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。虽然中微子的存在在20世纪50年代就被确定，但直到最近十年才证明中微子的质量不为零。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次重的粒子--电子至少轻10万倍，那么如何解释质量尺度上的巨大差异呢？中微子质量的起源是否与其他粒子有本质的不同？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，这种过程被称为无中微子双β衰变。双β衰变是原子核中的两个中子转化为两个质子，同时发射两个电子以保存电荷。这种反应，如果发生在自然界中，将通过中微子的交换来介导。因此，衰变的半衰期是中微子绝对质量的量度，这个量仍然是未知的。此外，许多试图将三种基本力统一为一种的大统一理论，要么预测双β衰变应该发生，要么预测它应该被禁止。因此，一个能够观察到或排除这一过程的实验将在我们对物质最基本水平的理解中发挥关键作用。在这一提议中，PI要求资金在马里兰州大学成立一个新的小组，合作进行EXO-200实验，该实验旨在寻找氙-136的无中微子双β衰变，并大大扩展和拓宽了实验所用的工具，以监测、识别和消除氙中的负电性杂质源。 实验将于2007年开始获取数据。除了上述基本科学目标外，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也有潜在的应用。PI还建议在新墨西哥州的卡尔斯巴德和弗吉尼亚州的谢南多厄河谷的公立学校启动一项推广计划，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.