Search for double-β decay of 76 Ge to excited states of 76 Se with the MAJORANA DEMONSTRATOR

Author(s): Arnquist, IJ; Avignone, FT; Barabash, AS; Barton, CJ; Bertrand, FE; Blalock, E; Bos, B; Busch, M; Buuck, M; Caldwell, TS; Chan, YD; Christofferson, CD; Chu, PH; Clark, ML; Cuesta, C; Detwiler, JA; Drobizhev, A; Edwards, TR; Edwins, DW; Efremenko, Y; Ejiri, H; Elliott, SR; Gilliss, T; Giovanetti, GK; Green, MP; Gruszko, J; Guinn, IS; Guiseppe, VE; Haufe, CR; Henning, R; Hervas Aguilar, D; Hoppe, EW; Hostiuc, A; Kidd, MF; Kim, I; Kouzes, RT; Lopez, AM; Lopez-Castano, JM; Martin, EL; Martin, RD; Massarczyk, R; Meijer, SJ; Mertens, S; Myslik, J; Oli, TK; Othman, G; Paudel, LS; Pettus, W; Poon, AWP; Radford, DC; Reine, AL; Rielage, K; Ruof, NW; Saykl, B; Stortini, MJ; Tedeschi, D; Varner, RL; Vasilyev, S; Wilkerson, JF; Wiseman, C; Xu, W; Yu, CH; Zhu, BX | Abstract: The majorana demonstrator is a neutrinoless double-β decay search consisting of a low-background modular array of high-purity germanium detectors, ∼2/3 of which are enriched to 88% in Ge76. The experiment is also searching for double-beta decay of Ge76 to excited states (e.s.) in Se76. Ge76 can decay into three daughter states of Se76, with clear event signatures consisting of a ββ-decay followed by the prompt emission of one or two γ rays. This results with high probability in multi-detector coincidences. The granularity of the demonstrator detector array enables powerful discrimination of this event signature from backgrounds. Using 41.9 kg yr of isotopic exposure, the demonstrator has set world leading limits for each e.s. decay of Ge76, with 90% CL lower half-life limits in the range of (0.75-4.0)×1024 yr. In particular, for the 2ν transition to the first 0+ e.s. of Se76, a lower half-life limit of 7.5×1023 yr at 90% CL was achieved.