Proton Charge Radius (PRad) Experiment at Jefferson Lab

The PRad experiment (E12-11-106) was recently performed with 1.1 and 2.2 GeV unpolarized electron beam on a windowless H$_2$ gas flow target in Hall B at Jefferson Lab. The experiment aims to investigate the proton radius puzzle by extracting the electric form factor of proton in an unprecedented low four-momentum transfer squared region, $Q^2 = 2\times10^{-4}-0.1~\rm{(GeV/c)}^2$, with a sub-percent precision. The PRad experiment utilizes a non-magnetic and calorimetric method with a high efficiency and high resolution calorimeter (HyCal) and two Gas Electron Multiplier (GEM) chambers. Its systematics are well controlled by two main advantages of this experiment: (1) The scattered electrons from M$\o$ller and $e-p$ elastic scattering are measured simultaneously, and the $e-p$ cross section will be normalized to the well-known M$\o$ller process; (2) The windowless gas flow target has no cell windows at both up- and downstream, which was one of the primary background sources in the previous $e-p$ elastic scattering experiments. Thus the PRad experiment has systematic uncertainties totally different from the previous magnetic spectrometric $e-p$ elastic scattering experiments. In this talk, we will present the details of the experimental method and preliminary analysis of the data.