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 efforts in studying its internal structure, there are still a number of puzzles surrounding the proton such as its spin, mass, and charge radius. The proton charge radius puzzle developed about ten years ago refers to a discrepancy of 5-7 standard deviations between the ultrahigh precise values of the proton charge radius determined from muonic hydrogen Lamb shift measurements and the CODATA values compiled from electron-proton scattering experiments and hydrogen spectroscopy measurements. In this talk I will briefly introduce the proton spin and mass puzzles first. I will then focus on the proton charge radius puzzle, the latest experimental results, and especially the PRad experiment at Jefferson Lab and its result.

HAIYAN GAO is the Henry Newson Professor of Physics at Duke University. She received her B.S. in physics from Tsinghua University in 1988 and her Ph.D. in Physics from the California Institute of Technology in 1994. She was a postdoctoral research associate at the University of Illinois, Urbana-Champaign from 1994 to 1996 prior to joining the Argonne National Laboratory as an Assistant Physicist. She was on the faculty at MIT from 1997-2002 before she joined the physics faculty at Duke in 2002 and became a full professor in 2008. She was named the Henry Newson Professor of Physics in 2012 at Duke. Her research interests cover nucleon structure, the search for QCD exotic states, fundamental symmetry studies at low energy, and the development of polarized gas targets. She has published many peer-reviewed papers and has given numerous invited talks internationally. She was the Chair of the Physics Department from 2011 to 2014 at Duke University, and served as the founding Vice Chancellor for Academic Affairs at Duke Kunshan University from Jan 2015-June 2019. She is a fellow of the American Physical Society (APS). She chaired and co-chaired many workshops and conferences and has served on many committees and advisory panels, including the executive board of the APS, the Executive Committee of the Division of Nuclear Physics of the APS, the National Academies of Sciences, Engineering and Medicine Committee on U.S.-Based Electron Ion Collider Science Assessment, the Nuclear Science Advisory Committee (NSAC) Long Range Plan Working Group, and the program advisory committees at Jefferson Lab, Brookhaven National Laboratory and Mainz. She has served on a number of editorial boards of physics journals, including the Editorial Board of Physical Review C. Currently she chairs the International Spin Physics Committee.