SuperCDMS Research and Development: WIMP Dark Matter Detector Performance, Scalability, and Surface Backgrounds

暗物质的发现对宇宙学、天体物理学、高能粒子物理学和我们对引力的理解都具有至关重要的意义。从星系和超星系团到遥远的超新星和宇宙微波背景辐射的广泛观测告诉我们，宇宙中近90%的物质以某种新的形式存在，与普通粒子不同。到目前为止，回到兹维基在20世纪30年代观察到的昏迷星团S，这种物质只有通过重力才能揭示自己，因为它既不发射也不吸收光线，因此被称为“暗物质”。我们打算继续验证的一个主要假设是，暗物质由弱相互作用的大质量粒子组成，即WIMP，这是一种假设的基本粒子，在大爆炸后不久普通物质的碰撞中产生。如果WIMP是暗物质，那么它们在我们银河系区域的局部密度使它们可以通过地面探测器中原子核的散射而被探测到。自然的WIMP候选者来自超对称性(SUSY)，这是粒子物理标准模型(SM)的扩展。SM的超对称扩展解决了粒子物理中的一些突出问题，其中包括所谓的规范层次问题。这些问题与天体物理学和宇宙学提出的谜题完全不同。也许不是巧合的是，超对称粒子正好具有作为暗物质的正确性质。在银晕中寻找WIMP暗物质，以及费米实验室的Tevatron和欧洲核子研究中心(CERN)大型强子对撞机正在建设的大型加速器实验，都是在以互补的方式搜索相同的基本物理。作为低温暗物质搜索(CDMS)合作的一部分，我们小组正在进行CDMSII暗物质实验，该实验目前在苏丹矿运行着一个5公斤重的探测器阵列。最新的CDMSII结果(2005年7月)设定了迄今为止最严格的界限--是世界上所有其他实验的十倍--关于银晕中是否存在WIMP，并已经开始约束一些可能存在超对称粒子的参数空间。在这个奖项的支持下，我们将用这个完整的阵列获得一年的数据，对这些数据的分析将允许灵敏度增加一个数量级。我们的团队在使用我们在凯斯的探测器测试设施以及在索丹建造和运营主要仪器的实验中发挥了重要作用。此外，还进行了背景研究和数据分析，这些研究和数据分析是提取科学的核心。到目前为止，已有四名案例研究生完成了S关于该实验的博士学位，目前有两名三年级学生在积极为探测器操作、蒙特卡罗模拟和分析做出贡献。在接下来的两年里，这些学生将与PI和研究助理(博士后)一起继续前往实验场，调整新的探测器阵列，操作并将数据带回家进行分析。当我们在矿山时，我们都将参加积极的外展计划，包括每天对实验室的公开参观。虽然Soudan的科学工作将消耗这笔拨款支持的大约80%的资源，但我们打算使用剩余的资源进行探测器研发，目标是拟议中的下一代实验SuperCDMS。这将主要涉及由我们在斯坦福大学的能源部支持的合作者制造的新探测器的低温测试，旨在改善背景拒绝和每个探测器模块更高的质量。在这项测试计划中，我们计划利用简化的探测器封装和更简单的冷电子设备可以同时进行测试的机会。我们还将继续去年开始的薄膜钨研究，通过本科高级项目。这项工作旨在更好地了解在我们的探测器中检测粒子相互作用的温度敏感薄膜的特性。目前，存在一个可以纠正的可变性问题，但需要大量的测试资源。胶片的制备是在一个共享的部门设施中以最低的成本进行的，并在我们的稀释冰箱中进行测试，同时进行探测器测试。我们研发工作的最后一个方面是开始开发一种新的检测技术来检测低能量的贝塔发射体，这代表了一个可能阻碍实验进展的背景。利用适度的资源和本科生的参与，我们可以帮助为未来的快速进步奠定基础。T.Shutt教授的团队和S.Golwala教授的团队在加州理工学院的存在，将极大地利用我们团队的努力。Shutt的团队正在为氙气实验开发低活性的MWPC元素，类似于Beta筛选器所需的元素，Golwala的团队可能有能力为原型建造提供硬件。

The discovery of dark matter is of fundamental importance to cosmology, astrophysics, high-energy particle physics and our understanding of gravity. A broad range of observations from galaxies and superclusters to distant supernovae and the cosmic microwave background radiation, tell us that nearly 90% of the matter in the universe is in some new form, different from ordinary particles. So far, going back to Zwicky's observations of the Coma cluster in the 1930's, this matter has revealed itself only through gravity, and is referred to as "dark matter" because it neither emits nor absorbs light. A leading hypothesis, which we propose to continue testing, is that the dark matter is comprised of Weakly Interacting Massive Particles, or WIMPs, a hypothetical elementary particle produced moments after the Big Bang in collisions of ordinary matter. If WIMPs are the dark matter, then their local density in our region of the Milky Way makes them detectable via scattering from atomic nuclei in a terrestrial detector. Natural WIMP candidates come from Supersymmetry (SUSY), an extension to the Standard Model (SM) of particle physics. Supersymmetric extensions of the SM solve a number of outstanding problems in particle physics, among them the so-called gauge hierarchy problem. These problems are completely distinct from riddles posed by astrophysics and cosmology. Perhaps not coincidently, SUSY particles have just the right properties to be the dark matter. The search for WIMP dark matter in the galactic halo and the large accelerator-based experiments at Fermilab's Tevatron and those under construction at CERN's Large Hadron Collider are, in a complementary fashion, searching for the same fundamental physics. Our group, as part of the Cryogenic Dark Matter Search (CDMS) collaboration, is conducting the CDMSII dark matter experiment which is currently running a 5-kg detector array in the Soudan Mine. The latest CDMSII result (July 2005) set the most stringent bound to date - by a factor of ten over all other experiments in the world - on the presence of WIMPs in the galactic halo, and has begun to constrain some of the parameter space where SUSY particles could lie. With support from this award, we will acquire a year of data with this full array, the analysis of which will allow an additional order of magnitude in sensitivity. Our group has plyaed a significant role in building the experiment using our detector test facility at Case, as well as building and operating the main apparatus at Soudan. In addition, have conducted background studies and data analyses that have been central to extracting the science. To date, four Case graduate students have completed their Ph.D.'s on the experiment, and currently two third year students on actively contributing to detector operations, Monte Carlo simulations and analysis. In the coming two years, these students, together with the PI's and research associates (postdocs), will continue travelling to the experimental site to tune up the new detector array, operate, and bring home the data to analyze. When at the mine, we will all participate in the active outreach program consisting of daily public tours of the laboratory. While the Soudan science effort will consume about 80% of the resources supported by this grant, we intend to use the remaining resources to carry out detector R&D aimed at a proposed next-generation experiment called SuperCDMS. This will primarily involve cryogenic testing of new detectors being fabricated by our DoE-supported collaborators at Stanford that aim to have improved rejection of backgrounds and higher mass per detector module. Within this testing program, we plan to take advantage of opportunities in which streamlined detector packaging and simpler cold electronics can be simultaneously tested. We will also continue thin film tungsten studies that were initiated last year through undergraduate senior projects. This work aims to better understand the properties of the temperature-sensitive films that detect particle interactions in our detectors. At present, there is a variability problem that can be corrected but requires significant testing resources. The preparation of the films is carried out at minimal cost in a shared departmental facility and tested in our dilution refrigerator simultaneously with detector-testing runs. The final aspect of our R&D work is to begin development of a new assay technique to detect low-energy beta emitters, which represent a background tha can hamper progress on the experiment. Using a modest level of resources and undergraduate involvement, we can help lay the groundwork for rapid progress in the future. The efforts of our group will be highly leveraged by the presence of Prof. T. Shutt's group at Case and Prof. S. Golwala's group at Caltech. Shutt's group is developing low-activity MWPC elements for the XENON experiment similar to what's needed for the beta screener, and Golwala's group may be in a position to provide hardware for prototype construction.