Charge Form Factor of the Neutron at Low Momentum Transfer from the ${}^2\overrightarrow{\mathrm{H}}(\overrightarrow{e},e^{\prime }n){}^1\mathrm{H}$ Reaction

We report new measurements of the neutron charge form factor at low momentum transfer using quasielastic electrodisintegration of the deuteron. Longitudinally polarized electrons at an energy of 850 MeV were scattered from an isotopically pure, highly polarized deuterium gas target. The scattered electrons and coincident neutrons were measured by the Bates Large Acceptance Spectrometer Toroid (BLAST) detector. The neutron form factor ratio $G_E^n/G_M^n$ was extracted from the beam-target vector asymmetry $A_{\mathrm{ed}}^V$ at four-momentum transfers $Q^2=0.14$, 0.20, 0.29, and $0.42(\mathrm{GeV}/c{)}^2$.

Figure 1

Measured (solid blue points) and calculated beam-target vector polarization observable $A_{\mathrm{ed}}^V$ (upper panel) and tensor asymmetry $A_d^T$ (lower panel) for the ${}^2\mathrm{H}(e,e^{\prime }n)p$ reaction at 850 MeV, a target orientation of ${\theta }_d=31.6°$ into the left sector of BLAST, and with neutrons detected in the right sector. The colored curves are Monte Carlo simulations based on the deuteron electrodisintegration model of Ref. [11] (dotted $\mathrm{\text{magenta}}=\mathrm{PWBA}$, short-dashed $\mathrm{\text{green}}=\mathrm{PWBA}+\mathrm{FSI}$, solid $\mathrm{\text{red}}=\mathrm{PWBA}+\mathrm{FSI}+\mathrm{MEC}+\mathrm{IC}+\mathrm{RC}$) using standard parameterizations for the nucleon form factors (see text). In addition, the corresponding curves for $G_E^n\equiv 0$ (dash-dotted red) and for elastic scattering from the free neutron (dashed black line) are shown.

Figure 2

World data on $G_E^n$ from double-polarization experiments [8]. The data correspond to neutron recoil polarization experiments with unpolarized ${}^2\mathrm{H}$ target (open triangles) and experiments with polarized ${}^2\mathrm{H}$ (open circles; $\mathrm{\text{solid red dots}}=\mathrm{\text{this work}}$) and ${}^3\mathrm{He}$ targets (open squares). The data are shown with statistical (small error bars) and with statistical and systematic errors added quadratically (large error bars). The “BLAST fit” (blue solid line) is a parameterization of all data displayed based on the sum of two dipoles shown with a one-sigma error band. The recent parameterization [25] (magenta dash-dotted line) is based on the form introduced in [21]. Also shown are recent results based on vector meson dominance and dispersion relations (red short-dashed [4] and green long-dashed lines [5]), and of a light-front cloudy bag model with relativistic constituent quarks [6] (cyan dotted line).