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}} – double-electron capture (0/2upnu text {ECEC}), electron capture with positron emission (0/2upnu text {EC}upbeta ^+) and double-positron emission (0/2upnu upbeta ^+upbeta ^+) – 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}^{2upnu text {EC}upbeta ^+} = (1.7 pm 0.6)cdot 10^{23},text {year}, the 2upnu text {EC}upbeta ^{+} decay of ^{124}Xe will likely be detected in upcoming Dark Matter experiments (e.g. XENONnT or LZ), and their major background will be upgamma -rays from detector construction materials. Second, searches for the 0upnu text {EC}upbeta ^+ 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}Xe could be achieved by the baseline design of the DARWIN observatory, or by extracting and using the ^{124}Xe from the tailings of the nEXO experiment. We demonstrate how a combination of ^{124}Xe results with those from 0upnu upbeta ^-upbeta ^- searches in ^{136}Xe could help to identify the neutrinoless decay mechanism.