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Observation of two-neutrino double electron capture in 124Xe with XENON1T

Nature volume 568, pages 532–535 (2019)Cite this article

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Abstract

Two-neutrino double electron capture (2νECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude1. Until now, indications of 2νECEC decays have only been seen for two isotopes2,3,4,5, 78Kr and 130Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance6,7. The 2νECEC half-life is an important observable for nuclear structure models8,9,10,11,12,13,14 and its measurement represents a meaningful step in the search for neutrinoless double electron capture—the detection of which would establish the Majorana nature of the neutrino and would give access to the absolute neutrino mass15,16,17. Here we report the direct observation of 2νECEC in 124Xe with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8 × 1022 years (statistical uncertainty, 0.5 × 1022 years; systematic uncertainty, 0.1 × 1022 years) is the longest measured directly so far. This study demonstrates that the low background and large target mass of xenon-based dark-matter detectors make them well suited for measuring rare processes and highlights the broad physics reach of larger next-generation experiments18,19,20.

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Fig. 1: Schematic of two-neutrino double electron capture.
Fig. 2: Fit of the background model to the measured energy spectrum.
Fig. 3: Zoom on the energy region of interest for 2νECEC in 124Xe.
Fig. 4: Observed 2νECEC half-life compared with theoretical predictions and other experiments.

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Data availability

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

We thank J. Menéndez for sharing his expertise in the theory of double β decay. We gratefully acknowledge support from the National Science Foundation, Swiss National Science Foundation, German Ministry for Education and Research, Max Planck Gesellschaft, Deutsche Forschungsgemeinschaft, Netherlands Organisation for Scientific Research (NWO), NLeSC, Weizmann Institute of Science, I-CORE, Pazy-Vatat, Initial Training Network Invisibles (Marie Curie Actions, PITNGA-2011-289442), Fundacao para a Ciencia e a Tecnologia, Region des Pays de la Loire, Knut and Alice Wallenberg Foundation, Kavli Foundation, Abeloe Graduate Fellowship, and Istituto Nazionale di Fisica Nucleare. Data processing was performed using infrastructures of the Open Science Grid and European Grid Initiative. We are grateful to Laboratori Nazionali del Gran Sasso for hosting and supporting the XENON project. B.K. was also at Albert Einstein Center for Fundamental Physics, University of Bern, Bern, Switzerland. S.K. is also at Kobayashi-Maskawa Institute, Nagoya University, Nagoya, Japan. J.A.M.L. is also at Coimbra Polytechnic – ISEC, Coimbra, Portugal.

Reviewer information

Nature thanks Jouni Suhonen and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

Author notes

  1. A list of participants and their affiliations appears at the end of the paper.

Authors and Affiliations

  1. Physics Department, Columbia University, New York, NY, USA

    E. Aprile, M. Anthony, P. de Perio, F. Gao, Z. Greene, J. Howlett, Q. Lin, G. Plante, A. Rizzo, Y. Zhang & T. Zhu

  2. Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova, Stockholm, Sweden

    J. Aalbers, V. C. Antochi, B. Bauermeister, J. Conrad, A. D. Ferella, J. Mahlstedt, K. Morå & B. Pelssers

  3. Nikhef and the University of Amsterdam, Science Park, Amsterdam, The Netherlands

    J. Aalbers, P. A. Breur, A. Brown, A. P. Colijn, M. P. Decowski & E. Hogenbirk

  4. Department of Physics and Astronomy, University of Bologna and INFN-Bologna, Bologna, Italy

    F. Agostini, P. Di Gangi, M. Garbini, G. Sartorelli & M. Selvi

  5. Institut für Physik & Exzellenzcluster PRISMA, Johannes Gutenberg-Universität Mainz, Mainz, Germany

    M. Alfonsi, U. Oberlack, M. Scheibelhut, S. Schindler & D. Wenz

  6. Institut für Kernphysik, Westfälische Wilhelms-Universität Münster, Münster, Germany

    L. Althueser, A. Fieguth, M. Murra, D. Schulte, M. Vargas, C. Weinheimer & C. Wittweg

  7. LIBPhys, Department of Physics, University of Coimbra, Coimbra, Portugal

    F. D. Amaro, J. M. R. Cardoso, J. A. M. Lopes, R. Peres, J. M. F. dos Santos & M. Silva

  8. New York University Abu Dhabi, Abu Dhabi, United Arab Emirates

    F. Arneodo, M. L. Benabderrahmane, G. Bruno, A. Di Giovanni & M. Messina

  9. Physik-Institut, University of Zurich, Zurich, Switzerland

    L. Baudis, A. Brown, C. Capelli, M. Galloway, S. Kazama, A. Kish, A. Manfredini, R. Peres, F. Piastra, S. Reichard & J. Wulf

  10. Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY, USA

    T. Berger, E. Brown & K. Odgers

  11. Max-Planck-Institut für Kernphysik, Heidelberg, Germany

    S. Bruenner, D. Cichon, G. Eurin, C. Hasterok, F. Joerg, M. Lindner, T. Marrodán Undagoitia, V. Pizzella, N. Rupp, J. Schreiner, H. Simgen & O. Wack

  12. Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot, Israel

    R. Budnik, R. Itay, G. Koltman, H. Landsman, L. Levinson, A. Manfredini, N. Priel & H. Qiu

  13. Physikalisches Institut, Universität Freiburg, Freiburg, Germany

    D. Coderre, A. Elykov, B. Kaminsky, S. Lindemann, D. Ramírez García, A. Rocchetti, N. Šarčević, M. Schumann & F. Toschi

  14. SUBATECH, IMT Atlantique, CNRS/IN2P3, Université de Nantes, Nantes, France

    J. P. Cussonneau, S. Diglio, J. Masbou, K. Micheneau, C. Therreau & D. Thers

  15. Department of Physics, University of California, San Diego, CA, USA

    J. Fei, F. Lombardi, K. Ni, Y. Wei & J. Ye

  16. INFN-Laboratori Nazionali del Gran Sasso and Gran Sasso Science Institute, L’Aquila, Italy

    W. Fulgione, A. Gallo Rosso, A. Molinario, J. Naganoma, R. Podviianiuk & Z. Wang

  17. INFN-Torino and Osservatorio Astrofisico di Torino, Torino, Italy

    W. Fulgione & G. Trinchero

  18. Department of Physics & Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL, USA

    L. Grandi, K. Miller, J. Pienaar, B. Riedel, E. Shockley, C. Tunnell & N. Upole

  19. Department of Physics ‘Ettore Pancini’, University of Napoli and INFN-Napoli, Naples, Italy

    M. Iacovacci, F. Marignetti & S. Mastroianni

  20. Department of Physics and Astronomy, Purdue University, West Lafayette, IN, USA

    A. Kopec, R. F. Lang & D. Masson

  21. LPNHE, Sorbonne Université, Université Paris Diderot, CNRS/IN2P3, Paris, France

    E. López Fune, L. Scotto Lavina & J. P. Zopounidis

  22. LAL, Université Paris-Sud, CNRS/IN2P3, Université Paris-Saclay, Orsay, France

    C. Macolino

  23. Department of Physics and Astronomy, Rice University, Houston, TX, USA

    J. Naganoma, P. Shagin & C. Tunnell

  24. Physics and Astronomy Department, University of California, Los Angeles, CA, USA

    H. Wang

Consortia

XENON Collaboration*

  • E. Aprile
  • , J. Aalbers
  • , F. Agostini
  • , M. Alfonsi
  • , L. Althueser
  • , F. D. Amaro
  • , M. Anthony
  • , V. C. Antochi
  • , F. Arneodo
  • , L. Baudis
  • , B. Bauermeister
  • , M. L. Benabderrahmane
  • , T. Berger
  • , P. A. Breur
  • , A. Brown
  • , A. Brown
  • , E. Brown
  • , S. Bruenner
  • , G. Bruno
  • , R. Budnik
  • , C. Capelli
  • , J. M. R. Cardoso
  • , D. Cichon
  • , D. Coderre
  • , A. P. Colijn
  • , J. Conrad
  • , J. P. Cussonneau
  • , M. P. Decowski
  • , P. de Perio
  • , P. Di Gangi
  • , A. Di Giovanni
  • , S. Diglio
  • , A. Elykov
  • , G. Eurin
  • , J. Fei
  • , A. D. Ferella
  • , A. Fieguth
  • , W. Fulgione
  • , A. Gallo Rosso
  • , M. Galloway
  • , F. Gao
  • , M. Garbini
  • , L. Grandi
  • , Z. Greene
  • , C. Hasterok
  • , E. Hogenbirk
  • , J. Howlett
  • , M. Iacovacci
  • , R. Itay
  • , F. Joerg
  • , B. Kaminsky
  • , S. Kazama
  • , A. Kish
  • , G. Koltman
  • , A. Kopec
  • , H. Landsman
  • , R. F. Lang
  • , L. Levinson
  • , Q. Lin
  • , S. Lindemann
  • , M. Lindner
  • , F. Lombardi
  • , J. A. M. Lopes
  • , E. López Fune
  • , C. Macolino
  • , J. Mahlstedt
  • , A. Manfredini
  • , F. Marignetti
  • , T. Marrodán Undagoitia
  • , J. Masbou
  • , D. Masson
  • , S. Mastroianni
  • , M. Messina
  • , K. Micheneau
  • , K. Miller
  • , A. Molinario
  • , K. Morå
  • , M. Murra
  • , J. Naganoma
  • , K. Ni
  • , U. Oberlack
  • , K. Odgers
  • , B. Pelssers
  • , R. Peres
  • , F. Piastra
  • , J. Pienaar
  • , V. Pizzella
  • , G. Plante
  • , R. Podviianiuk
  • , N. Priel
  • , H. Qiu
  • , D. Ramírez García
  • , S. Reichard
  • , B. Riedel
  • , A. Rizzo
  • , A. Rocchetti
  • , N. Rupp
  • , J. M. F. dos Santos
  • , G. Sartorelli
  • , N. Šarčević
  • , M. Scheibelhut
  • , S. Schindler
  • , J. Schreiner
  • , D. Schulte
  • , M. Schumann
  • , L. Scotto Lavina
  • , M. Selvi
  • , P. Shagin
  • , E. Shockley
  • , M. Silva
  • , H. Simgen
  • , C. Therreau
  • , D. Thers
  • , F. Toschi
  • , G. Trinchero
  • , C. Tunnell
  • , N. Upole
  • , M. Vargas
  • , O. Wack
  • , H. Wang
  • , Z. Wang
  • , Y. Wei
  • , C. Weinheimer
  • , D. Wenz
  • , C. Wittweg
  • , J. Wulf
  • , J. Ye
  • , Y. Zhang
  • , T. Zhu
  •  & J. P. Zopounidis

Contributions

The XENON1T detector was designed and constructed by the XENON Collaboration. Operation, data processing, calibration, Monte Carlo simulations of the detector and of theoretical models, and data analyses were performed by a large number of XENON Collaboration members, who also discussed and approved the scientific results. The paper was written by A.F. and C. Wittweg. It was reviewed and edited by the collaboration and all authors approved the final version of the manuscript.

Corresponding authors

Correspondence to A. Fieguth or C. Wittweg.

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Competing interests

The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Spatial distribution of events.

Interaction depth Z versus squared radius R2 for events with energies 80–140 keV. High-density areas correspond to the edges of the TPC, where most of external β and γ radiation is absorbed. The 1,502-kg fiducial volume is indicated by the solid red line. Further segmentation into an inner (1.0 t) and an outer (0.5 t) volume is marked by the black dashed line.

Extended Data Fig. 2 Energy resolution.

Ratio of the mean peak energy (μE) to the peak width (σE) for low-energy monoenergetic lines in selected LXe dark-matter experiments (LUX38 and XENON10039) and in the 1.5-t fiducial mass of the XENON1T detector. The relative resolution is defined as the σE/μE ratio of the Gaussian lines and is fitted using a phenomenological function (solid blue line). For XENON1T the data points are 83mKr (41.5 keV), 131mXe (163.9 keV), 129mXe (236.2 keV), 214Pb (351.9 keV) and 208Tl (510.8 keV). Only statistical uncertainties are shown for XENON1T (smaller than the markers). The energy of the 2νECEC peak is indicated by the black dashed line.

Extended Data Fig. 3 125I time evolution.

Fit of the 125I model to data in a 2σ energy interval around the mean energy of the 125I peak in 10-d bins with Poisson uncertainties. Periods with an increased 125I decay rate are attributed to artificial activations from neutron calibrations, equipment tests and a dedicated activation study. The decrease of the rate to the background level corresponds to an effective iodine decay constant of τ = 9.1 d. The best fit is shown as a solid black line. The green (yellow) bands mark the 1σ (2σ) model uncertainties resulting from the Poisson uncertainties of the 125Xe data underlying the model. The pink bands indicate the data selection for the 2νECEC search, where the decay rate has returned to the background level.

Extended Data Fig. 4 χ2 curve for the number of measured 2νECEC events.

By comparing the best-fit value of N2νECEC = 126 events to a null result one obtains \(\sqrt{{\rm{\Delta }}{\chi }^{2}}=4.4\).

Extended Data Table 1 Systematic uncertainties
Full size table

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XENON Collaboration*. Observation of two-neutrino double electron capture in 124Xe with XENON1T. Nature 568, 532–535 (2019). https://doi.org/10.1038/s41586-019-1124-4

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