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比的精度测量质子电荷半径；(B)以前所未有的1.4%的精度从PrimeX-II数据集中提取pi-零介子的寿命；以及(C)开发和准备经批准的实验以测量新开发的D厅的ETA介子的寿命。最近，一项来自缪子-氢实验的新结果，其前所未有的不到0.1%的精确度表明，质子大小比以前所知的小了7个标准差。这一实验事实引发了物理学中众所周知的“质子半径危机”。为了解决这一危机，该奖项的PI领导的一个合作开发了一个独特设计的电子散射实验，该实验在JLab获得了PAC38的批准，主要条件是开发一种新的无窗口气体氢靶。轻准标量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)，以迄今最好的精度(总计2.8%)测量了pi-零介子寿命。结果已发布并包含在粒子数据组平均中。为了达到前所未有的1.4%的预测精度，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。根据这项奖励，分析过程将完成，结果将公布。在国际和平研究所的领导下，开发了一项以目前最高精度(3%)测量ETA介子寿命的奖项，并批准在JLab的D厅运行。我们将为未来几年的实验做准备。精确的质子大小知识对于理解强子物质的结构是至关重要的。此外，它对于原子物理，特别是氢原子的光谱研究也是非常重要的。目前发展的质子大小实验情况限制了里德堡常数的测定，里德堡常数是迄今为止物理学中最基本和最精确的常数之一。拟议中的实验，以其独立的新颖方法和预计的精度，有可能直接显著改变里德伯格常数的当前值(如果它证实了最近的缪子-氢结果)，或者质疑缪子-氢系统中基本理论计算的充分性。在后一种情况下，它也可能预示着超越标准模型的新物理。轻准标量介子的寿命携带着有关对称性的直接信息，特别是在物理相互作用理论中，它们的部分破缺效应。因此，对这些重要数量的精确了解将作为我们对自然的理解的基准测试。特别是，预计对ETA寿命的高精度测量将以一种直接和最模型无关的物理方式显著改善轻夸克质量比。夸克质量和它们的比例从实验中输入到当代粒子物理中。该项目将在非洲裔美国学生目前代表性不足的领域为他们提供高级和最高级别的科学培训。

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.