Measurements of the Proton Elastic-Form-Factor Ratio<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>μ</mml:mi><mml:mi>p</mml:mi></mml:msub><mml:msubsup><mml:mi>G</mml:mi><mml:mi>E</mml:mi><mml:mi>p</mml:mi></mml:msubsup><mml:mo>/</mml:mo><mml:msubsup><mml:mi>G</mml:mi><mml:mi>M</mml:mi><mml:mi>p</mml:mi></mml:msubsup></mml:math>at Low Momentum Transfer

High-precision measurements of the proton elastic form-factor ratio, mu pG p E/G p M, have been made at four-momentum transfer, Q2, 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 Q2 range the deviation from unity is primarily due to G p E being smaller than expected.