The “Proton Radius Puzzle,” initiated by the 2010 muonic hydrogen measurement, highlighted a significant discrepancy (about 7\sigma) with respect to values derived from electron scattering and ordinary hydrogen spectroscopy. While recent measurements, such as the PRad experiment at JLab and some of updated hydrogen spectroscopies, look to favor a smaller proton charge radius, consistent with the muonic result, inconsistencies with earlier electron scattering data remain unresolved. In addition, discrepancies among recent 1S–3S transition hydrogen spectroscopy resultsn suggest the presence of unquoted systematic uncertainties. Consequently, a precise determination of the proton charge radius is still critical to resolve this long-standing issue.
In this seminar, I will report on the current status of our ULQ2 (Ultra-Low Q2) project at the Research Center for Accelerator and Radioisotope Science (RARiS; former ELPH), Tohoku University. Utilizing a 60-MeV electron linac, we have measured the elastic electron–proton scattering cross section in the lowest-ever momentum transfer region, Q^2 = 0.0003–0.008 (GeV/c)^2. A key feature of this project is an absolute cross-section measurement relative to the well-known ^{12}C cross section using a CH_2 target, aiming to control systematic uncertainties at the 10^{-3} level. This approach is expected to provide the least model-dependent determination of the proton charge radius from electron scattering.
In addition to the proton measurement, we have also performed elastic electron–deuteron scattering measurement under the same kinematics, providing the world’s lowest-Q^2 data for the deuteron. A puzzle similar to that of the proton has been pointed out, and our measurement enables a determination of the deuteron charge radius. From this e+d data, we are challenging the determination of the neutron charge radius via electron scattering for the first time by exploiting the fourth moments of the deuteron charge distribution.
I will first review the current status of the Proton Radius Puzzle and recent global experimental efforts. I will then present the details of the ULQ2 project, including the challenges for the neutron charge radius.