中国物理学会高能物理分会第十一届全国会员代表大会暨学术年会

In 2010, a new method using muonic hydrogen spectroscopy led to a proton charge radius ($r_{p}$) result that was nearly ten times more precise but significantly smaller than results obtained using the two traditional methods, namely $e-p$ elastic scattering and ordinary Hydrogen spectroscopy. This discrepancy triggered the so-called "proton charge radius puzzle". To investigate this discrepancy, the PRad collaboration performed a new experiment in 2016 in Hall B at the Thomas Jefferson National Accelerator Facility. With both 1.1 and 2.2 GeV electron beams, the experiment measured the $e-p$ elastic scattering cross sections in an unprecedentedly low values of momentum transfer squared region ($Q^2 = 2.1\times10^{-4} - 0.06~\rm{(GeV/c)}^2$), with a sub-percent precision. The PRad experiment utilized a magnetic-spectrometer-free setup, which was based on a large acceptance and high resolution calorimeter (HyCal), a plane of two large-area Gas Electron Multiplier (GEM) detectors, and a windowless H$_2$ gas-flow target. In this talk, I will discuss details of the data analysis and present the results of this experiment. I will also focus on the newly approved PRad-II experiment. Through a number of major upgrades to the experimental setup and analysis method, the new experiment aims to reduce the total uncertainty of $r_{p}$ by nearly a factor of 4 compared to that of PRad.