  22. Battistoni, G. et al. in Hadronic Shower Simulation Workshop (eds Albrow, M. & Raja, R. ) 31–49 (American Institute of Physics Conf. Ser. Vol. 896,. (2007)

  23. Ferrari, A., Sala, P. R., Fasso, A. & Ranft, J. FLUKA: A Multi-Particle Transport Code (Program version 2005) (Tech. Rep. CERN-2005–010, SLAC-R-773, INFN-TC-05–11, CERN, 2005)

    Book  Google Scholar 

  24. Wilson, W. B. SOURCES 4A: A Code for Calculating (α,n) Spontaneous Fission, and Delayed Neutron Sources and Spectra (Tech. Rep. LA-13639-MS, Los Alamos National Laboratory, 1999)

    Google Scholar 

  25. Bhat, M. in Nuclear Data for Science and Technology (ed. Qaim, S. ) 817–821 (Research Reports in Physics, Springer, 1992)

    Book  Google Scholar 

  26. Rodríguez, T. R. & Martínez-Pinedo, G. Energy density functional study of nuclear matrix elements for neutrinoless ββ decay. Phys. Rev. Lett. 105, 252503 (2010)

    ADS  Article  Google Scholar 

  27. Menéndez, J., Poves, A., Caurier, E. & Nowacki, F. Disassembling the nuclear matrix elements of the neutrinoless ββ decay. Nucl. Phys. A 818, 139–151 (2009)

    ADS  Article  Google Scholar 

  28. Barea, J., Kotila, J. & Iachello, F. Nuclear matrix elements for double-β decay. Phys. Rev. C 87, 014315 (2013)

    ADS  Article  Google Scholar 

  29. Šimkovic, F., Rodin, V., Faessler, A. & Vogel, P. 0νββ and 2νββ nuclear matrix elements, quasiparticle random-phase approximation, and isospin symmetry restoration. Phys. Rev. C 87, 045501 (2013)

    ADS  Article  Google Scholar 

  30. Forero, D. V., Tórtola, M. & Valle, J. W. F. Global status of neutrino oscillation parameters after Neutrino-2012. Phys. Rev. D 86, 073012 (2012)

    ADS  Article  Google Scholar 

  31. Ackerman, N. et al. (EXO-200 Collaboration). Observation of two-neutrino double-beta decay in 136Xe with EXO-200. Phys. Rev. Lett. 107, 212501 (2011)

    ADS  CAS  Article  Google Scholar 

  32. Wilks, S. S. The large-sample distribution of the likelihood ratio for testing composite hypotheses. Ann. Math. Stat. 9, 60–62 (1938)

    Article  Google Scholar 

  33. Cowan, G. Statistical Data Analysis (Oxford Science Publications, Clarendon, 1998)

    Google Scholar 

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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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