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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.

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 222Rn 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 222Rn activity concentration of 10μBq/kg in 3.2t of xenon. The knowledge of the distribution of the 222Rn sources allowed us to selectively eliminate problematic components in the course of the experiment. The predictions from the emanation measurements were compared to data of the 222Rn activity concentration in XENON1T. The final 222Rn activity concentration of (4.5±0.1)μBq/kg in the target of XENON1T is the lowest ever achieved in a xenon dark matter experiment.

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.

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 magnitude . Until now, indications of 2νECEC decays have only been seen for two isotopes , Kr and Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance . The 2νECEC half-life is an important observable for nuclear structure models 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 . Here we report the direct observation of 2νECEC in 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  years (statistical uncertainty, 0.5 × 10  years; systematic uncertainty, 0.1 × 10  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 .

A technique for manufacturing thin-film resistors on cylindrical substrates is demonstrated. These devices are aimed for application in rare-event detectors that must minimize radioactive backgrounds from trace impurities in electronic components inside the detector. Cylindrical, conducting Ni films were created via Electron Beam Deposition, using a mechanism that rotates the substrate, to demonstrate proof of principle and measure the resistivity on axis and in azimuth. These films are characterized by measurements using a facsimile of the Van Der Pauw method combined with electrostatic simulations. In the two cylindrical samples made we observe anisotropic electrical behavior with resistivities of 1392.5, 888.5 \\(n \\Omega m\\) around the azimuth and of 81.9, 72.8 \\(n \\Omega m\\) along the axis of the sample. We show that this anisotropy is not caused just by the electron beam evaporation by measuring a planar rectangle sample made in the same process but without spinning which has estimated resistivities of 66.5, and 71.9 \\(n \\Omega m\\) in both directions, and calculated resistivity using the standard Van der Pauw equation of \\(66.1\\pm2.8\\) \\(n \\Omega m\\). In spite of the anisotropy in the cylindrical samples, we show that these films can be used as resistors.

Silicon photomultipliers are regarded as a very promising technology for next-generation, cutting-edge detectors for low-background experiments in particle physics. This work presents systematic reflectivity studies of Silicon Photomultipliers (SiPM) and other samples in liquid xenon at vacuum ultraviolet (VUV) wavelengths. A dedicated setup at the University of M\"unster has been used that allows to acquire angle-resolved reflection measurements of various samples immersed in liquid xenon with 0.45{\\deg} angular resolution. Four samples are investigated in this work: one Hamamatsu VUV4 SiPM, one FBK VUV-HD SiPM, one FBK wafer sample and one Large-Area Avalanche Photodiode (LA-APD) from EXO-200. The reflectivity is determined to be 25-36% at an angle of incidence of 20{\\deg} for the four samples and increases to up to 65% at 70{\\deg} for the LA-APD and the FBK samples. The Hamamatsu VUV4 SiPM shows a decline with increasing angle of incidence. The reflectivity results will be incorporated in upcoming light response simulations of the nEXO 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\\(\\). 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 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\\)).