MRI: Development of a High Sensitivity Instrument to Search for CP Violation in Positronium Decay

尽管我们目前对宇宙中基本力和粒子（标准模型）的理解在许多方面都非常成功，但仍有许多重大问题尚未得到解答。一个重要的问题与物质和反物质行为的某些差异有关（所谓的cp违反效应）。这种效应已经在被称为介子的奇异粒子的衰变中被观察到，但迄今为止所观察到的差异太小，无法解释，例如，为什么宇宙中的物质比反物质多得多。由于这个和其他原因，人们认为标准模型是不完整的。因此，对其他类型的cp违反效应的实验搜索可以提供可能有助于扩展标准模型的信息。该奖项的目标是设计和建造一种新的仪器，以寻找电子及其反粒子正电子湮灭过程中的cp违反效应。该项目将为维滕贝格大学（Wittenberg University）的本科生提供研究培训机会，该校物理学专业的女性比例远高于全国平均水平。这些学生将与教师密切合作，开发、组装和测试最先进的仪器，并将学习如何在科学前沿进行研究。该装置还将为密歇根州立大学的至少一名研究生提供一个研究项目。该奖项旨在开发一种高效、紧凑、对称和高速率的仪器，该仪器将能够在张量极化正电子（Ps）的衰变中寻找CP破坏。已知CP违背发生在K介子和B介子的弱衰变中，并在标准模型（SM）中通过Cabibbo-Kobayashi-Maskawa （CKM）夸克混合矩阵的CP违背相进行描述。由于发现了中微子振荡，类似的机制也被期望存在于中微子扇区。在SM - CP违反效应被强烈抑制的观测值中寻找新的CP违反机制被认为是寻找新物理的最敏感的探针之一。它们还有望为宇宙中物质比反物质占优势的起源提供可能的微观解释。即将开发的仪器将使测量正电子的自旋和正电子的3- γ衰变中两个最高能量伽玛的动量的cp违反相关性方面有一个数量级的改进成为可能。该仪器由夹在两个薄塑料闪烁体之间的正电子源组成，这些闪烁体被高孔隙率介质包围，正电子可以在其中有效地形成正电子。这个正电子产生和标记系统将被48个LYSO闪烁晶体包围，这些晶体覆盖了一个大的立体角，并被分割，以选择最敏感的衰变事件。整个探测系统将位于密歇根州立大学（MSU）国家超导回旋加速器实验室（NSCL）现有超导磁体的热孔内。相关项的提取需要确定正电子的张量极化，这将从正电子的衰变时间谱中推导出来。开始信号由其中一个薄闪烁体发出，停止信号由一对LYSO晶体同时发出。该项目的优点在于系统的统计敏感性和系统敏感性。该小组希望在几个小时内得到目前0.002的极限，并在合理的测量时间内将其提高一个数量级。此外，所提出的仪器的对称性，以及所有构型同时测量的事实，消除了在最精确的先前测量中占主导地位的系统效应。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。

Although our present understanding of the fundamental forces and particles in the universe (the Standard Model) is remarkably successful in many respects, there are still significant questions that remain unanswered. One important question has to do with certain differences in how matter and anti-matter behave (so-called CP-violating effects). Such effects have been observed in the decays of exotic particles called mesons, but the differences that have so far been observed are much too small to explain, for example, why there is much more matter in the universe than anti-matter. For this and other reasons, it is believed that the Standard Model is incomplete. Thus, experimental searches for other types of CP-violating effects can provide information that might help to extend the Standard Model. The goal of this award is to design and build a new instrument to search for CP-violating effects in the annihilation of electrons and their anti-particles, positrons. This project will provide research training opportunities for undergraduate students at Wittenberg University, where the proportion of women physics majors is well above the national average. These students will work closely with faculty to develop, assemble, and test state-of-the-art instrumentation and will learn how to do research at the frontiers of science. The proposed apparatus will also enable a research project for at least one graduate student at Michigan State University.This award is to develop a highly efficient, compact, symmetric, and high rate capable instrument which will enable the search for CP violation in the decay of tensor polarized Positronium (Ps). CP violation is known to occur in the weak decays of the K and B mesons and is described in the Standard Model (SM) through the CP-violating phase of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. Since the discovery of neutrino oscillations, a similar mechanism is also expected to exist in the neutrino sector. Searches for new mechanisms of CP violation in observables where SM CP-violating effects are strongly suppressed are considered among the most sensitive probes to search for new physics. They are also expected to provide possible microscopic explanations for the origin of the dominance of matter over anti-matter in the universe. The instrument to be developed will make possible an order of magnitude improvement in the measurement of a CP-violating correlation involving the spin of positronium and the momenta of the two most energetic gammas in the 3-gamma decay of positronium. The instrument consists of a positron source sandwiched between two thin plastic scintillators surrounded by a high porosity medium where positrons efficiently form positronium. This positronium production and tagging system will be surrounded by 48 LYSO scintillation crystals covering a large solid angle and segmented so as to select the most sensitive decay events. The entire detection system will be located inside the warm bore of an existing superconducting magnet available at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU). The extraction of the correlation term requires the determination of the tensor polarization of positronium, which will be deduced from the decay time spectrum of positronium. The start signal will be given by one of the thin scintillators and the stop signal by a pair of LYSO crystals in coincidence. The merit of the project resides in both the statistical and systematic sensitivity of the system. The group expects to get the current limit of 0.002 in a few hours and to improve it by one order of magnitude within a reasonable measuring time. In addition, the symmetry of the proposed instrument, and the fact that all configurations are measured simultaneously, eliminate the dominant systematic effects in the most precise previous measurement.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.