University of Chicago Contribution to PICO-500 Engineering and Construction

确定暗物质的问题已经困扰了我们80多年。在此期间，这种神秘物质存在的天文证据大量涌现。它既不能吸收光也不能发射光，但它对发光物体（如恒星、星系和更大的结构）的引力作用使它的存在变得明显。如果由新的基本粒子组成，安置在地下实验室里的专用探测器——在那里不受宇宙辐射的影响——就有可能解决这个长期存在的谜题，这个谜题现在是粒子物理学、宇宙学和天文学的核心。PICO-500是这些探测器之一，由有史以来最大的气泡室之一组成，位于加拿大SNOLAB实验室地下一英里多的地方。它被设计为对大多数已知的辐射源不敏感，而对粒子暗物质的特征相互作用具有高度响应。该奖项将允许一名美国博士后在这个有前途的暗物质探测器的建设、调试和第一批数据的提取中发挥核心作用。像PICO气泡室中使用的过热液体提供了任何暗物质探测器对电子反冲的最佳辨别，主要的环境背景限制了这些搜索的灵敏度。PICO探测器还具有对无处不在的α发射器诱导的信号的有效声学抑制。这导致了PICO项目的暗物质灵敏度持续增加的记录，因为它的气泡室已经变大了。最近的现象学工作强调了含氟PICO靶点的重要性：它们对于全面探索暗物质粒子相互作用的所有可能模式是必要的。本提案中描述的工作的完成将把PICO的灵敏度扩展到1E-42 cm^2的自旋相关截面（比目前的限制提高100）。这将通过这个通道提供对超对称暗物质的详尽探索，特别是对少数GeV/c^2粒子，这是一个非常有现象学意义的质量范围。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The problem of the identification of Dark Matter has been with us for more than eight decades. In the interim, astronomical evidence for the existence of this mysterious form of matter has poured in. Unable to absorb or emit light, its presence is made evident by its gravitational pull on luminous objects such as stars, galaxies, and larger structures. If composed of new elementary particles, dedicated detectors housed in underground laboratories -protected there from cosmic radiation- stand a chance of solving this long-standing riddle, now central to particle physics, cosmology and astronomy. PICO-500 is one of these detectors, consisting of one of the largest bubble chambers ever built and housed more than a mile underground in the Canadian SNOLAB laboratory. It is by design insensitive to most known sources of radiation, while highly responsive to the characteristic interactions expected from particle dark matter. This award will allow a US postdoc to play a central role in the construction, commissioning, and extraction of first data from this promising dark matter detector.Superheated liquids like those used in PICO bubble chambers provide the best discrimination of any dark matter detector against electron recoils, the dominant environmental background limiting the sensitivity of these searches. PICO detectors additionally feature an efficient acoustic rejection against signals induced by ubiquitous alpha emitters. This has resulted in a track-record of continuous increase in dark matter sensitivity from the PICO program, as its bubble chambers have grown in size. Recent phenomenological work has emphasized the importance of fluorine-containing PICO targets: these are necessary for a comprehensive exploration of all possible modes of dark matter particle interaction. The completion of the work described in this proposal will extend the sensitivity of PICO down to spin-dependent cross sections of 1E-42 cm^2 (an improvement of 100 beyond present limits). This will provide an exhaustive exploration of supersymmetric dark matter via this channel, and in particular for few GeV/c^2 particles, a mass range of much phenomenological interest.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.