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Author ORCID Identifier



Open Access Dissertation

Document Type


Degree Name

Doctor of Philosophy (PhD)

Degree Program


Year Degree Awarded


Month Degree Awarded


First Advisor

Andrea Pocar

Subject Categories

Elementary Particles and Fields and String Theory | Nuclear


This thesis presents searches for several modes of double beta decays of 136Xe to two low-lying excited levels of 136Ba. For each final level, both 2ν and 0ν double beta decay processes are considered. The data and general techniques developed by the EXO-200 collaboration are used, including a Machine Learning process to improve sensitivity in multivariate space. EXO-200 is an experimental program searching for 0νββ decay in a time projection chamber filled with 175 kg of liquid Xenon enriched to 80% 136Xe, functioning as both source and detector. Experimental searches of double beta decay with 136Xe and other isotopes are motivated by non Standard Model neutrinoless decay (0νββ) possibilities. If found this would imply the Majorana nature of neutrinos, provide access to the currently unknown absolute mass scale of the neutrino, and violate lepton conservation by 2 units. Nuclear matrix elements necessary to connecting observed neutrinoless halflife to the neutrino mass scale depend on nuclear models. Observation of both the 2ν 0+ excited and 0+ ground state in a single element may constrain these models. Sensitivity studies of search performance are described, and final sensitivities from 7.86 · 10^23 years to 2.00 · 10^25 years are reported.