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.