Search for Majoron-emitting modes of ¹³⁶Xe double beta decay with the complete EXO-200 dataset

A search for Majoron-emitting modes of the neutrinoless double beta decay of ¹³⁶Xe is performed with the full EXO-200 dataset. This dataset consists of a total ¹³⁶Xe exposure of 234.1 kg·yr, and includes data with detector upgrades that have improved the energy threshold relative to previous searches. A lower limit of T₁/₂ > 4.3 × 10²⁴ yr at 90% C.L. on the half-life of the spectral index n = 1 Majoron decay was obtained, a factor of 3.6 more stringent than the previous limit from EXO-200 and a factor of 1.6 more stringent than the previous best limit from KamLAND-Zen. This limit corresponds to a constraint on the Majoron-neutrino coupling constant of |⟨g_ee⟩| < (0.4–0.9) × 10⁻⁵. The lower threshold and the additional data taken resulted in a factor 8.4 improvement for the n = 7 mode compared to the previous EXO-200 search. This search provides the most stringent limits to date on the Majoron-emitting decays of ¹³⁶Xe with spectral indices n = 1, 2, 3, and 7.

Physical Review D, 104, 112002 (2021)