Search for Dark Matter with PICO (PICASSO/COUPP)

寻找宇宙中神秘失踪的质量已经成为当今粒子物理学和宇宙学面临的巨大挑战之一。天文观测表明，宇宙中只有1%的物质是发光的，现在有令人信服的证据表明，宇宙中85%的引力可追溯物质是一种新的外来物质，不同于我们周围含有质子、中子和电子的“普通”物质。这种所谓的冷暗物质（CDM）被认为是由所谓的wimp（弱相互作用大质量粒子）组成的，这是一种中性粒子，大约比质子重10到1000倍，它们彼此之间以及与普通物质之间的相互作用极其微弱。毕加索*暗物质实验转向了寻找弱相互作用大质量粒子。它基于液体到蒸汽的相变，这种相变可以由WIMP与亚稳态过热液滴中的原子核相互作用引起。在目前阶段，毕加索合作项目已经在加拿大安大略省萨德伯里的SNOLAB地下实验室安装了32个探测器，总目标质量为2.6千克数据。并不断地获取数据。2012年发表的研究结果使毕加索站在了直接依赖自旋的暗物质搜索的最前沿。一项针对整套探测器的新物理分析目前已经完成，预计将比2012年的结果有实质性的改进。新的分析也扩展到自旋独立扇区，在那里新的PICASSO数据开始探测CoGENT/DAMA/CRESST联合年度调制区域。毕加索发现的过热液体技术强大的背景辨别特性保证了“背景盲”操作长达吨年的曝光。因此，在2012年12月，过热液体领域的两个领先小组，PICASSO和COUPP在新的PICO合作中联合起来，共同致力于在SNOLAB进行500公斤的暗物质实验。在250L的气泡室中，将过热的C4F10或C3F8作为活性目标，PICO-250L将专注于自旋依赖的质子相互作用，在一年的实时数据采集中，预期灵敏度为3 × 10-7 pb。在与自旋无关的扇区中，对于质量约为20gevc -2的WIMP，灵敏度将达到4 × 10- 10pb左右，灵敏度增加到几个GeVc-2的质量。PICO-250L符合下一代暗物质实验（G2）的定义，没有其他G2实验在自旋依赖相互作用和轻WIMP质量领域具有类似的灵敏度。目前，背景源似乎得到了很好的控制；PICO 500的工程设计进展顺利。14 / 15财年的工作将集中在建设上，预计在15/16财年进行调试。加拿大毕卡索/ PICO特遣队由五个机构的研究小组组成：阿尔伯塔大学、劳伦森大学、蒙特里萨大学、皇后大学和多伦多大学。在这份拨款申请中，我们要求为完成PICASSO现阶段的工作提供运营资金，该阶段应于2013年夏季结束，并要求为我们在PICO-250L的工作提供运营资金。加拿大的超对称天体研究项目

The quest for the mysterious missing mass of the Universe has become one of the big challenges for today's particle physics and cosmology. Astronomical observations show that only 1% of the matter of the Universe is luminous and there is now convincing evidence that 85% of all gravitationally traceable matter in the Universe is of a new exotic kind, different from the "ordinary" matter surrounding us with its protons, neutrons and electrons. This so-called Cold Dark Matter (CDM) is thought to consist of so-called WIMPs (Weakly Interacting Massive Particles), neutral particles, about ten to thousand times heavier than a proton and which interact extremely weakly with each other and ordinary matter. The PICASSO* dark matter experiment is tuned to the search for WIMPs. It is based on liquid-to-vapour phase transitions which can be induced by WIMP interactions with nuclei in metastable superheated droplets. In its present phase the PICASSO collaboration has installed 32 detectors with a total target mass of 2.6 kg data in the SNOLAB underground laboratory at Sudbury, Ontario, Canada. and is continuously taken data. Results published in 2012 had placed PICASSO at the forefront of direct spin-dependent dark matter searches. A new physics analysis of an entire set of detectors is presently finalized and a substantial improvement with respect to our 2012 results is anticipated. The new analysis extends also to the spin-independent sector where the new PICASSO data start to probe the joint CoGENT/DAMA/CRESST annual modulation region. The powerful background discrimination features of the superheated liquid technique discovered by PICASSO promise “background blind” operation for up to tonne - year exposures. Therefore in December 2012 the two leading groups in the field of superheated liquids, PICASSO and COUPP joint forces within the new PICO collaboration to work together towards a 500 kg dark matter experiment at SNOLAB. With superheated C4F10 or C3F8 as the active target in a 250L bubble chamber, PICO-250L would focus on spin-dependent proton interactions, with an expected sensitivity at the level of 3 x 10-7 pb for one year of live data taking. In the spin-independent sector the sensitivity reach would be around 4 x 10-10 pb for WIMP masses around 20 GeVc-2, with an increased sensitivity down to masses of a few GeVc-2. PICO-250L meets the definition of a next generation dark matter experiment (G2) and no other G2 experiment would have comparable sensitivities in the sectors of spin-dependent interactions and light WIMP masses. At the moment background sources appear well under control; the engineering design of PICO 500 is progressing well. Work during FY14 / 15 will concentrate on construction, with commissioning foreseen in FY15/16. The Canadian PICASSO / PICO contingent consists of research groups of five institutions: University of Alberta, Laurentian University, Université de Montréal, Queen’s University and University of Toronto. In this grant request we ask for operating funds for the completion of the present phase of PICASSO, which should be concluded by summer 2013, and for operating funds to support our work in PICO-250L. *Project In Canada to Search for Supersymmetric Objects Projet d'Identification de CAnditats Supersymetriques Sombres