Search for Two-neutrino Double-Beta Decay of $^{136}\rm Xe$ to the $0^+_1$ excited state of $^{136}\rm Ba$ with the Complete EXO-200 Dataset
Creators: name: 200 Collaboration
name: Kharusi, S. Al
name: Anton, G.
name: Badhrees, I.
name: Barbeau, P. S.
name: Beck, D.
name: Belov, V.
name: Bhatta, T.
name: Breidenbach, M.
name: Brunner, T.
name: Cao, G. F.
name: Cen, W. R.
name: Chambers, C.
name: Cleveland, B.
name: Coon, M.
name: Craycraft, A.
name: Daniels, T.
name: Darroch, L.
name: Daugherty, S. J.
name: Davis, J.
name: Delaquis, S.
name: Der Mesrobian-Kabakian, A.
name: DeVoe, R.
name: Dilling, J.
name: Dolgolenko, A.
name: Dolinski, M. J.
name: Echevers, J.
name: Fairbank, W.
name: Fairbank, D.
name: Farine, J.
name: Feyzbakhsh, S.
name: Fierlinger, P.
name: Fu, Y. S.
name: Fudenberg, D.
name: Gautam, P.
name: Gornea, R.
name: Gratta, G.
name: Hall, C.
name: Hansen, E. V.
name: Hoessl, J.
name: Hufschmidt, P.
name: Hughes, M.
name: Iverson, A.
name: Jamil, A.
name: Jessiman, C.
name: Jewell, M. J.
name: Johnson, A.
name: Karelin, A.
name: Kaufman, L. J.
name: Koffas, T.
name: Krücken, R.
name: Kuchenkov, A.
name: Kumar, K. S.
name: Lan, Y.
name: Larson, A.
name: Lenardo, B. G.
name: Leonard, D. S.
name: Li, G. S.
name: Li, S.
name: Li, Z.
name: Licciardi, C.
name: Lin, Y. H.
name: MacLellan, R.
name: McElroy, T.
name: Michel, T.
name: Mong, B.
name: Moore, D. C.
name: Murray, K.
name: Njoya, O.
name: Nusair, O.
name: Odian, A.
name: Ostrovskiy, I.
name: Perna, A.
name: Piepke, A.
name: Pocar, A.
name: Retière, F.
name: Robinson, A. L.
name: Rowson, P. C.
name: Runge, J.
name: Schmidt, S.
name: Sinclair, D.
name: Skarpaas, K.
name: Soma, A. K.
name: Stekhanov, V.
name: Tarka, M.
name: Thibado, S.
name: Todd, J.
name: Tolba, T.
name: Totev, T. I.
name: Tsang, R.
name: Veenstra, B.
name: Veeraraghavan, V.
name: Vogel, P.
name: Vuilleumier, J. -L.
name: Wagenpfeil, M.
name: Watkins, J.
name: Weber, M.
name: Wen, L. J.
name: Wichoski, U.
name: Wrede, G.
name: Wu, S. X.
name: Xia, Q.
name: Yahne, D. R.
name: Yang, L.
name: Yen, Y. -R.
name: Zeldovich, O. Ya.
name: Ziegler, T.
Publisher: name: arXiv
Year: 2023
Subjects: subject: High Energy Physics - Experiment (hep-ex)
subject: Nuclear Experiment (nucl-ex)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: A new search for two-neutrino double-beta ($2νββ$) decay of $^{136}\rm Xe$ to the $0^+_1$ excited state of $^{136}\rm Ba$ is performed with the full EXO-200 dataset. A deep learning-based convolutional neural network is used to discriminate signal from background events. Signal detection efficiency is increased relative to previous searches by EXO-200 by more than a factor of two. With the addition of the Phase II dataset taken with an upgraded detector, the median 90$\%$ confidence level half-life sensitivity of $2νββ$ decay to the $0^+_1$ state of $^{136}\rm Ba$ is $2.9 \times 10^{24}~\rm yr$ using a total $^{136}\rm Xe$ exposure of $234.1~\rm kg~yr$. No statistically significant evidence for $2νββ$ decay to the $0^+_1$ state is observed, leading to a lower limit of $T^{2ν}_{1/2}(0^+ \rightarrow 0^+_1) &gt; 1.4\times10^{24}~\rm yr$ at 90$\%$ confidence level, improved by 70$\%$ relative to the current world's best constraint.
description: 9 pages, 7 figures, 2 tables