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The results of the experimental search for two-neutrino $2K$-capture in $^{124}$Xe with a large copper proportional counter obtained by processing the data for an exposure of 37.7 kg$times$day are presented. The experimental setup is located at the Underground Low-Background Laboratory of the Baksan Neutrino Observatory at a depth of 4900 m w.e. The combination of methods of selection of useful signals with a unique set of characteristics and the event topology taken into account allowed us to suppress the background in the energy region of interest. A new half-life limit for $2K(2 u)$-capture in $^{124}$Xe was determined: T$_{1/2}geq7.7cdot10^{21}$ yrs (90% C.L.).
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Two-neutrino double electron capture is a process allowed in the Standard Model of Particle Physics. This rare decay has been observed in $^{78}$Kr, $^{130}$Ba and more recently in $^{124}$Xe. In this publication we report on the search for this process in $^{124}$Xe and $^{126}$Xe using the full exposure of the Large Underground Xenon (LUX) experiment, in a total of of 27769.5~kg-days. No evidence of a signal was observed, allowing us to set 90% C.L. lower limits for the half-lives of these decays of $2.0times10^{21}$~years for $^{124}$Xe and $1.9times10^{21}$~years for $^{126}$Xe.
Two-neutrino double electron capture is a rare nuclear decay where two electrons are simultaneously captured from the atomic shell. For $^{124}$Xe this process has not yet been observed and its detection would provide a new reference for nuclear matrix element calculations. We have conducted a search for two-neutrino double electron capture from the K-shell of $^{124}$Xe using 7636 kg$cdot$d of data from the XENON100 dark matter detector. Using a Bayesian analysis we observed no significant excess above background, leading to a lower 90 % credibility limit on the half-life $T_{1/2}>6.5times10^{20}$ yr. We also evaluated the sensitivity of the XENON1T experiment, which is currently being commissioned, and find a sensitivity of $T_{1/2}>6.1times10^{22}$ yr after an exposure of 2 t$cdot$yr.
Two-neutrino double electron capture ($2 u$ECEC) is a second-order Weak process with predicted half-lives that surpass the age of the Universe by many orders of magnitude. Until now, indications for $2 u$ECEC decays have only been seen for two isotopes, $^{78}$Kr and $^{130}$Ba, and instruments with very low background levels are needed to detect them directly with high statistical significance. The $2 u$ECEC half-life provides an important input for nuclear structure models and its measurement represents a first step in the search for the neutrinoless double electron capture processes ($0 u$ECEC). A detection of the latter would have implications for the nature of the neutrino and give access to the absolute neutrino mass. Here we report on the first direct observation of $2 u$ECEC in $^{124}$Xe with the XENON1T Dark Matter detector. The significance of the signal is $4.4sigma$ and the corresponding half-life $T_{1/2}^{2 utext{ECEC}} = (1.8pm 0.5_text{stat}pm 0.1_text{sys})times 10^{22};text{y}$ is the longest ever measured directly. This study demonstrates that the low background and large target mass of xenon-based Dark Matter detectors make them well suited to measuring other rare processes as well, and it highlights the broad physics reach for even larger next-generation experiments.
We conducted an improved search for the simultaneous capture of two $K$-shell electrons on the $^{124}$Xe and $^{126}$Xe nuclei with emission of two neutrinos using 800.0 days of data from the XMASS-I detector. A novel method to discriminate $gamma$-ray/$X$-ray or double electron capture signals from $beta$-ray background using scintillation time profiles was developed for this search. No significant signal was found when fitting the observed energy spectra with the expected signal and background. Therefore, we set the most stringent lower limits on the half-lives at $2.1 times 10^{22}$ and $1.9 times 10^{22}$ years for $^{124}$Xe and $^{126}$Xe, respectively, with 90% confidence level. These limits improve upon previously reported values by a factor of 4.5.
We describe the Monte Carlo (MC) simulation package of the `2K-CAPTURE setup and discuss the agreement of its output with data. The `2K-CAPTURE MC simulates the energy loss of particles in detector and components of the passive shield and generates the resulting response in working volume large proportional counter (LPC). The simulation accounts for absorption, reemission, and scattering of both photons and neutrons and tracks them until they either are absorbed. The algorithm proceeds with a detailed simulation of the electronics chain. The MC is tuned using data collected with radioactive calibration source deployed in the internal channel of the installation. The simulation reproduces the energy response of the detector corresponding to distribution of the generated pointwise clusters of a charge of primary ionization in LPC.
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