Measurement of Proton Electromagnetic Form Factors at High Momentum Transfers

Abstract

Elastic electron-proton scattering cross sections have been measured using the internal beam of the 6-BeV Cambridge Electron Accelerator at laboratory scattering angles between 31° and 90° for values of the four-momentum transfer squared ranging from $q^2=0.389\mathrm{} \mathrm{to} 6.81$ ${(\mathrm{B}\mathrm{e}\mathrm{V}/\mathit{c})}^2$ ($q^2=10\mathrm{} \mathrm{to} 175$ ${\mathrm{F}}^{-2\mathrm{}}$). Incident electron energies ranged from 1.0 to 6.0 BeV. Scattered electrons from an internal liquid-hydrogen target were momentum-analyzed using a single quadrupole spectrometer capable of momentum analysis up to 3.0 BeV/c. Čerenkov and shower counters were used to help reject pion and low-energy background. The cross sections presented are absolute cross sections with experimental errors ranging from 6.8% to 20%. Separation of proton electromagnetic form factors have been made for all but the two highest momentum transfer points, using the Rosenbluth formula. Both form factors, $G_{\mathrm{Ep}}$ and $G_{\mathrm{Mp}}$, were observed to continue to decrease as the momentum transfer increases. An upper limit to the possible asymptotic values of the proton electromagnetic form factors has been established.