A search for exotic physics processes in double beta decay with EXO-200

The Standard Model of Elementary Particles (SM) has proved to be robust and compatible with many different experiments, but neutrinos have exhibited physics beyond the SM and remain an active topic of research. Neutrinoless double beta decay (0nubetabeta) is a theoretical process beyond the SM of great experimental interest due to the possibility of answering fundamental questions about the neutrino. A measurement of 0nubetabeta would decipher the nature of the neutrino, could resolve the neutrino mass hierarchy, and would show that lepton number is not a conserved quantity.;The EXO-200 experiment uses 175 kg of liquified xenon enriched in 136Xe to search for 0nubetabeta. With 99.8 kg˙yr of 136Xe exposure, a long electron lifetime, and a set of high-level analysis techniques, a limit of T(0nu/1/2) > 1.1 x 1025 yr is placed on the 0nubetabeta process. The high precision, low background experiment also provides a good candidate environment to search for other physics beyond the SM, two of which are described in this thesis.;An alternate 0nubetabeta process that can be sought in 136Xe is 0nubetabeta with the emission of a theoretical particle called a Majoron. A search for four Majoron-emitting modes of 0nubetabeta is performed, with limits on the Majoron-neutrino coupling constants presented. Another theoretical phenomenon that could present itself in double beta decay is a spectral perturbation due to neutrino coupling with a Lorentz-violating tensor field predicted in the Standard-Model Extension framework. The first direct search for the oscillation-free, momentum-independent neutrino coupling coefficient in this framework is presented, setting a limit on the timelike component of -1.73 · 10-5 GeV < a((3)/of) < 2.56 ˙ 10-6 GeV.;The EXO-200 detector has proven to be a sensitive probe to physics beyond the SM, but in order to reach half-lives of 0nubetabeta that resolve the neutrino hierarchy within the next few decades, a larger detector with a significant increase in mass is required. The upgraded nEXO experiment is currently in the research and development stage. The first simulations of nEXO and the resulting sensitivity reach are presented.