Measurement of the Neutron Electric Form FactorGE,nin the QuasifreeH2

Abstract

The electric form factor of the neutron G E,n has been measured in the quasifree 2 H͑ e, e 0 n͒p reaction using the 855 MeV polarized cw electron beam of the Mainz Microtron MAMI. The polarization of the scattered neutrons was analyzed in a polarimeter consisting of two walls of plastic scintillators. The precession of the neutron spin in a magnetic field was used for the first time to circumvent the measurement of the effective analyzing power of the polarimeter and the beam polarization. In this way G E,n could be determined with little model dependence and experimental uncertainties. The result G E,n ͑0.34 GeV 2 ͞c 2 ͒ 0.0611 6 0.0069 stat ͑ 10.0069 20.0055 ͒ syst is larger than previously assumed.The internal structure of protons and neutrons at low energies is still an unsolved problem. At momentum transfers of the order Q 2 ϳ L 2 QCD , where L QCD ഠ 250 MeV 2 ͞c 2 is the scale parameter of quantum chromodynamics (QCD), perturbative solutions are not possible. Rather, the strictly nonlinear QCD has to be solved in the framework of lattice gauge theory or approximated by effective field theories. Sensitive tests of these solutions require the detailed comparison with low energy observables, such as the excitation spectrum of the nucleon or the electric and magnetic form factors. In particular, the form factors, which are measured by elastic electronnucleon scattering, contain the information on the spatial distribution of charge and magnetization in the nucleon. The electric form factor of the neutron G E,n is of special significance since the charge of the neutron vanishes and a nonzero form factor must come from a nonuniform spatial distribution of valence quarks or the sea of correlated quark-antiquark pairs, e.g., pions.The experimental information on G E,n is still unsatisfactory for two reasons. First, the lack of a free neutron target requires the use of nuclear targets thus introducing binding effects. Second, the standard Rosenbluth separation of the longitudinal cross section arising from the electric form factor G E,n ͑Q 2 ͒ and the transverse cross section due to the magnetic form factor G M,n ͑Q 2 ͒ inare limited because G 2 E,n ͑Q 2 ͒ ø tG 2 M,n ͑Q 2 ͒. Here s Mott denotes the Mott cross section, t Q 2 ͞4M 2 , and e ͓1 1 2͑1 1 t͒ tan 2 q e ͞2͔ 21 . Recently, such a separation of G E,n in the range Q 2 Ӎ 1.75 4 GeV 2 ͞c 2 yielded data compatible with G E,n ϵ 0 [1].

Role of mesons in the electromagnetic form factors of the nucleon

“…4 very well. The values obtained from Rosenbluth separations [45,56,57,59,[69][70][71][72][73][74][75][76][77] would be much higher than those, but are not plotted because of their greater sensitivity to the two-photon corrections and the nuclear target model dependence. Figures 1-6 show not only the data at low Q 2 , the main focus of this study, but also over an expanded range to see the small structures in the data and models better.…”

“…For the neutron the fit function was reduced to the sum of two dipoles, fit to the world polarized data as of 2008 of Refs. [53,54,70,71,[73][74][75]77,[98][99][100][101]. Again note the addition of early polarized measurements to the data set of Sec.…”

A polarization relation and the measurement of the longitudinal response in pseudoscalar meson electroproduction off the nucleon

“…In the past few years, elastic and quasielastic scattering experiments yielded decisive results concerning the electromagnetic structure of proton [1] and neutron [2][3][4][5][6] by taking advantage of double polarization observables. These provide also high sensitivity to the longitudinal electromagnetic coupling of nucleon resonances [7].…”

Electromagnetic form factors of nucleons with QCD constraints: Systematic study of space- and time-like regions

“…The solid curve is the result for case I and the dashed one for case II. Data points are taken from Meyerhoff et al[22], Rohe et al[30], Herberg et al[24], Zhu et al[31], Passchier et al[25], Ostrick et al[26], and Madey et al[41].…”