Measurement ofGEp/GMp

Abstract

The ratio of the electric and magnetic form factors of the proton G(E(p))/G(M(p)), which is an image of its charge and magnetization distributions, was measured at the Thomas Jefferson National Accelerator Facility (JLab) using the recoil polarization technique. The ratio of the form factors is directly proportional to the ratio of the transverse to longitudinal components of the polarization of the recoil proton in the elastic e(-->)p---> e(-->)p reaction. The new data presented span the range 3.5< Q(2)< 5.6 GeV(2) and are well described by a linear Q(2) fit. Also, the ratio sqrt[Q(2)] F(2(p))/F(1(p)) reaches a constant value above Q(2) = 2 GeV(2).

Revealing Dressed Quarks via the Proton’s Charge Distribution

“…=P k is proportional to G E ðQ 2 Þ=G M ðQ 2 Þ [8,9]. A series of such experiments [7,[10][11][12][13] (6), and since the proton's magnetic form factor is reliably known on a spacelike domain that extends to Q 2 % 30 GeV 2 [14,15], the evolution of this ratio exposes novel features of the proton's charge distribution, as expressed in G E ðQ 2 Þ. An explanation of the discrepancy between the Rosenbluth and polarization transfer results for the ratio is currently judged to lie in two-photon-exchange corrections to the Born approximation, which affect the polarization transfer extraction of G E ðQ 2 Þ=G M ðQ 2 Þ far less than they do the ratio inferred via Rosenbluth separation [16].…”

Proton Form Factor Puzzle and the CEBAF Large Acceptance Spectrometer (CLAS) Two-Photon Exchange Experiment

“…The results indicate that the charge and the magnetization distribution inside the proton are similar. In contrast, the results from recent polarization transfer measurements [12,13,14,15], using a polarized electron beam to measure the polarization of the recoil proton shows an approximately linear drop of the ratio from unity with increasing Q 2 . The results from both techniques are shown in Fig.…”

“…A number of recent experiments at Jefferson Lab and MIT-Bates [12,13,14,15,27,28] have measured the proton form factor ratio R up to Q 2 = 8. …”

“…The Mott cross section is the cross section for scattering of a spin 1 2 electron from a point-like spinless particle, and is given as, 14) where α ≈ 1 137…”

“…More recent methods employ polarization degrees of freedom. In these methods, the electric to magnetic form factor ratio is determined by measuring the longitudinal and transverse polarization transferred to the proton by the polarized electron beam [12,13,14,15]. Both of these methods assume an exchange of a single virtual photon in the scattering process.…”