Detection prospects for the second-order weak decays of $$^{124}$$Xe in multi-tonne xenon time projection chambers

Abstract

We investigate the detection prospects for two-neutrino and neutrinoless second-order weak decays of $$^{124}{\mathrm{Xe}}$$ 124 Xe – double-electron capture ($$0/2\upnu \text {ECEC}$$ 0 / 2 ν ECEC ), electron capture with positron emission ($$0/2\upnu \text {EC}\upbeta ^+$$ 0 / 2 ν EC β + ) and double-positron emission ($$0/2\upnu \upbeta ^+\upbeta ^+$$ 0 / 2 ν β + β + ) – in multi-tonne xenon time projection chambers. We simulate the decays in a liquid xenon medium and develop a reconstruction algorithm which uses the multi-particle coincidence in these decays to separate signal from background. This is used to compute the expected detection efficiencies as a function of position resolution and energy threshold for planned experiments. In addition, we consider an exhaustive list of possible background sources and find that they are either negligible in rate or can be greatly reduced using our topological reconstruction criteria. In particular, we draw two conclusions: First, with a theoretical half-life of $$T_{1/2}^{2\upnu \text {EC}\upbeta ^+} = (1.7 \pm 0.6)\cdot 10^{23}\,\text {year}$$ T 1 / 2 2 ν EC β + = ( 1.7 ± 0.6 ) · 10 23 year , the $$2\upnu \text {EC}\upbeta ^{+}$$ 2 ν EC β + decay of $$^{124}$$ 124 Xe will likely be detected in upcoming Dark Matter experiments (e.g. XENONnT or LZ), and their major background will be $$\upgamma $$ γ -rays from detector construction materials. Second, searches for the $$0\upnu \text {EC}\upbeta ^+$$ 0 ν EC β + decay mode will likely be background-free, and new parameter space may be within reach. To this end we investigate two different scenarios of existing experimental constraints on the effective neutrino mass. The necessary 500 kg-year exposure of $$^{124}$$ 124 Xe could be achieved by the baseline design of the DARWIN observatory, or by extracting and using the $$^{124}$$ 124 Xe from the tailings of the nEXO experiment. We demonstrate how a combination of $$^{124}$$ 124 Xe results with those from $$0\upnu \upbeta ^-\upbeta ^-$$ 0 ν β - β - searches in $$^{136}$$ 136 Xe could help to identify the neutrinoless decay mechanism.

Study of Excess Electronic Recoil Events in XENON1T

“…Events/(t y keV) Figure 3.9: Background model for 2νECEC of 124 Xe in SR1 including three monoenergetic peaks at 64.3, 36.7, and 9.8 keV with fixed branching ratios. The overall normalization of the three peaks was unconstrained in the fit, but the predicted values are given by the half-life calculation in [161]. Being an intrinsic background to xenon, this component is constant in time.…”

“…For this analysis, the event selection and consideration of time dependence allow us to include all three peaks in the background model. The predicted rates of the peaks are taken from an updated half-life [161] with fixed branching ratios from [88]; the overall rate was not constrained in the fit since the half-life was derived from the same dataset. Due to changing distance between the Earth and the Sun, the neutrino flux in the detector is expected to modulate by a subtle ∼ 7% peak-to-peak.…”

Domain-informed neural networks for interaction localization within astroparticle experiments

“…While the (x, y) coordinate is not directly used for physical analysis, it is useful for diagnostic analysis for the detector. A high position resolution is crucial for identifying the topology of interactions (Wittweg et al, 2020). Thus the distribution of localization error, ˆ…”

The Xenon Road to Direct Detection of Dark Matter at LNGS: The XENON Project

“…Both isotopes decay via second order weak processes though (double-beta and double-electron capture, respectively). They actually allow for new physics searches in the neutrino sector through their neutrinoless decay modes even without isotopic enrichment [50][51][52]. The two odd-spin isotopes 129 Xe (spin 1/2) and 131 Xe (spin 3/2), sensitive to the SD channel, are present with a large abundance of 26.4% and 21.2%, respectively.…”