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

EXO-200 was a leading double beta decay experiment consisting of a single-phase, enriched liquid xenon time projection chamber filled with an admixture of 80.672% ¹³⁶Xe and 19.098% ¹³⁴Xe. The detector operated at the Waste Isolation Pilot Plant between 2010 and 2018 and was designed to search for double beta decay of ¹³⁶Xe. Data were acquired in two phases separated by a period of detector upgrades. We report on the search for 0νββ and 2νββ decay of ¹³⁴Xe with Phase II EXO-200 data, with median 90% CL exclusion sensitivity to half-life T(0ν) ≥ 3.7 × 10²³ yr and T(2ν) ≥ 2.6 × 10²¹ yr, respectively. No statistically significant signal is observed for either decay mode. We set a world-leading lower limit on the half-life of the neutrinoless decay mode of ¹³⁴Xe of T(0ν) ≥ 8.7 × 10²³ yr (90% CL) and the second strongest constraint on the two-neutrino decay of T(2ν) ≥ 2.9 × 10²¹ yr (90% CL), a threefold improvement over the EXO-200 Phase I measurement. New constraints are also set for the 2νββ and 0νββ decays of ¹³⁴Xe to the lowest excited state of ¹³⁴Ba.

Physical Review Letters, 136, 232502 (2026)