Letter | Published:

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

## 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.

## Access optionsAccess options

from\$8.99

All prices are NET prices.

## Data availability

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

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## References

1. 1.

Winter, R. G. Double K capture and single K capture with positron emission. Phys. Rev. 100, 142–144 (1955).

2. 2.

Gavrilyuk, Y. M. et al. Indications of 2ν2K capture in 78Kr. Phys. Rev. C 87, 035501 (2013).

3. 3.

Ratkevich, S. S. et al. Comparative study of the double-K-shell-vacancy production in single- and double-electron-capture decay. Phys. Rev. C 96, 065502 (2017).

4. 4.

Meshik, A. P., Hohenberg, C. M., Pravdivtseva, O. V. & Kapusta, Y. S. Weak decay of 130Ba and 132Ba: geochemical measurements. Phys. Rev. C 64, 035205 (2001).

5. 5.

Pujol, M., Marty, B., Burnard, P. & Philippot, P. Xenon in archean barite: weak decay of 130Ba, mass-dependent isotopic fractionation and implication for barite formation. Geochim. Cosmochim. Acta 73, 6834–6846 (2009).

6. 6.

Gavriljuk, Y. M. et al. 2K-capture in 124Xe: results of data processing for an exposure of 37.7 kg day. Phys. Part. Nucl. 49, 563–568 (2018).

7. 7.

Abe, K. et al. Improved search for two-neutrino double electron capture on 124Xe and 126Xe using particle identification in XMASS-I. Progr. Theor. Exp. Phys. 2018, 053D03 (2018).

8. 8.

Suhonen, J. Double beta decays of 124Xe investigated in the QRPA framework. J. Phys. G Nucl. Phys. 40, 075102 (2013).

9. 9.

Aunola, M. & Suhonen, J. Systematic study of beta and double beta decay to excited final states. Nucl. Phys. A 602, 133–166 (1996).

10. 10.

Singh, S., Chandra, R., Rath, P. K., Raina, P. K. & Hirsch, J. G. Nuclear deformation and the two neutrino double-β-decay in 124,126Xe, 128,130Te, 130,132Ba and 150Nd isotopes. Eur. Phys. J. A 33, 375–388 (2007).

11. 11.

Hirsch, M., Muto, K., Oda, T. & Klapdor-Kleingrothaus, H. V. Nuclear structure calculation of β+β+, β+/EC and EC/EC decay matrix elements. Z. Phys. A 347, 151–160 (1994).

12. 12.

Rumyantsev, O. A. & Urin, M. H. The strength of the analog and Gamow–Teller giant resonances and hindrance of the 2νββ-decay rate. Phys. Lett. B 443, 51–57 (1998).

13. 13.

Pirinen, P. & Suhonen, J. Systematic approach to β and 2νββ decays of mass A = 100–136 nuclei. Phys. Rev. C 91, 054309 (2015).

14. 14.

Coello Pérez, E. A., Menéndez, J. & Schwenk, A. Two-neutrino double electron capture on 124Xe based on an effective theory and the nuclear shell model. Preprint at https://arxiv.org/abs/1809.04443 (2018).

15. 15.

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

16. 16.

Bernabeu, J., De Rujula, A. & Jarlskog, C. Neutrinoless double electron capture as a tool to measure the ν e mass. Nucl. Phys. B 223, 15–28 (1983).

17. 17.

Sujkowski, Z. & Wycech, S. Neutrinoless double electron capture: a tool to search for Majorana neutrinos. Phys. Rev. C 70, 052501 (2004).

18. 18.

Aprile, E. et al. Physics reach of the XENON1T dark matter experiment. J. Cosmol. Astropart. Phys. 1604, 027 (2016).

19. 19.

Mount, B. J. et al. LUX-ZEPLIN (LZ). Report No. LBNL-1007256 (Lawrence Berkeley National Laboratory, 2017).

20. 20.

Aalbers, J. et al. DARWIN: towards the ultimate dark matter detector. J. Cosmol. Astropart. Phys. 1611, 017 (2016).

21. 21.

Doi, M. & Kotani, T. Neutrinoless modes of double beta decay. Prog. Theor. Phys. 89, 139–159 (1993).

22. 22.

Cullen, D. Program RELAX: A Code Designed to Calculate Atomic Relaxation Spectra of X-Rays and Electrons. Report No. UCRL-ID–110438 (Lawrence Livermore National Laboratory, 1992).

23. 23.

Buchmüller, W., Peccei, R. & Yanagida, T. Leptogenesis as the origin of matter. Annu. Rev. Nucl. Part. Sci. 55, 311–355 (2005).

24. 24.

Nesterenko, D. A. et al. Double-beta transformations in isobaric triplets with mass numbers A = 124, 130, and 136. Phys. Rev. C 86, 044313 (2012).

25. 25.

Aprile, E. et al. Search for two-neutrino double electron capture of 124Xe with XENON100. Phys. Rev. C 95, 024605 (2017).

26. 26.

Aprile, E. et al. The XENON1T dark matter experiment. Eur. Phys. J. C 77, 881 (2017).

27. 27.

Aprile, E. et al. Dark matter search results from a one tonne × year exposure of XENON1T. Phys. Rev. Lett. 121, 111302 (2018).

28. 28.

Aprile, E. et al. Conceptual design and simulation of a water Cherenkov muon veto for the XENON1T experiment. J. Instrum. 9, P11006 (2014).

29. 29.

Aprile, E. et al. Material radioassay and selection for the XENON1T dark matter experiment. Eur. Phys. J. C 77, 890 (2017).

30. 30.

Aprile, E. et al. Removing krypton from xenon by cryogenic distillation to the ppq level. Eur. Phys. J. C 77, 275 (2017).

31. 31.

de Laeter, J. et al. Atomic weights of the elements. Review 2000 (IUPAC technical report). Pure Appl. Chem. 75, 683–800 (2003).

32. 32.

Linstrom, P. & Mallard, W. G. E. NIST Chemistry WebBook, NIST Standard Reference Database Number 69 https://doi.org/10.18434/T4D303 (2018).

33. 33.

Zhang, H. et al. Dark matter direct search sensitivity of the PandaX-4T experiment. Sci. China Phys. Mech. Astron. 62, 31011 (2019).

34. 34.

Manalaysay, A. et al. Spatially uniform calibration of a liquid xenon detector at low energies using 83mKr. Rev. Sci. Instrum. 81, 073303 (2010).

35. 35.

Conti, E. et al. Correlated fluctuations between luminescence and ionization in liquid xenon. Phys. Rev. B 68, 054201 (2003).

36. 36.

Aprile, E., Giboni, K. L., Majewski, P., Ni, K. & Yamashita, M. Observation of anti-correlation between scintillation and ionization for MeV gamma-rays in liquid xenon. Phys. Rev. B 76, 014115 (2007).

37. 37.

Szydagis, M. et al. NEST: a comprehensive model for scintillation yield in liquid xenon. J. Instrum. 6, P10002 (2011).

38. 38.

Akerib, D. S. et al. Signal yields, energy resolution, and recombination fluctuations in liquid xenon. Phys. Rev. D 95, 012008 (2017).

39. 39.

Aprile, E. et al. The XENON100 dark matter experiment. Astropart. Phys. 35, 573–590 (2012).

## 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.

### 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
• , 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

• H. Wang

### 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.

### Competing interests

The authors declare no competing interests.

### Corresponding authors

Correspondence to A. Fieguth or C. Wittweg.

## Extended data figures and tables

1. ### 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.

2. ### 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.

3. ### 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.

4. ### 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 $$\sqrt{{\rm{\Delta }}{\chi }^{2}}=4.4$$.

### DOI

https://doi.org/10.1038/s41586-019-1124-4