University of Chicago Contribution to PICO-500 Engineering and Construction

暗物质的识别问题已经困扰我们80多年了。在此期间，关于这种神秘物质存在的天文证据蜂拥而至。它不能吸收或发射光线，它的存在因其对恒星、星系和更大结构等发光物体的引力而变得明显。如果由新的基本粒子组成，专门的探测器被安置在地下实验室--在那里受到宇宙辐射的保护--就有机会解开这个长期存在的谜团，这个谜团现在是粒子物理学、宇宙学和天文学的核心。Pico-500就是这样的探测器之一，它由有史以来建造的最大的气泡室之一组成，安置在加拿大SNOLAB实验室地下一英里以上的地方。根据设计，它对大多数已知的辐射源不敏感，但对粒子暗物质预期的特征相互作用高度敏感。这一奖项将允许一名美国博士后在构建、调试和提取这一前景光明的暗物质探测器的第一批数据方面发挥核心作用。像皮科气泡室中使用的过热液体提供了所有暗物质探测器中对电子反冲的最佳分辨能力，主要环境背景限制了这些搜索的灵敏度。Pico探测器还具有针对无处不在的阿尔法发射器诱导的信号的有效声学抑制功能。这导致了皮科计划对暗物质敏感度不断增加的记录，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皮科靶的重要性：这些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。这项提议中描述的工作的完成将把Pico的灵敏度扩展到1E-42厘米^2的自旋相关截面(比目前的限制提高了100)。这将通过这个渠道提供对超对称暗物质的详尽探索，特别是对极少数GeV/c^2粒子，这是一个具有许多现象学兴趣的质量范围。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.