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\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} for values of the four-momentum transfer squared ranging from ${q}^{2}=0.389 \mathrm{to} 6.81$ ${(\mathrm{B}\mathrm{e}\mathrm{V}/\mathit{c})}^{2}$ (${q}^{2}=10 \mathrm{to} 175$${\mathrm{F}}^{\ensuremath{-}2}$). 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. \ifmmode \check{C}\else \v{C}\fi{}erenkov 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.