2 neutrino double electron capture is a second order weak interaction process with a half lives that surpasses the era of the Universe by orders of magnitude1. Until today, indications of 2&nu, ECEC decays have only been seen for 2 isotopes2, 3, 4, 5, 78Kr and 130Ba, and instruments with very low background levels are needed to find them directly with high statistical significance6, 7. The 2&nu, ECEC half life is important visible for nuclear structure models8, 9, 10, 11, 12, 13, 14 and its measurement is a significant step in the search for neutrinoless double electron capture, the detection of which would set the majoranian character of the neutrino and would give access to the complete neutrino mass15, 16, 17.Here we report the direct observation of 2&nu, ECEC in 124Xe with the XENON1T matter detector.
The significance of the signal is 4.4 standard deviations and the corresponding half life of 1.8 ×, 1022 years is the longest measured directly so far. This analysis demonstrates that the low background and large target mass of xenon based dark matter detectors make them ideal to measuring infrequent processes and highlights the broad physics reach of next generation experiments18, 19, 20.