Calibration Study and Preliminary Results of PRad Experiment

The latest measurements of the proton radius through muonic hydrogen Lamb shift show a discrepancy of $7\sigma$ from a global analysis of standard hydrogen Lamb shift and elastic $ep$-scattering. In order to understand this \textit{proton radius puzzle}, the PRad experiment\footnote{The PRad experiment is supported in part by NSF MRI award PHY-1229153} successfully took in last June some elastic $ep$-scattering data at very low $Q^2$ ($2 \cdot 10^{-4}$ to $10^{-1}$~GeV$^2$) with very accurate angle and energy measurements to minimize the systematic uncertainties. % Before measuring the cross-sections that will be used to extract the electromagnetic form factor $G_E(Q^2)$ and the proton radius, a very careful calibration of the electromagnetic calorimeter (\textit{HyCal}) must be performed to get a good energy resolution and separate $ep$-events from M$\o$ller events especially at low angle. We will present an extended study of the electromagnetic calorimeter calibration of this experiment as well as some preliminary results on $ep$- and $ee$-scattering processes extracted from the data\footnote{This work is supported in part by NSF award PHY-0855543}.