High-precision measurement of the proton elastic form factor ratio mu(p)G(E)/G(M) at low Q(2)

X. Zhan
J. Arrington
K. Hafidi
D. S. Armstrong, William & Mary
B. Hahn, William & Mary
J. Katich, William & Mary


We report a new, high-precision measurement of the proton elastic form factor ratio mu(p)G(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 a total uncertainty of approximately 1%, the new data clearly show that the deviation of the ratio mu(p)G(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 an electric (magnetic) form factor roughly 2% smaller (1% larger) than the previous global fit in this Q(2) range. We obtain new extractions of the proton electric and magnetic radii, which are < r(E)(2) > (1/2) = 0.875 +/- 0.010 fm and < r(M)(2) > (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. (C) 2011 Published by Elsevier B.V.