Measurements of the proton elastic-form-factor ratio μpGEp/GMp at low momentum transfer

High-precision measurements of the proton elastic form-factor ratio, mu(p)G(E)(p)/G(M)(p), have been made at four-momentum transfer, Q(2), values between 0.2 and 0.5 GeV2. The new data, while consistent with previous results, clearly show a ratio less than unity and significant differences from the central values of several recent phenomenological fits. By combining the new form-factor ratio data with an existing cross-section measurement, one finds that in this Q(2) range the deviation from unity is primarily due to G(E)(p) being smaller than expected.