Measurement of the Proton Elastic Form Factor Ratio at Low Q[sup 2

Abstract

A new measurement of the proton elastic form factor ratio μpGE/GM in the range of Q2 = 0.3–0.7 (GeV/c)2 was made at Jefferson lab (JLab) Hall A using recoil polarimetry. With unprecedented (∼1%) precision, the new data show that the proton form factor ratio strongly deviates from unity in this low momentum transfer region, indicating a significant difference between the proton charge and magnetic distribution at large length scale. The new result is a few percent lower than previous world polarization data. The updated global fit indicates a 2% decrease in GE and 1% increase in GM in this low Q2 region.