High-precision measurement of the proton elastic form factor ratio μ_pG_E/G_M at low Q²

We report a new, high-precision measurement of the proton elastic form factor ratio μ_pG_E/G_M for the four-momentum transfer squared Q²=0.3-0.7(GeV/c)². The measurement was performed at Jefferson Lab (JLab) in Hall A using recoil polarimetry. With a total uncertainty of approximately 1%, the new data clearly show that the deviation of the ratio μ_pG_E/G_M from unity observed in previous polarization measurements at high Q² continues down to the lowest Q² value of this measurement. The updated global fit that includes the new results yields an electric (magnetic) form factor roughly 2% smaller (1% larger) than the previous global fit in this Q² range. We obtain new extractions of the proton electric and magnetic radii, which are 〈rE2v1/2=0.875±0.010 fm and 〈rM2〉1/2=0.867±0.020 fm. The charge radius is consistent with other recent extractions based on the electron-proton interaction, including the atomic hydrogen Lamb shift measurements, which suggests a missing correction in the comparison of measurements of the proton charge radius using electron probes and the recent extraction from the muonic hydrogen Lamb shift.