High Precision Measurement of the Proton Elastic Form Factor Ratio $μ_pG_E/G_M$ at low $Q^2$

We report a new high precision measurement of the proton elastic form factor ratio $\mu_pG_E/G_M$ for the four-momentum transfer squared $Q^2$ = 0.3--0.7~ (GeV$/c$)$^2$. The measurement was performed at Jefferson Lab (JLab) in Hall A using recoil polarimetry. With the achieved $\sim$1% total uncertainty, the new data clearly show that the deviation of the ratio $\mu_pG_E/G_M$ from unity observed in previous polarization measurements at high $Q^2$ continues down to the lowest $Q^2$ value of this measurement. The updated global fit that includes the new results yields in this $Q^2$ range an electric (magnetic) form factor $\sim$2% smaller ($\sim$1% larger) than the previous global fit. We obtain new extractions of the proton electric and magnetic radii, which are $^{1/2}=0.875\pm0.010$ fm and $^{1/2}=0.867\pm0.020$ fm.