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Search for Double Beta Decays of $^{134}$Xe with EXO-200 Phase II

S. Al Kharusi, G. Anton, I. Badhrees, P. S. Barbeau, V. Belov, T. Bhatta, M. Breidenbach, T. Brunner, G. F. Cao, W. R. Cen, C. Chambers, B. Cleveland, M. Coon, A. Craycraft, T. Daniels, L. Darroch, S. J. Daugherty, J. Davis, S. Delaquis, A. Der Mesrobian-Kabakian, R. DeVoe, A. Dolgolenko, M. J. Dolinski, J. Echevers, B. Eckert, W. Fairbank, D. Fairbank, J. Farine, S. Feyzbakhsh, P. Fierlinger, Y. S. Fu, D. Fudenberg, P. Gautam, R. Gornea, G. Gratta, C. Hall, E. V. Hansen, J. Hoessl, P. Hufschmidt, M. Hughes, A. Iverson, A. Jamil, C. Jessiman, M. J. Jewell, A. Johnson, A. Karelin, L. J. Kaufman, T. Koffas, R. Krücken, A. Kuchenkov, K. S. Kumar, Y. Lan, A. Larson, B. G. Lenardo, D. S. Leonard, G. S. Li, S. Li, Z. Li, C. Licciardi, Y. H. Lin, R. MacLellan, T. McElroy, T. Michel, B. Mong, D. C. Moore, K. Murray, O. Njoya, O. Nusair, A. Odian, I. Ostrovskiy, H. Peltz Smalley, A. Perna, A. Piepke, A. Pocar, F. Retière, A. L. Robinson, P. C. Rowson, S. Schmidt, D. Sinclair, K. Skarpaas, A. K. Soma, V. Stekhanov, M. Tarka, S. Thibado, J. Todd, T. Tolba, T. I. Totev, R. Tsang, B. Veenstra, V. Veeraraghavan, P. Vogel, J. -L. Vuilleumier, M. Wagenpfeil, J. Watkins, M. Weber, L. J. Wen, U. Wichoski, G. Wrede, S. X. Wu, Q. Xia, D. R. Yahne, L. Yang, Y. -R. Yen, O. Ya. Zeldovich, T. Ziegler

Abstract

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% $^{136}$Xe and 19.098% $^{134}$Xe. The detector operated at WIPP between 2010 and 2018 and was designed to search for double beta decay of $^{136}$Xe. Data was acquired in two phases separated by a period of detector upgrades. We report on the search for $0νββ$ and $2νββ$ decay of $^{134}$Xe with Phase II EXO-200 data, with median 90% C.L. exclusion sensitivity $T_{1/2}^{0ν} \geq 3.7\times 10^{23}$ yr and $T_{1/2}^{2ν} \geq 2.6 \times 10^{21}$ 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 $^{134}$Xe of $T_{1/2}^{0ν} \geq 8.7\times10^{23}$ (90% C.L.) and the second strongest constraint on the two-neutrino decay of $T_{1/2}^{2ν} \geq 2.9\times10^{21}$ (90% C.L.), a 3-fold improvement over the EXO-200 Phase I measurement. New constraints are also set for the $2νββ$ and $0νββ$ decays of $^{134}$Xe to the lowest excited state of $^{134}$Ba.

Search for Double Beta Decays of $^{134}$Xe with EXO-200 Phase II

Abstract

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% Xe and 19.098% Xe. The detector operated at WIPP between 2010 and 2018 and was designed to search for double beta decay of Xe. Data was acquired in two phases separated by a period of detector upgrades. We report on the search for and decay of Xe with Phase II EXO-200 data, with median 90% C.L. exclusion sensitivity yr and 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 Xe of (90% C.L.) and the second strongest constraint on the two-neutrino decay of (90% C.L.), a 3-fold improvement over the EXO-200 Phase I measurement. New constraints are also set for the and decays of Xe to the lowest excited state of Ba.

Paper Structure

This paper contains 6 equations, 2 figures, 2 tables.

Figures (2)

  • Figure 1: Source agreement in the single-site energy dimension for sources positioned near the cathode. Prominent peaks include 667 keV (137Cs), 1173 and 1332 keV (60Co), 351, 609, and 1764 keV (226Ra), and 2615 keV (228Th).
  • Figure 2: Best fit to the single-site (SS, main panel) and multi-site (MS, inset) energy spectra, shown over the energy range used for the analysis. The MS dataset includes events with partial 3D spatial reconstruction, see text for details. The fits to both $2\nu$ and $0\nu$$\beta\beta$ decays to the ground state (GS) and excited state (ES) of $^{134}$Ba all return zero signal counts with $\chi^{2}_{red,SS}=1.57$ and $\chi^{2}_{red,MS}=1.09$, respectively. The expected spectra of the $\beta\beta$ signals are shown as follows: i) at the measured half-life lower limit ($2\nu$ GS mode, shaded), ii) magnified 5-fold (2$\nu$ ES mode) and iii) magnified 50-fold to make them visible ($0\nu$ modes, line-dotted).