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Two-neutrino double electron capture (2 ν ECEC) is a second-order weak-interaction process with a predicted half-life that surpasses the age of the Universe by many orders of magnitude 1 . Until now, indications of 2 ν ECEC decays have only been seen for two isotopes 2 – 5 , 78 Kr and 130 Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance 6 , 7 . The 2 ν ECEC half-life is an important observable for nuclear structure models 8 – 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 mass 15 – 17 . Here we report the direct observation of 2 ν ECEC in 124 Xe with the XENON1T dark-matter detector. The significance of the signal is 4.4 standard deviations and the corresponding half-life of 1.8 × 10 22  years (statistical uncertainty, 0.5 × 10 22  years; systematic uncertainty, 0.1 × 10 22  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 experiments 18 – 20 . Two-neutrino double electron capture is observed experimentally in 124 Xe with the XENON1T detector, yielding a half-life of 1.8 × 10 22 years.

We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column was integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant [Formula omitted]Rn background originating from radon emanation. After inserting an auxiliary [Formula omitted]Rn emanation source in the gas loop, we determined a radon reduction factor of [Formula omitted] (95% C.L.) for the distillation column by monitoring the [Formula omitted]Rn activity concentration inside the XENON100 detector.

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.

Abstract We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column was integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant²²²222 Rn background originating from radon emanation. After inserting an auxiliary²²²222 Rn emanation source in the gas loop, we determined a radon reduction factor ofR > 27R > 27 (95% C.L.) for the distillation column by monitoring the²²²222 Rn activity concentration inside the XENON100 detector.

We report the results of a search for the inelastic scattering of weakly interacting massive particles (WIMPs) in the XENON1T dark matter experiment. Scattering off \\(^{129}\\)Xe is the most sensitive probe of inelastic WIMP interactions, with a signature of a 39.6 keV de-excitation photon detected simultaneously with the nuclear recoil. Using an exposure of 0.89 tonne-years, we find no evidence of inelastic WIMP scattering with a significance of more than 2\\(\\sigma\\). A profile-likelihood ratio analysis is used to set upper limits on the cross-section of WIMP-nucleus interactions. We exclude new parameter space for WIMPs heavier than 100 GeV/c\\({}^2\\), with the strongest upper limit of \\(3.3 \\times 10^{-39}\\) cm\\({}^2\\) for 130 GeV/c\\({}^2\\) WIMPs at 90\\% confidence level.

The selection of low-radioactive construction materials is of utmost importance for the success of low-energy rare event search experiments. Besides radioactive contaminants in the bulk, the emanation of radioactive radon atoms from material surfaces attains increasing relevance in the effort to further reduce the background of such experiments. In this work, we present the \\(^{222}\\)Rn emanation measurements performed for the XENON1T dark matter experiment. Together with the bulk impurity screening campaign, the results enabled us to select the radio-purest construction materials, targeting a \\(^{222}\\)Rn activity concentration of 10 \\(\\mu\\)Bq/kg in 3.2 t of xenon. The knowledge of the distribution of the \\(^{222}\\)Rn sources allowed us to selectively eliminate critical components in the course of the experiment. The predictions from the emanation measurements were compared to data of the \\(^{222}\\)Rn activity concentration in XENON1T. The final \\(^{222}\\)Rn activity concentration of (4.5 \\(\\pm\\) 0.1) \\(\\mu\\)Bq/kg in the target of XENON1T is the lowest ever achieved in a xenon dark matter experiment.

XENONnT is a dark matter direct detection experiment, utilizing 5.9 t of instrumented liquid xenon, located at the INFN Laboratori Nazionali del Gran Sasso. In this work, we predict the experimental background and project the sensitivity of XENONnT to the detection of weakly interacting massive particles (WIMPs). The expected average differential background rate in the energy region of interest, corresponding to (1, 13) keV and (4, 50) keV for electronic and nuclear recoils, amounts to \\(12.3 \\pm 0.6\\) (keV t y)\\(^{-1}\\) and \\((2.2\\pm 0.5)\\times 10^{-3}\\) (keV t y)\\(^{-1}\\), respectively, in a 4 t fiducial mass. We compute unified confidence intervals using the profile construction method, in order to ensure proper coverage. With the exposure goal of 20 t\\(\\,\\)y, the expected sensitivity to spin-independent WIMP-nucleon interactions reaches a cross-section of \\(1.4\\times10^{-48}\\) cm\\(^2\\) for a 50 GeV/c\\(^2\\) mass WIMP at 90% confidence level, more than one order of magnitude beyond the current best limit, set by XENON1T. In addition, we show that for a 50 GeV/c\\(^2\\) WIMP with cross-sections above \\(2.6\\times10^{-48}\\) cm\\(^2\\) (\\(5.0\\times10^{-48}\\) cm\\(^2\\)) the median XENONnT discovery significance exceeds 3\\(\\sigma\\) (5\\(\\sigma\\)). The expected sensitivity to the spin-dependent WIMP coupling to neutrons (protons) reaches \\(2.2\\times10^{-43}\\) cm\\(^2\\) (\\(6.0\\times10^{-42}\\) cm\\(^2\\)).

Direct dark matter detection experiments based on a liquid xenon target are leading the search for dark matter particles with masses above \\(\\sim\\) 5 GeV/c\\(^2\\), but have limited sensitivity to lighter masses because of the small momentum transfer in dark matter-nucleus elastic scattering. However, there is an irreducible contribution from inelastic processes accompanying the elastic scattering, which leads to the excitation and ionization of the recoiling atom (the Migdal effect) or the emission of a Bremsstrahlung photon. In this letter, we report on a probe of low-mass dark matter with masses down to about 85 MeV/c\\(^2\\) by looking for electronic recoils induced by the Migdal effect and Bremsstrahlung, using data from the XENON1T experiment. Besides the approach of detecting both scintillation and ionization signals, we exploit an approach that uses ionization signals only, which allows for a lower detection threshold. This analysis significantly enhances the sensitivity of XENON1T to light dark matter previously beyond its reach.

We report results from searches for new physics with low-energy electronic recoil data recorded with the XENON1T detector. With an exposure of 0.65 t-y and an unprecedentedly low background rate of \\(762\\) events/(t y keV) between 1 and 30 keV, the data enables sensitive searches for solar axions, an enhanced neutrino magnetic moment, and bosonic dark matter. An excess over known backgrounds is observed at low energies and most prominent between 2 and 3 keV. The solar axion model has a 3.4\\(\\) significance, and a 3D 90% confidence surface is reported for axion couplings to electrons, photons, and nucleons. This surface is inscribed in the cuboid defined by \\(g_ae<3.8 10^-12\\), \\(g_aeg_an^eff<4.8 10^-18\\), and \\(g_aeg_a<7.710^-22 GeV^-1\\), and excludes either \\(g_ae=0\\) or \\(g_aeg_a=g_aeg_an^eff=0\\). The neutrino magnetic moment signal is similarly favored over background at 3.2\\(\\) and a confidence interval of \\(_ ın (1.4,2.9)10^-11_B\\) (90% C.L.) is reported. Both results are in strong tension with stellar constraints. The excess can also be explained by \\(\\) decays of tritium at 3.2\\(\\) with a trace amount that can neither be confirmed nor excluded with current knowledge of its production and reduction mechanisms. The significances of the solar axion and neutrino magnetic moment hypotheses are reduced to 2.0\\(\\) and 0.9\\(\\), respectively, if an unconstrained tritium component is included in the fitting. With respect to bosonic dark matter, the excess favors a monoenergetic peak at (\\(2.30.2\\)) keV (68% C.L.) with a 3.0\\(\\) global (4.0\\(\\) local) significance. We also consider the possibility that \\(^37\\)Ar may be present in the detector and yield a 2.82 keV peak. Contrary to tritium, the \\(^37\\)Ar concentration can be tightly constrained and is found to be negligible.

Xenon dual-phase time projection chambers designed to search for Weakly Interacting Massive Particles have so far shown a relative energy resolution which degrades with energy above \\(\\sim\\)200 keV due to the saturation effects. This has limited their sensitivity in the search for rare events like the neutrinoless double-beta decay of \\(^{136}\\)Xe at its \\(Q\\)-value, \\(Q_{\\beta\\beta}\\simeq\\) 2.46 MeV. For the XENON1T dual-phase time projection chamber, we demonstrate that the relative energy resolution at 1 \\(\\sigma/\\mu\\) is as low as (0.80\\(\\pm\\)0.02) % in its one-ton fiducial mass, and for single-site interactions at \\(Q_{\\beta\\beta}\\). We also present a new signal correction method to rectify the saturation effects of the signal readout system, resulting in more accurate position reconstruction and indirectly improving the energy resolution. The very good result achieved in XENON1T opens up new windows for the xenon dual-phase dark matter detectors to simultaneously search for other rare events.