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

寻找轻子数违反零的中微子双β衰变（bb 0 n）是当今粒子物理的主要挑战之一。要观察这种衰变，需要中微子是马约拉纳粒子。EXO-200合作项目目前正在使用部署在美国新墨西哥州废物隔离试验厂WIPP矿的单片液体时间投影室（TPC）探测器寻找同位素136氚中的bb 0 n衰变。它装有约200公斤的浓缩铀，其中136公斤浓缩到80%以上。记录探测器中每个相互作用的能量和位置，从而将潜在的bb 0 n信号与背景区分开来。到目前为止，尚未观察到bb 0 n信号，因此半衰期限值为T1/2>1.1x1025年（90% C.L.）。麦吉尔大学我最近加入了EXO-200协作，我代表麦吉尔在EXO-200协作委员会。我建议在麦吉尔大学建立一个控制中心。培训学生并支持远程数据采集。根据合作计划和协议，EXO-200至少需要两年的数据，将bb 0 n的灵敏度提高了2倍以上。 新一代的可再生热塑性塑料nEXO的开发已经开始。该探测器将由5吨浓缩液组成。一个潜在的地点是萨德伯里的SNOLab设施。在寻找136 Ba中的bb 0 n的一个独特功能是可以提取和识别136 Ba衰变的子体136 Ba。如果探测器内部发生类似bb 0 n的事件，则提取衰变体积并探测Ba。如果发现了Ba，则该事件可以被确定为136的bb衰变，而不产生Ba的背景则被拒绝。这种Ba标记技术与传统的背景限制检测器相比将大大提高检测器的灵敏度。 Ba标记在液体和气体TPC检测器中是可能的。这两种可能性都是nEXO合作使用几种技术追求的。一种可能的技术是将Ba从氦气中提取到真空环境中。在斯坦福大学时，Dan Fudenberg（斯坦福大学的博士生）和我开发了一种技术，可以在高达10 bar的压力下从惰性气体中提取离子。我计划把这个原理验证实验的核心部分从斯坦福大学搬到麦吉尔大学。这是一个独特的系统，代表了目前高压离子萃取研究的前沿。一旦搬迁，我将实施一个实用的气体标记系统所需的ppb灵敏度的其余步骤。麦吉尔大学的实验将为nEXO合作中的Ba标记发展做出重大贡献，并有可能成为该实验的主要贡献。它提供了一个独特的机会，培养学生在尖端技术。 我请求提供资金来支持EXO-200的运行和Xe气体中Ba标记技术的开发。此外，我将系统地研究SiPM在nEXO中的光子探测。

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.