Nuclear Physics Studies at Fermilab and PSI

该基金支持的主要物理项目是研究受质子大小影响的核子和原子核的性质。由于质子是核子的一种，是原子核的主要组成部分之一，因此准确地知道它的大小对于理解原子核的基本结构至关重要。然而，通过两种独立的方法测量质子的大小却产生了不同的结果。这种差异的来源一直是这个项目希望解决的一个长达10年的难题。研究将在两个地方进行：一个在芝加哥附近的费米国家加速器实验室（Fermilab），作为SeaQuest合作项目的一部分；另一个在瑞士苏黎世附近的保罗·谢勒研究所，作为MUSE合作项目的一部分。本科生和研究生的教育和培训，以及针对公众的外展，是这个研究计划的一个重要方面。费米实验室的实验集中在drel - yan产生的介子对上，以确定核子中的轻反夸克分布。它将更好地理解轻反夸克如何分布在非极化和极化核子中，以及轨道角动量如何有助于核子自旋。这些信息将为质子的理论模型提供直接输入，并阐明质子自旋的起源。MUSE实验将同时测量电子和介子与质子之间的弹性散射，这是由一束含有正电荷和负电荷的电子、介子和介子的光束提供的。该实验旨在解决用电子和介子散射测量的质子尺寸无法解释的巨大差异。这种差异是完全出乎意料的，因为质子的大小不应该取决于它是用电子还是μ子测量的。这种方法以前从未被尝试过。此外，密歇根小组将在MUSE实验中领导液氢靶的操作，并在SeaQuest实验中领导分析工作。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。

The main physics program supported by this grant is directed at the study of those properties of nucleons and nuclei that are influenced by the size of the proton. As the proton is a nucleon, and one of the principal components of nuclei, knowing its size precisely is critical to understanding fundamental nuclear structure. Yet measurement of the proton's size through two independent means has yielded different results. The source of this difference has been a 10-year old puzzle that this project hopes to solve. Research will be carried out at two places: one at the Fermi National Accelerator Laboratory (Fermilab) near Chicago as part of a collaboration called SeaQuest, and the other at the Paul Scherrer Institute near Zurich, Switzerland, as part of a collaboration called MUSE. Education and training of both undergraduate and graduate students, as well as outreach aimed at the general public, is an important aspect of this research program. The experiment at Fermilab is focused on Drell-Yan production of dimuon pairs to determine the light anti-quark distributions in the nucleon. It will provide a better understanding how light anti-quarks are distributed in unpolarized and polarized nucleons, and how orbital angular momentum might contribute to the nucleon spin. This information will provide direct input to theoretical models of the proton, and shed light on the origin of the proton's spin. The MUSE experiment will simultaneously measure elastic scattering with electrons and muons off protons which is afforded by a beam that contains positively and negatively charged electrons, muons and pions. The experiment aims to resolve the unexplained large discrepancy of the size of the proton measured with electron and muon scattering. This discrepancy was entirely unexpected, as the proton's size should not depend on whether it is measured with electrons or muons. This approach has never been attempted before. In addition, the Michigan group will lead the operation of a liquid hydrogen target at the MUSE experiment, and lead the analysis effort at the SeaQuest experiment.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.