High Precision Experiments to Study Light Neutral Meson Decay Rates and the Proton Charge Radius Using Electromagnetic Probes

该奖项包括杰斐逊实验室（JLab）的三个项目：（a）开发一个新的实验，以低于百分之一的精度测量质子电荷半径;（B）从PrimEx-II数据集中以前所未有的1.4%的精度提取π零介子的寿命;以及（c）开发和准备批准的实验，以测量新开发的Hall D中的η介子的寿命。 质子大小作为物理学中最基本的物理量之一，其准确性一直是物理学研究的重要内容。 最近，一个μ子-氢实验的新结果，其前所未有的精度低于0.1%，表明质子的大小比以前已知的小7个标准差。 这一实验事实引发了物理学中著名的“质子半径危机”。 为了解决这一危机，由该奖项的PI领导的合作开发了一个独特设计的电子散射实验，该实验得到了JLab的PAC 38的批准，其主要条件是开发一种新的无窗气体氢靶。 对轻赝标介子寿命的精确测量仍然是该奖项的重要组成部分。 PrimEx合作进行了第一次测量（PrimEx-I）的π-零介子寿命的最佳精度到目前为止（2.8%）。 结果已发表并包含在粒子数据组平均值中。 为了达到前所未有的1.4%精度，我们的合作升级了实验装置，并在2010年秋季进行了第二次（PrimEx-II）实验。 根据这项合同，将完成分析过程，并公布结果。 在PI的领导下，开发了一个测量eta介子寿命的奖项，该奖项具有迄今为止最好的精度（3%），并被批准在JLab的D厅运行。 我们将在接下来的几年里准备这个实验。质子大小的精确知识对于理解强子物质的结构至关重要。 此外，它对原子物理学，特别是氢原子的光谱学非常重要。 里德伯常数是迄今为止物理学中已知的最基本、最精确的常数之一。 所提出的实验，其独立的新方法和预计的精度，有一个直接的潜力，要么显着改变里德伯常数的当前值（如果它证实了最近的μ子氢的结果）或质疑μ子氢系统的基础理论计算的充分性。 在后一种情况下，它也可能表明标准模型之外的新物理学。 在物理学的相互作用理论中，轻赝标介子的寿命携带着关于对称性的直接信息，特别是它们的部分破缺效应。 因此，这些重要量的精确知识将作为我们理解自然的基准测试。 特别是，预期的高精度测量的eta寿命将显着提高轻夸克质量的比例在物理学中直接和最模型无关的方式。 夸克质量及其比值是从实验中输入到当代粒子物理中的。 该项目将为非洲裔美国学生提供先进和最高的科学培训，目前他们在这一领域的代表性不足。

The award includes three projects at Jefferson Lab (JLab): (a) developing a new experiment to measure the proton charge radius with a sub-percent precision; (b) extracting the lifetime of the pi-zero meson with an unprecedented 1.4% precision from the PrimEx-II data set; and (c) developing and preparing the approved experiment to measure the lifetime of the eta meson in the newly developing Hall D. As one of the most fundamental quantities, accurate knowledge of the proton size was always important in physics. Very recently, a new result from a muonic-hydrogen experiment, with its unprecedented less than 0.1% precision, stated that the proton size is up to 7 standard deviations smaller than it was previously known to be. This experimental fact triggered the well-known "proton radius crisis" in physics. To address this crisis, a collaboration led by the PI of this award developed a uniquely designed electron-scattering experiment that was approved by PAC38 at JLab with the major condition of developing a new windowless gas hydrogen target. Precision measurements of the light pseudo-scalar meson lifetimes continue to be an important part of this award. The PrimEx collaboration performed the first measurement (PrimEx-I) of the pi-zero meson lifetime with the best precision up to date (2.8% in total). The results have been published and included in the Particle Data Group average. To reach the projected unprecedented 1.4% precision, our collaboration upgraded the experimental setup and performed the second (PrimEx-II) experiment in the fall of 2010. Under this award, the analysis process will be completed, and the results published. Under the leadership of the PI, a award to measure the eta meson lifetime with the best up to now precision (3%) was developed and approved to run in Hall D at JLab. We will prepare this experiment to run for the next few years.Precise knowledge of the proton size is critically important for understanding of the structure of hadronic matter. In addition, it is very important for atomic physics, in particular, the spectroscopy of atomic hydrogen. The currently developed experimental situation with the proton size limits the determination of the Rydberg constant, one of the basic and the most precisely known constant in physics up to now. The proposed experiment, with its independent novel approach and projected precision, has a direct potential to either significantly shift the current value of the Rydberg constant (if it confirms recent muonic-hydrogen result) or question the sufficiency of the basic theory calculations in muonic-hydrogen system. In the latter case, it may also indicate new physics beyond the Standard Model. The lifetimes of the light pseudo-scalar mesons carry direct information about the symmetries, and in particular, their partial breaking effects in the theory of interaction in physics. Therefore, the precise knowledge of these important quantities will serve as benchmark tests to our understanding of Nature. In particular, the anticipated high precision measurement of the eta lifetime will significantly improve the ratio of light quark masses in a direct and most model independent way in physics. The quark masses and their ratios are input into the contemporary particle physics from the experiment. This project will provide advanced and highest scientific training for African-American students in a field where they are currently under-represented.