EAGER Proposal: Preliminary Studies for a Dark Photon Search

粒子物理学是发现物质和能量的基本成分的科学，涉及从最小到最大尺度的宇宙探索。20世纪粒子物理学的一个显著成就是标准模型的发展，它令人印象深刻地描述了广泛的基本粒子，如夸克和轻子，以及它们之间的相互作用（力）。但是标准模型是不完整的，我们知道在它之外还有重要的物理学。证据是什么？例如，天文观测表明，标准模型的已知粒子只占宇宙总物质的1/6。剩下的被称为“暗物质”，因为我们不能直接看到它。为了寻找暗物质，许多实验正在进行中或正在计划阶段，暗物质可能是一种粒子，也可能是多种粒子，或者完全是其他的东西。一种假设的暗物质粒子被称为暗光子（标记为A），它预计会与标准模型的光子（我们大多数人都知道是伽马射线）相互作用。智力价值：这项探索性研究的早期概念补助金（EAGER）支持由Peter wititch教授领导的康奈尔大学小组进行的初步研究，以评估在康奈尔威尔逊同步加速器实验室进行“高风险-高回报”暗光子（a’）搜索实验的技术可行性、最佳设计、物理范围和大致成本。预期的目标是在反应中产生A‘: (e+) + (e-) -光子+ A’。一个正电子（e+）束将从同步加速器中提取出来，并引导到液态氢目标上，这将提供目标电子的来源（目标电子是氢原子内部的成分）。出射光子是在晶体电磁量热计中检测到的，而a '本身不是直接检测到的，而是通过其在测量的缺失质量分布中的潜在特征“观察”到的。这种暗光子搜索的策略将与现有的和计划中的实验在几个方面有所不同：(a)暗光子是通过缺失质量技术识别的，因此搜索不像大多数竞争实验那样依赖于特定的衰变模式；(b)没有关于暗光子寿命的假设，这是许多使用长束转储或电子-正电子顶点的实验所内置的；(c)暗光子完全不需要衰变。这项研究需要的唯一关键假设是暗光子应该与普通光子相互作用。该提案解决了最近发布的粒子物理项目优先小组报告（称为P5）的两个科学驱动因素：暗物质和寻找新粒子和相互作用。康奈尔储存环的正电子束提供的独特设施将使这个小组能够探索暗物质物理的新体制，也可能有助于解释所谓的“g-2异常”，这是当前实验研究中一个潜在的相关和非常有趣的话题。在标准模型之外寻找这种物理现象，将面临基本的对称性、质量的本质、空间的维度以及我们宇宙的宇宙学起源。更广泛的影响：该提案包括在与该EAGER项目相关的前沿研究技术方面对本科生和研究生进行培训。此外，该提案的主要研究人员都为康奈尔大学的一个广泛的外展项目做出了贡献：到当地的K-12学校，包括为教师、教育工作者和高中生提供的校内项目和校外研讨会；对公众；对本科生也是如此。

Particle Physics is the science of discovery of the fundamental constituents of matter and energy and is involved in the exploration of the Universe from the smallest to the largest scales. A remarkable achievement of Twentieth Century Particle Physics was the development of the Standard Model that describes impressively well a broad spectrum of fundamental particles such as quarks and leptons and the interactions (the forces) among them. But the Standard Model is incomplete and we know there is important physics that lies beyond it. What is the evidence? As an example, astronomical observations indicate that the known particles of the Standard Model make up only 1/6 of the total matter in the Universe. The remainder is called "Dark Matter" because we don't see it directly. A number of experiments are underway or in the planning stage to look for the Dark Matter, which might be one type of particle or possibly many or perhaps something else entirely. One hypothesized Dark Matter particle is called the Dark Photon (labeled the A') and it is expected to interact with the photon of the Standard Model (known to most of us as a gamma ray).Intellectual Merit: This EArly concept Grant for Exploratory Research (EAGER) supports preliminary studies by a Cornell group led by Professor Peter Wittich to assess the technical feasibility, optimal design, physics reach, and approximate cost of a "high risk-high payoff" dark photon (A') search experiment at the Cornell Wilson Synchrotron Laboratory. The intended goal is to produce the A' in the reaction: (e+) + (e-) - photon + A'. A positron (e+) beam is to be extracted from the synchrotron and directed onto a liquid hydrogen target that will provide the source of target electrons (which are constituents within the hydrogen atoms). The outgoing photon is detected in a crystal electromagnetic calorimeter and the A' itself is not detected directly, but rather is "observed" via its potential signature in the measured missing mass distribution.The strategy of this dark photon search will differ from existing and planned experiments in several respects: (a) the dark photon is identified by a missing-mass technique, so the search does not depend on a specific decay mode, as most competing experiments do; (b) there is no assumption about dark photon lifetime, as is built into many experiments that use long beam dumps or electron-positron vertexing; and (c) there is no requirement that the dark photon decay at all. The only critical assumption needed for this search is that the dark photon should interact with ordinary photons.This proposal addresses two of the science drivers of the recently released Report of the Particle Physics Project Prioritization Panel (called P5): Dark Matter and the Search for New Particles and Interactions. The unique facility provided by the Cornell storage ring's positron beam would allow this group to probe a new regime in Dark Matter physics, and also to possibly help explain the so-called "g-2 anomaly", a potentially related and highly interesting topic of current experimental research. Searching for this kind of physics beyond the Standard Model confronts fundamental symmetries, the nature of mass, the dimensionality of space, and the cosmological origins of our universe.Broader Impacts: The proposal includes training of undergraduate and graduate students in cutting-edge research techniques associated with this EAGER project. Additionally the principal investigators on the proposal all contribute to an extensive program of outreach at Cornell: to local K-12 schools, including both in-school programs and out-of-school workshops for teachers, educators, and high school students; to the general public; and to undergraduates.