Results of the MAJORANA DEMONSTRATOR's Search for Double-Beta Decay of ⁷⁶Ge to Excited States of ⁷⁶Se

The MAJORANA DEMONSTRATOR is searching for double-beta decay of ⁷⁶Ge to excited states (E.S.) in ⁷⁶Se using a modular array of high purity Germanium detectors. ⁷⁶Ge can decay into three E.S.s of ⁷⁶Se. The E.S. decays have a clear event signature consisting of a ββ-decay with the prompt emission of one or two γ-rays, resulting in with high probability in a multi-site event. The granularity of the DEMONSTRATOR detector array enables powerful discrimination of this event signature from backgrounds. Using 21.3 kg-y of isotopic exposure, the DEMONSTRATOR has set world leading limits for each E.S. decay, with 90% CL lower half-life limits in the range of (0.56 2.1) ⋅ 10²⁴ y. In particular, for the 2v transition to the first 0⁺ E.S. of ⁷⁶Se, a lower half-life limit of 0.68 ⋅ 10²⁴ at 90% CL was achieved.