Recoil Polarization Measurements of the Proton Electromagnetic Form Factor Ratio toQ2=8.5  GeV2

Abstract

Among the most fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dynamics of the nucleon's quark constituents; indeed, recent proton data have attracted intense theoretical interest. In this Letter, we report new measurements of the proton electromagnetic form factor ratio using the recoil polarization method, at momentum transfers Q2=5.2, 6.7, and 8.5 GeV2. By extending the range of Q2 for which G(E)(p) is accurately determined by more than 50%, these measurements will provide significant constraints on models of nucleon structure in the nonperturbative regime.

Baryons as relativistic three-quark bound states

“…Traditionally they have been extracted under the assumption of one-photon exchange via the Rosenbluth cross section in (4.40). It came as quite a surprise when the polarisation transfer measurements at Jefferson Lab revealed a ratio G p E /G p M that falls off and points towards a zero crossing at larger Q 2 [568][569][570][571], in stark contrast to the Rosenbluth measurements which had predicted a constant ratio. The question of the zero crossing for G p E is not yet settled because the experimental errors are still too large, but the situation is expected to change in the near future with the Jefferson Lab 12 GeV program where measurements in Halls A, B and C will extend the data range for G p E , G n E , and G n M up to the 10 -14 GeV 2 region [572].…”

“…In this respect, new experiments with polarised beams and/or polarised targets performed at Jefferson Lab allowed to extract the ratio G E /G M directly. However, the resulting ratio disagrees with the scaling predictions and shows a falloff with t, even pointing towards a zero crossing at larger t [568][569][570][571].…”

Hadron Structure from the Feynman-Hellmann Theorem

“…We demonstrate that application of the technique provides a dramatic improvement in On the left, the ratio G E /G M for the proton from application of the Feynman-Hellmann method, from a variational analysis of three-point functions, and from experiment [37,4,3]. On the right, the scaled pion form factor Q 2 F π from the Feynman-Hellmann technique and from experiment [7].…”

The OLYMPUS experiment

“…Recently, measurements of the electric to magnetic form factor ratio of the proton, µ p G p E /G p M , using polarization techniques [1][2][3][4][5][6][7][8][9][10][11][12][13] have shown a dramatic discrepancy at high four-momentum transfer, Q 2 , in comparison with the ratio obtained using the traditional Rosenbluth technique in unpolarized cross section measurements [14][15][16][17][18][19], highlighted in Fig. 1 by a selection of data sets.…”