MRI: Development of a Windowless Hydrogen Gas Flow Target for a High Precision Measurement of the Proton Charge Radius

质子和中子（统称为核子）是原子核的基本组成部分，它们通过强大的力在原子核内结合在一起。质子半径的精确知识对于理解核子的基础夸克和胶子是至关重要的，夸克和胶子是被称为量子色动力学（QCD）的公认强作用力理论的自由度。最近关于质子大小的争论也保证了对质子半径的高精度测量，这是由新的超高精度测量引起的，在介子氢中，质子半径明显小于通常接受的从正常（电子）氢中测量得到的值。为了解决这个“质子半径危机”，我们开发了一种新的电子散射实验，使用高分辨率量热计和无窗气体流动目标可以达到前所未有的精度。由诺福克州立大学、杜克大学、密西西比州立大学和北卡罗来纳农工大学组成的一个合作项目将开发和构建一个无窗的氢气流动目标，以便以低于百分之一的精度测量质子电荷半径。该实验将在杰弗逊实验室进行，这将是同类实验中的第一个目标。无窗目标与高分辨率量热计相结合，将使我们能够获得0.0001至0.001 gev平方范围内的动量转移平方，这比以往任何电子散射实验都要低。实验将采用一种新颖的方法，利用高分辨率量热计同时测量电子-质子弹性散射截面和穆勒散射截面。这种新方法将有助于在电子散射实验中首次实现前所未有的精度和几乎与模型无关的质子电荷半径提取。

Protons and neutrons (collectively called nucleons) are fundamental building blocks of the atomic nuclei, and they are held together inside nuclei by the strong force. Precise knowledge of the proton's radius is critically important for understanding nucleons in terms of the underlying quarks and gluons, which are the degrees of freedom of the accepted theory of the strong force called quantum chromodynamics (QCD). High precision measurement of the proton's radius is also warranted by the recent controversy over the size of the proton, triggered by the new ultra-high precision measurement in muonic hydrogen, which is significantly smaller than the commonly accepted value obtained from measurement in normal (electronic) hydrogen. To address this "proton radius crisis," we have developed a novel electron scattering experiment that can achieve an unprecedented precision using a high resolution calorimeter and a windowless gas flow target.A Collaboration consisting of Norfolk State University, Duke University, Mississippi State University, and North Carolina A&T State University will develop and construct a windowless hydrogen gas flow target in order to measure the proton charge radius with sub-percent precision. The experiment will be carried out at Jefferson Lab, where this target will be the first of its kind. The windowless target in conjunction with a high resolution calorimeter will allow us to access momentum transferred squared ranging from 0.0001 to 0.001 GeV-squared, which is lower than any previous electron scattering experiment. The experiment will employ a novel approach of simultaneous measurements of electron-proton elastic scattering cross section and Moller scattering cross section using a high resolution calorimeter. This novel approach will help achieve the unprecedented precision and an almost model independent extraction of the proton charge radius for the first time in electron scattering experiments.