A Liquid Hydrogen Target for the MUSE Experiment

众所周知，质子和中子是由夸克和胶子组成的。夸克和胶子产生了一个有限大小的质子，它比单个原子的大小大约小1万倍。这个项目的目标是测量质子的大小。一个持续存在的难题是，当用氢原子测量质子的大小时，质子的大小似乎不同于它的对应物，称为μ子-氢，其中氢原子中的电子被它更重的表兄μ子所取代。该项目将专门建造用于实验的设备，以提供对质子半径的独立测量。除了该项目的直接科学目标外，这些实验还为学生和青年科学家提供了在现代科学实验的国际合作中工作的经验和培训，并提供了最先进的技术。用介子氢测量的质子半径与用电子-质子散射测量的质子半径之间存在7个标准差的差异，这可能是粒子物理标准模型之外的物理现象，也可能是传统物理的新方面或从实验数据中提取半径的问题。μ子散射实验（MUSE）旨在解决质子半径之谜。MUSE使用瑞士保罗·谢勒研究所（PSI）的混合介子、介子和电子束。MUSE将通过介子-质子散射和电子-质子散射来确定质子半径，此外还将用正轻子和负轻子进行这些反应。这不仅减少了系统的不确定性，而且还提供了对双光子交换贡献的灵敏度，这可能是早期实验中看到的一些差异的原因。低温靶是确定质子电荷半径实验的重要组成部分，可以说是实验装置中最复杂的组成部分。密歇根大学的研究小组将在2018年夏天开始为PSI的MUSE实验提供一个功能正常的液氢目标。

It is well known that protons and neutrons are made from constituents, called quarks and gluons. The quarks and gluons give rise to a finite size of the proton, which is about 10,000 times smaller than the size of a single atom. The goal of this project is to measure the size of the proton. An on-going puzzle is that the proton size appears to be different when measured with atomic hydrogen as compared with its counterpart, called muonic-hydrogen, where the electron in a hydrogen atom is replaced by its heavier cousin, the muon. This project will specifically build equipment to be used in an experiment to provide an independent measurement of the proton radius. In addition to the direct scientific goals of the project, the experiments provide students and young scientists experience and training in working in the international collaborations of modern scientific experiments, with state of the art technology.The seven standard deviation discrepancy between the proton radius measured with muonic hydrogen and with electron-proton scattering may point to physics beyond the standard model of particle physics, or result from novel aspects of conventional physics or from issues in extracting the radius from experimental data. The Muon Scattering Experiment (MUSE) aims to resolve the proton radius puzzle. MUSE uses a mixed pion, muon and electron beam at the Paul Scherrer Institute (PSI) in Switzerland. MUSE will determine the proton radius through both muon-proton scattering and electron-proton scattering, in addition to performing these reactions with positive and negative leptons. This not only reduces systematic uncertainties but also provides sensitivity to two-photon exchange contributions which may be responsible for some of the discrepancies seen in earlier experiments. The cryogenic target that will be constructed with the award funds is an essential component of the experiment to determine the charge radius of the proton, and arguably the most complex component of the experimental setup. The Michigan group will embark on delivering a functioning liquid hydrogen target to the MUSE experiment at PSI by summer 2018.