Studies of Nucleons at Fermilab and PSI

该奖项支持的主要物理学项目是研究核子的自旋结构和解决一个关于质子真实的大小的10年之久的难题。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SpinQuest合作的一部分，另一个在瑞士苏黎世附近的Paul Scherrer研究所，作为MUSE合作的一部分。费米实验室的实验重点是质子-质子散射中的一个特殊过程，称为Drell-Yan过程，它可以识别核子中的轻反夸克，并确定这些反夸克如何对质子的自旋做出贡献。瑞士的实验旨在解决用电子和μ子散射测量的质子大小的无法解释的巨大差异，其中μ子是电子的重表亲。这种差异完全出乎意料，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。该研究计划将培训博士后研究员和研究生和本科生的低温技术和目标。初级研究人员参与这项工作是很重要的，以确保在核物理界和其他方面有足够的专门知识，以支持未来对低温仪器的需要。每个小组成员都有机会参与硬件、模拟和分析项目，学习多种编程语言和大型数据集分析的宝贵技能。芝加哥附近费米实验室的SpinQuest实验将使用质子-质子散射中的Drell-Yan过程对海夸克进行首次Sivers函数测量，以确定u-bar和d-bar Sivers分布的符号和大小。这些研究可能揭示海夸克的轨道角动量对质子自旋的贡献，并使量子色动力学的基本预测的测量成为QCD规范制定的核心，称为Sivers函数的“符号变化”。PSI的密歇根小组的研究旨在重新评估质子电荷半径和仍然存在的差异，方法是对质子的弹性电子和μ子散射进行首次同时测量，这是由包含带正电和带负电的电子，μ子和π子的光束提供的。这种方法以前从未尝试过，并且很有希望解决质子半径之谜。由于他们在建造和运行低温目标方面的专业知识，密歇根州的小组将领导MUSE的非极化目标工作，并支持费米实验室的极化目标工作。该计划建立在该小组所获得的专业知识的基础上，是该小组过去几年工作的自然延伸。它提供了硬件，模拟和分析项目的平衡组合，这将使学生和博士后受益，它将使U-M集团在PSI的MUSE实验中发挥关键作用，并在费米实验室的Drell-Yan计划中发挥重要作用。本科生、研究生和博士后研究员的教育和培训，以及面向公众的推广活动，是该研究项目的一个重要方面。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。

The main physics program supported by this award is the study of the spin structure of nucleons and solving a 10-year old puzzle about the real size of the proton. Research will be carried out at two places: one at the Fermi National Accelerator Laboratory (Fermilab) near Chicago as part of a collaboration called SpinQuest, and the other at the Paul Scherrer Institute near Zurich, Switzerland, as part of a collaboration called MUSE. The experiment at Fermilab is focused on a special process in proton-proton scattering, called the Drell-Yan process, which can identify light antiquarks in the nucleon and determine how those antiquarks contribute to the spin of the proton. The experiment in Switzerland aims to resolve the unexplained large discrepancy of the size of the proton measured with electron and muon scattering, where the muon is a heavy cousin of the electron. This discrepancy was entirely unexpected, as the proton's size should not depend on whether it is measured with electrons or muons. This research program will train postdoctoral fellows and graduate and undergraduate students on cryogenic techniques and targets. The involvement of junior researchers in this effort is important to ensure that there will be sufficient expertise in the nuclear physics community and beyond to support the future need for cryogenic instrumentation. Every group member has the opportunity to get involved in hardware, simulation and analysis projects and learn valuable skills in several programming languages and in the analysis of large data sets.The SpinQuest experiment at Fermilab near Chicago will perform the first Sivers function measurement on sea quarks using the Drell-Yan process in proton-proton scattering to determine the sign and magnitude of the u-bar and d-bar Sivers distributions. These studies may shed light on the contribution of orbital angular momentum from sea quarks to the proton spin and enable measurements of a fundamental prediction of Quantum Chromodynamics that goes to the heart of the gauge formulation of QCD, known as the "sign change" of the Sivers function. The studies of the Michigan group at PSI aim to reassess the proton charge radius and the discrepancies that remain by performing the first simultaneous measurement of elastic electron and muon scattering off the proton, which is afforded by a beam that contains positively and negatively charged electrons, muons and pions. This approach has never been attempted before and carries high promise to resolve the proton radius puzzle. Due to their specific expertise in building and running cryogenic targets, the Michigan group will lead the unpolarized target effort at MUSE and support the polarized target effort at Fermilab. This program builds on the expertise the group has gained and is a natural extension of the group's work over the past years. It provides a balanced mix of hardware, simulation and analysis projects, which will benefit students and postdocs, and it will allow the U-M group to play a key role in the MUSE experiment at PSI and have a strong presence in the Drell-Yan program at Fermilab. Education and training of undergraduate and graduate students and postdoctoral fellows, as well as outreach aimed at the general public, is an important aspect of this research program.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.