Enriched Xenon Observatory for double beta decay

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The Enriched Xenon Observatory (EXO) collaboration is engaged in a search for the neutrinoless double beta decay (0?ßß) of 136Xe. Observation of this decay would prove that neutrinos are their own anti-­-pariticle (Majorana), provide information on the scale and ordering of neutrino masses, and give the first evidence for lepton number non-­-conservation. EXO is currently operating an enriched xenon liquid-­-phase time projection chamber (EXO200) near Carlsbad, New Mexico. With data collected from this experiment, EXO made the first measurement of the regular double beta decay of 136Xe and has produced world-­-leading measurements in the search for the neutrinoless decay. Canadian groups have contributed significantly to these efforts providing both hardware and analysis expertise to the project. For example, Canadians have taken lead responsibility for elements of detector design and engineering, calibration systems, and analysis coordination. Prof. Jacques Farine (Laurentian) is currently chair of the EXO Scientific Board and Prof. David Sinclair (Carleton) is serving on the EXO Executive Committee. EXO200 will continue collecting data for at least 2 more years and expects to remain at the forefront of the search for the 0?ßß decay. To substantially improve sensitivity in the search for 0?ßß beyond EXO200, it will be necessary for EXO to scale up its detector technology and decrease the background levels. Based on the success of EXO200, the collaboration is pursuing development of a next-­-generation liquid-­-phase detector (nEXO). With the addition of the TRIUMF group to EXO, the Canadians are well-­-positioned to lead significant components in the development of the next phase of EXO. This work will include design of calibration systems, evaluation of UV sensitive photo-­-detectors, Monte Carlo simulation, detector engineering, and continued Barium tagging development. The SNOLAB ‘cryopit’ could provide an excellent home for this next phase of EXO, and the collaboration is pursuing concepts for nEXO with this in mind. While a liquid-­-phase detector might prove the best option for the next scale of EXO, there are distinct advantages that might be gained from a gas-­-phase detector. Hence the Canadian groups will continue to pursue detector R&D and, in particular, barium tagging for the gas-­-phase possibility.