Search for neutrino-less double beta decay with EXO-200 and nEXO

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The search for lepton-number violating zero neutrino double-beta decays (bb0n) is one of the main challenges in today’s particle physics. Observation of this decay would require the neutrino to be a Majorana particle. The EXO-200 collaboration is currently searching for bb0n decay in the isotope 136Xe using a monolithic liquid-Xe time-projection chamber (TPC) detector deployed at the Waste Isolation Pilot Plant WIPP mine in New Mexico, USA. It is filled with ~200kg of Xe enriched in 136Xe to more than 80%. The energy and position of each interaction in the detector are recorded, allowing a potential bb0n signal to be distinguished from backgrounds. So far no bb0n signal has been observed, resulting in a half-life limit of T1/2>1.1x1025 years (90% C.L.). McGill U. recently joined the EXO-200 collaboration, and I represent McGill on the EXO-200 collaboration board. I propose to establish a control center at McGill U. to train students and support remote data taking. It is planned and agreed on by the collaboration that EXO-200 takes data for a minimum of two years, improving the sensitivity to bb0n by more than a factor of 2. The development of nEXO, a next-generation Xe TPC, has started. This detector will consist of 5T of enriched liquid Xe. A potential location is the SNOLab facility in Sudbury. A unique feature in the search for bb0n in Xe is the possibility to extract and identify the 136Xe decay daughter, 136Ba. If a bb0n like event occurs inside the detector, the decay volume is extracted and probed for Ba. If Ba is found, the event can be identified as a bb-decay of 136Xe, while backgrounds that do not produce Ba are rejected. This Ba-tagging technique will greatly enhance the sensitivity of the detector compared to a conventional, background limited detector. Ba-tagging is possible in liquid and gaseous TPC detectors. Both possibilities are being pursued by the nEXO collaboration using several techniques. One possible technique is the Ba-extraction from a Xe gas into a vacuum environment. While at Stanford, Dan Fudenberg (PhD student at Stanford) and I developed a technique to extract ions from a noble gas at pressures up to 10 bar. I plan to move the central parts of this proof-of-principle experiment from Stanford to McGill. This is a unique system, and represents currently the leading edge of high-pressure ion-extraction studies. Once relocated, I will implement the remaining steps required for a practical gas tagging system with a ppb sensitivity. The experiment at McGill will provide a significant contribution to the Ba-tagging developments in the nEXO collaboration and has the potential to become a leading contribution to the experiment. It offer a unique opportunity to train students in cutting-edge technologies. I request funding to support the operation of EXO-200 and the development of a Ba-tagging technique in Xe gas. Furthermore, I will systematically investigate SiPMs for photon detection in nEXO.