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 124Xe – double-electron capture (0 / 2 ν ECEC), electron capture with positron emission (0 / 2 ν EC β +) and double-positron emission (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 T1/22νECβ+=(1.7±0.6)·1023year, the 2 ν EC β + decay of 124Xe will likely be detected in upcoming Dark Matter experiments (e.g. XENONnT or LZ), and their major background will be γ -rays from detector construction materials. Second, searches for the 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 124Xe could be achieved by the baseline design of the DARWIN observatory, or by extracting and using the 124Xe from the tailings of the nEXO experiment. We demonstrate how a combination of 124Xe results with those from 0 ν β -β - searches in 136Xe could help to identify the neutrinoless decay mechanism.