The Proton Radius Puzzle

Dr. Mahbub Khandaker
Department of Physics
Idaho State University

Among quantities describing the structure of the nucleon, the electromagnetic form factors are the most fundamental quantities -- they are sensitive to the distribution of charge and magnetization within the nucleon.  Precise information about the proton charge radius is particularly important because it is a crucial input to high precision tests of quantum electrodynamics (QED).  Traditionally, the proton’s rms charge radius has been determined by electron scattering and atomic hydrogen spectroscopy.  Recently, the extremely precise extraction of the proton radius obtained from spectroscopy of muonic hydrogen disagrees significantly with that extracted from electronic hydrogen or elastic electron-proton scattering.  This discrepancy is the “proton radius puzzle”.  We are preparing a new high precision measurement of the proton radius using electron scattering at Jefferson Lab to address this puzzle.  This experiment, with its independent novel approach and projected precision, will have a direct potential to either significantly shift the current value of the proton radius, or question the sufficiency of QED calculations in muonic hydrogen, or probe new physics beyond the Standard Model.  The description and current status of the experiment will be presented in this talk.