Search for Neutrinoless Double Beta Decay with EXO-200

中微子是物理学中最神秘、最难以捉摸的基本粒子。尽管它们的存在早在20世纪50年代就被确定了，但直到最近十年才证明中微子的质量是非零的。中微子质量的发现引发了一些有趣的问题。例如，我们知道最重的中微子比第二重的粒子——电子——至少轻10万倍。如何解释这种巨大的质量差异呢？中微子质量的起源与其他粒子有根本的不同吗？这与中微子是物质中唯一不带电荷的成分这一事实有关吗？中微子物理学家得出结论，这些问题可以通过研究一种罕见的核过程来解决，这种过程被称为中微子双衰变。双衰变是原子核中的两个中子转化为两个质子，同时释放两个电子以保持电荷。这种反应，如果在自然界中发生，将由一个中微子的交换来调解。因此，衰变的半衰期是对中微子绝对质量的一种度量，而中微子的绝对质量仍然是未知的。此外，许多试图将三种基本力统一为一的大统一理论要么预测会发生双衰变，要么预测它应该被禁止。因此，一个能够观察或排除这一过程的实验将在我们理解物质的最基本层面上发挥关键作用。在这项提案中，PI要求在马里兰大学成立一个新的小组，合作开展EXO-200实验，这是一个寻找氙-136的中微子双β衰变的实验，并大大扩展和拓宽实验中使用的工具，以监测、识别和消除氙中电负性杂质的来源。该实验将于2007年开始收集数据。除了上述基本科学目标之外，这里提出的研究将产生更广泛的影响。为EXO-200开发的辐射探测器技术在核医学和国土安全等其他领域具有潜在的应用前景。PI还建议在新墨西哥州卡尔斯巴德和弗吉尼亚州谢南多厄山谷的公立学校开展一项推广计划，这两个地区都是服务不足的地区。

Neutrinos are the most mysterious and elusive of the fundamental particles known to physics. Although their existence was conclusively established in the 1950's, only within the last decade has it been demonstrated that neutrinos have a non-zero mass. The discovery of neutrino mass raises some intriguing questions. For example, we know that the heaviest neutrino is at least a factor of 100,000 times lighter than the next heaviest particle, the electron. What can explain this wide disparity in mass scales? Is the origin of neutrino mass fundamentally different from the other particles? And is this related to the fact that neutrinos are the only constituents of matter that carry no electric charge?Neutrino physicists have concluded that these questions can best be addressed by studying a rare nuclear process known as neutrinoless double beta decay. Double beta decay is the conversion of two neutrons in an atomic nucleus into two protons, while emitting two electrons to conserve charge. This reaction, if it occurs in nature, would be mediated by the exchange of a neutrino. Consequently the half life of the decay is a measure of the absolute mass of the neutrino, a quantity which is still unknown. In addition, many grand unification theories, which attempt to unite three of the fundamental forces into one, either predict that double beta decay should occur, or predict that it should be forbidden. Therefore an experiment which can observe or rule out this process will play a key role in our understanding of matter at its most basic level.In this proposal, the PI requests funds to start a new group at the University of Maryland to collaborate on EXO-200, an experiment to search for the neutrinoless double beta decay of Xenon-136, and to significantly expand and broaden the tools used by the experiment to monitor, identify, and eliminate sources of electronegative impurities in the xenon. The experiment will begin taking data in 2007.In addition to the basic science goals described above, the research proposed here will have a significantly broader impact. The radiation detector technology being developed for EXO-200 has potential applications in other fields such as nuclear medicine and homeland security. The PI also proposes to initiate an outreach program in the public schools in Carlsbad New Mexico and the Shenandoah Valley of Virginia, both of which are underserved areas.