Electric and magnetic form factors of the proton

Abstract

This paper describes a precise measurement of electron scattering off the proton at momentum transfers of 0.003 Q 2 1 GeV 2 . The average point-to-point error of the cross sections in this experiment is ∼0.37%. These data are used for a coherent new analysis together with all world data of unpolarized and polarized electron scattering from the very smallest to the highest momentum transfers so far measured. The extracted electric and magnetic form factors provide new insight into their exact shape, deviating from the classical dipole form, and of structure on top of this gross shape. The data reaching very low Q 2 values are used for a new determination of the electric and magnetic radii. An empirical determination of the two-photon-exchange correction is presented. The implications of this correction on the radii and the question of a directly visible signal of the pion cloud are addressed.

Evaluation of the strength of electron-proton scattering data for determining the proton charge radius

“…Because of the importance of the electron-proton scattering data to this puzzle, the data has been extensively scrutinized and discussed [5,[8][9][10][11][12][13][14][15][16][17][18][19][20]. The present work reanalyzes the scattering data and concludes that it is consistent with a much larger range of R E values than obtained by others.…”

“…A third determination of R E can be obtained from precise measurements of the the cross sections for elastic scattering between electrons and protons. The most precise electronproton scattering experiment is the recent measurement [4] of the MAMI collaboration, and their analysis [5] leads to R E = 0.879(8) fm, which disagrees with the muonic hydrogen value by 4.6 standard deviations.CODATA [3] uses a combination of the scattering and hydrogen values to obtain R E , and its value differs from the muonic hydrogen value by 7 standard deviations. This disagreement has now widely been referred to as the proton size puzzle [6].…”

“…A third determination of R E can be obtained from precise measurements of the the cross sections for elastic scattering between electrons and protons. The most precise electronproton scattering experiment is the recent measurement [4] of the MAMI collaboration, and their analysis [5] leads to R E = 0.879 (8) fm, which disagrees with the muonic hydrogen value by 4.6 standard deviations.…”

“…This disagreement has now widely been referred to as the proton size puzzle [6]. Many papers have discussed this puzzle, including many that have proposed physics beyond the standard model [7].Because of the importance of the electron-proton scattering data to this puzzle, the data has been extensively scrutinized and discussed [5,[8][9][10][11][12][13][14][15][16][17][18][19][20]. The present work reanalyzes the scattering data and concludes that it is consistent with a much larger range of R E values than obtained by others.…”

The Proton Radius Puzzle

“…The rate of decrease of the electric and magnetic form factors at Q 2 = 0 are directly proportional to the rms electric and magnetic radii. For an example of a detailed study using this technique, see Bernauer et al (2014).…”