Collaborative Research: Equipment for the PSI MUSE Experiment

众所周知，质子和中子是由称为夸克和胶子的成分组成的。夸克和胶子产生了有限大小的质子，大约是单个原子大小的一万倍。这个项目的目标是测量质子的大小。一个正在进行的谜题是，当用原子氢测量质子大小时，与其对应的被称为穆斯介子-氢的质子大小似乎不同，后者是由氢原子中的电子被其更重的表亲--Muon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，这些实验还利用最先进的技术，为学生和年轻科学家提供在现代科学实验的国际合作中工作的经验和培训。用缪子氢和电子-质子散射测量的质子半径之间的七个标准差可能指向粒子物理标准模型之外的物理，或者是传统物理的新方面，或者是从实验数据中提取半径的问题。缪子散射实验(MUSE)旨在解决质子半径之谜。缪斯使用的是瑞士保罗谢勒研究所(PSI)的混合介子、介子和电子束。除了与正轻子和负轻子进行这些反应外，MUSE还将通过Muon-质子散射和电子-质子散射来确定质子半径。这不仅减少了系统的不确定性，而且提供了对双光子交换贡献的敏感性，这可能是早期实验中看到的一些差异的原因。该合作项目在2016年9月至2018年8月期间为缪斯的建设提供支持。PSI还将进行正在进行的测试和调试活动，包括2017年底用部分光谱仪进行的彩排。

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 that may be responsible for some of the discrepancies seen in earlier experiments. This collaborative project supports the construction of MUSE over the period September 2016 - August 2018. There will also be ongoing testing and commissioning activities at PSI, including a dress rehearsal run with a partial spectrometer in late 2017.