Search for Double Beta Decays of ¹³⁴Xe with EXO-200 Phase II

The discovery of neutrinoless double beta decay (0νββ) would imply that neutrinos are Majorana fermions, i.e. their own antiparticles, probing the relationship between matter and antimatter. EXO-200 was a leading double beta decay experiment that searched for this decay in ¹³⁶Xe. EXO-200 operated between 2010 and 2018 at the underground WIPP site in New Mexico and consisted of a single-phase enriched liquid xenon TPC containing ~200 kg of an admixture of 81% ¹³⁶Xe and 19% ¹³⁴Xe. As another double beta emitter, ¹³⁴Xe presents an opportunity to search for 0νββ in a second xenon isotope within the same detector, which can reduce theoretical uncertainties on the ratio of the decay probabilities. We present a search for 0νββ and 2νββ decays of ¹³⁴Xe with Phase II EXO-200 data. No statistically significant signal is observed for either mode, and we set lower bounds on the half-life of the 0νββ and 2νββ decays of ¹³⁴Xe to the ground state of ¹³⁴Ba as well as 2νββ decay to the lowest 2⁺ excited state.