Abstract: Nucleons (protons and neutrons) are the building blocks of atomic nuclei and are responsible for more than 99% of the visible matter in the universe. Despite decades of effort in studying its internal structure, there are still a number of puzzles surrounding the proton, such as its spin, mass, and charge radius. While major progress has been made in the last three decades in understanding the proton spin “crisis,” which originated in the late 1980s in the European Muon Collaboration experiment, the nature of proton spin remains puzzling. Another puzzle developed about ten years ago that concerns the proton charge radius, which refers to a 5-7 sigma discrepancy among the ultrahigh-precision value of the proton charge radius determined from muonic hydrogen Lamb shift measurements, the values determined from electron-proton scattering experiments, and the CODATA value of electronic hydrogen spectroscopy measurements.
In this talk I will introduce these puzzles first, and then focus on the latest developments concerning the proton spin and charge radius.