Puzzling out the Proton with Precision Scattering Experiments

质子是物质的基本组成部分之一，即使在发现100年后，它们仍然没有被完全理解。事实上，质子的精确半径是一个谜，对这个量的多次测量并不完全一致。质子半径的精确测量是重要的，因为它决定了与其他粒子的相互作用以及散射的横截面。质子的半径甚至影响氢原子的原子光谱，这与里德伯常数的值密切相关，里德伯常数对所有原子光谱的解释至关重要，因为电子和质子波函数重叠，半径是确定重叠量的重要参数。 这项研究旨在解决这个“质子半径之谜”。瑞士Paul Scherrer研究所的μ子质子散射实验（MUSE）将从质子上散射一束电子和μ子，以测量质子的形状。本科生和研究生以及博士后研究人员的工作是这些努力的核心，他们将获得精确核物理实验，现代探测器和加速器技术以及模拟和分析算法的知识和经验。这项国际研究工作将使他们接触到来自世界各地的研究人员，促进文化交流，并为他们提供发展沟通和领导能力的机会。MUSE将利用Paul Scherrer研究所提供的全球独特光束来测量轻子-质子截面。这是第一次测量μ子-质子散射，精确到足以解决质子半径难题核心的4%效应。通过测量两种电荷的μ子以及在很宽的运动学范围内的电子和正电子，MUSE将提供关键数据来验证现有的e-p散射结果，通过比较两种电荷来测试辐射校正，包括双光子交换，并通过比较两个轻子家族来寻找违反轻子普适性的情况。Bernauer博士的团队将在软件开发和数据分析中发挥关键作用。与德国美因茨的MAMI-A1和MESA-MAGIX合作开展的其他活动旨在减少质子电形状因子的主要系统不确定性，并将在梅萨准备未来的实验，该实验将把磁形状因子的不确定性减少两个数量级。这将导致质子的磁半径和泽马赫半径的空前精确。后一个量也可以在光谱学中获得，并且与电荷半径一样，是原子物理和核物理之间的一个罕见的重叠点。该奖项反映了NSF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。

Protons are one of the basic building blocks of matter and even 100 years after their discovery, they are still not fully understood. In fact, the precise radius of the proton is a puzzle and multiple measurements of this quantity do not all agree. A precise measurement of a proton’s radius is important because it determines the interaction with, and cross-section for scattering, other particles. The proton’s radius even affects the atomic spectroscopy of the hydrogen atom, which is tightly linked to the value of the Rydberg constant that has been so critical to the interpretation of all atomic spectra, because the electron and proton wave functions overlap and the radius is an important parameter in determining the amount of overlap. The research supported by this grant aims to resolve this “Proton Radius puzzle”. The Muon Proton Scattering Experiment (MUSE) at the Paul Scherrer Institute, Switzerland, will scatter a beam of electrons and muons off protons to measure the shape of the proton. The work of undergraduate and graduate students as well as postdoctoral researchers is central to these efforts, and they will gain knowledge and experience in precision nuclear physics experiments, modern detector and accelerator techniques, as well as in simulation and analysis algorithms. This international research effort will expose them to researchers from all over the world, fostering cultural exchange and give them opportunity to develop their communication and leadership skills.MUSE will make use of the worldwide unique beam available at the Paul Scherrer Institute to measure the lepton-proton cross section. It is the first measurement of muon-proton scattering that is precise enough to address the 4% effect at the heart of the proton radius puzzle. Through the measurement of muons of both charges, as well as electrons and positrons over a wide kinematic range, MUSE will provide crucial data to verify the existing e-p scattering results, test radiative corrections including two-photon-exchange by comparison of the two charges, and search for a violation of lepton universality by comparison of the two lepton families. Dr. Bernauer’s group will play a crucial role in the development of the software and the analysis of the data. Additional activities with the MAMI-A1 and MESA-MAGIX collaborations in Mainz, Germany, aim to reduce the leading systematic uncertainties on the proton electric form factor and will prepare a future experiment at MESA which will reduce the uncertainty of the magnetic form factor by two orders of magnitude. This will lead to an unprecedented precision on both the magnetic radius and the Zemach radius of the proton. The latter quantity is also accessible in spectroscopy and is, like the charge radius, a rare point of overlap between atomic and nuclear physics.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.