The Size of the Proton

Dr. Mehdi Meziane
Department of Physics
Duke University


Nucleons are the building blocks of the atomic nucleus, which contribute more than 99.9% of the mass of the visible universe.  Among quantities describing the structure of the nucleon, the electromagnetic form factors, which are related to the spatial distribution of charge and magnetization within the nucleon, are the most fundamental.  Elastic electron-proton scattering is a fantastic tool to probe the proton structure and thus to extract those form factors.  When the four-momentum transfer squared of the scattering reaches zero, the charge radius of the proton can be determined from the corresponding electric form factor. The recent high precision measurements of the proton charge radius performed at Paul Scherrer Institute from muonic hydrogen Lamb shift puzzled the hadronic physics community.  A value was extracted, 7-sigma smaller than the previous determinations obtained from electron-proton scattering experiments and based on precision spectroscopy of electronic hydrogen.  An additional extraction of the proton charge radius from electron scattering at Mainz is also in good agreement with these "electronic" determinations.  In this colloquium, after introducing the framework around electron-proton scattering, I will present the latest developments on the proton charge radius measurements.