A Search for the Double-Beta Decay of Xe-136 to an Excited State of Ba-136 with EXO-200

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Yen, Yung-Ruey
Hall, Carter R
This thesis presents a search for the two neutrino double beta decay (&beta;&beta;2&nu;) of <super>136</super>Xe to the 0<super>+</super><sub>1</sub> excited state of <super>136</super>Ba using data from the EXO-200 detector collected between 2011 and 2012. The &beta;&beta;2&nu; decay to the excited state is a process that have been observed for other double beta decay nuclei. An observation of this decay would shed some light on the validities of the various nuclear physics models. Located at the Waste Isolation Pilot Plant (WIPP) near Carlsbad, NM, EXO-200 is a liquid xenon time projection chamber filled with 200 kg of 80.6% isotopically enriched <super>136</super>Xe. The liquid xenon serves both as the decay source and the detection medium. Maximum likelihood fits of the sum energy spectra based on Monte Carlo simulations are used to constrain the number of &beta;&beta;2&nu; decay to the 0<super>+</super><sub>1</sub> excited state of <super>136</super>Ba. A half-life lower limit on this decay of 1.2 &middot; 10<super>23</super> year at 90% C.L is set, still a couple orders of magnitude from our expected theoretical rate of 2.5 &middot; 10<super>25</super> year from the applying the calculated phase space factor and the nuclear matrix element suppressions on the measured &beta;&beta;2&nu; decay to the ground state. A developing analysis using a new energy variable designed specifically for the search of the decay to the excited state is also discussed.