Absolute electron-proton cross sections at low momentum transfer measured with a high pressure gas target system

Absolute differential cross sections for elastic electron-proton scattering have been measured in a four-momentum transfer range up to 1.4 fm^?2. Using a high pressure gas target system, we have obtained highly accurate data with a small normalization error of 0.5%. The electromagnetic form factors G_E and G_M have been extracted and the rms charge radius has been determined to be $\text{〈r}{}^2{}_{\mathrm{<mtext>E</mtext>}}\text{〉}{}_{\mathrm{<mtext>p</mtext>}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 0.862±0.012}\text{fm}$. The shape of the isovector spectral function near threshold shows a significant non-resonant contribution of the two-pion state. This enhancement is so strong that the derivative at q² = 0 differs considerably from the usual vector meson dominance model value. This result is in good agreement with theoretical predictions.