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# 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.

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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

### 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.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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

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

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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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• ### Sensitivity of the NEXT experiment to Xe-124 double electron capture

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Journal of High Energy Physics (2021)

• ### MetroMMC: Electron-Capture Spectrometry with Cryogenic Calorimeters for Science and Technology

• P. C.-O. Ranitzsch
• , D. Arnold
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Journal of Low Temperature Physics (2020)

• ### Dark-matter detector observes exotic nuclear decay

• Jouni Suhonen

Nature (2019)

• ### Ultra-sensitive radionuclide analyses: new frontiers in radioanalytics

• Pavel P. Povinec

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