Direct Measurement of QED Radiative Effects in PRad Experiment

The PRad experiment at Jefferson Lab has precisely measured the cross sections for elastic electron-proton scattering at four momentum transfer square $2\times 10^{-4} < Q^2 < 6\times 10^{-2}$ GeV$^2$. The proton's charge radius was extracted based on the measured proton form factors at very low $Q^2$. This experiment utilized a high-precision, hybrid calorimeter (HyCal) to precisely determine the energy and position of electrons and photons. The HyCal's inner array of PbWO$_4$ modules can well separate two incident particles with a distance greater than about 30 mm (of the module size) on the detection plane. Together with the GEM detectors placed in front of the HyCal, which serve as a veto detector for neutral particles, the PRad detector system is capable of measuring and identifying the scattered electrons and the radiative photons simultaneously from the elastic $ep$ and $ee$ scattering processes. In this proceeding, the preliminary results of the direct radiative effects measurement from the PRad experiment are discussed; the expected improvements with the upcoming PRad-II experiment are also presented.