The proton charge radius (r_p) is one of the important bench-mark quantities in physics. Precise knowledge of its value is critically important for the understanding of the underlying quark-gluon structure of the nucleon as well as in atomic physics, − especially in the spectroscopy of atomic hydrogen. The recent result from Lamb shift measurements from the muonic hydrogen: r_p = 0.8409(4) fm, with its unprecedented less than 0.1% precision, is currently up to eight standard deviations smaller than the average value from all previous experiments. This result triggered the well-known “proton radius puzzle'' in nuclear and atomic physics. So far, all theoretical efforts and more precise simulations have failed to explain this discrepancy on the value of this fundamental quantity. This situation urgently requires new high precision measurements to understand the source of the discrepancy. The Jefferson Lab Proton Radius (PRad) experiment collected data in 2016 for a high precision determination of the proton charge radius through the electron-proton elastic scattering process using a novel non-magnetic-spectrometer method. Scattered electrons were detected in a high resolution, large acceptance electromagnetic calorimeter, as well as in a pair of large area Gas Electron Multiplier (GEM) detectors which provided precision coordinate determination. A windowless hydrogen gas flow target was used to ensure that there was no target window background. Systematic uncertainty in the ep cross section measurement was controlled by normalizing it to the simultaneously measured well known Moller cross section. The final result from PRad experiment has been accepted for publication in Nature. The result will be presented in this talk.
Abha Rajan