Puzzling out the Proton with Precision Scattering Experiments

质子是物质的基本组成部分之一，即使在它们被发现100年后，人们仍然没有完全理解它们。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。对质子半径的精确测量很重要，因为它决定了质子与其他粒子的相互作用和散射的横截面。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都至关重要，因为电子和质子的波函数重叠，而半径是决定重叠量的重要参数。这项资助的研究旨在解决这个“质子半径难题”。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，他们将获得精密核物理实验，现代探测器和加速器技术以及模拟和分析算法方面的知识和经验。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技能的机会。缪斯将利用保罗·谢勒研究所全球独一无二的光束来测量轻子-质子的横截面。这是第一次对介子-质子散射的测量，其精度足以解决质子半径谜题核心的4%效应。通过测量两种电荷的介子，以及在宽运动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包括双光子交换在内的辐射修正，并通过比较两种轻子族来寻找违反轻子普适性的现象。伯纳尔博士的团队将在软件开发和数据分析方面发挥关键作用。与德国美因茨的MAMI-A1和MESA- magix合作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，并将为MESA的未来实验做准备，该实验将把磁性形状因子的不确定性降低两个数量级。这将使质子的磁半径和泽马赫半径达到前所未有的精度。后者在光谱学中也可以得到，并且像电荷半径一样，是原子物理学和核物理学之间罕见的重叠点。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。

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.