Measurement of the Neutron Electric to Magnetic Form Factor Ratio at $Q^2\mathbf{=}1.58{\mathrm{GeV}}^2$ Using the Reaction ${}^3\overrightarrow{\mathrm{He}}(\overrightarrow{e},e^{\prime }n)pp$

A measurement of beam helicity asymmetries in the reaction ${}^3\overrightarrow{\mathrm{He}}(\overrightarrow{e},e^{\prime }n)pp$ is performed at the Mainz Microtron in quasielastic kinematics to determine the electric to magnetic form factor ratio of the neutron $G_E^n/G_M^n$ at a four-momentum transfer $Q^2=1.58{\mathrm{GeV}}^2$. Longitudinally polarized electrons are scattered on a highly polarized ${}^3\mathrm{He}$ gas target. The scattered electrons are detected with a high-resolution magnetic spectrometer, and the ejected neutrons are detected with a dedicated neutron detector composed of scintillator bars. To reduce systematic errors, data are taken for four different target polarization orientations allowing the determination of $G_E^n/G_M^n$ from a double ratio. We find ${\mu }_n{G}_E^n/G_M^n=0.250\pm 0.058(\mathrm{stat})\pm 0.017(\mathrm{syst})$.

Figure 1

Asymmetries of the reaction ${}^3\overrightarrow{\mathrm{He}}(\overrightarrow{e},e^{\prime }n)pp$ were measured for four different target polarization orientations. The asymmetries obtained in setups 2 and 4 that correspond to a polarization direction in the scattering plane and perpendicular to the (mean) momentum transfer (${\theta }^{*}=90°$; ${\varphi }^{*}=0°$ and 180°, respectively) are sensitive on $G_E^n/G_M^n$. Asymmetries measured in setups 1 and 3 (${\theta }^{*}=0°$ and 180°) are used for normalization.

Figure 2

(a) Experimental raw asymmetries obtained in the four setups for different proton rejection criteria without any corrections. For instance, veto 1 denotes the asymmetries obtained when only the timing signals of the neutron detector veto counters were used for proton rejection. For analysis, veto 5 was used, which included an additional cut on the reconstructed energy deposited in the detector bars of the first layer. (b) Total number of accepted events. (c) Relative proton contamination estimated by analysis of scattering data on a hydrogen target. (d) Ratio of $A_{\perp }$ (statistically weighted mean value of the two asymmetries measured in setups 2 and 4) and $A_{\parallel }$, which is essentially used to extract $g$.

Figure 3

The result for ${\mu }_n{G}_E^n/G_M^n$ of this experiment along with results from other double polarization experiments [3, 4, 5, 6, 7, 8, 9, 10, 11, 12]. The uncertainties shown are the statistical and the systematic errors added in quadrature. Also shown are the results of recent calculations based on general parton distributions [32] (solid line), dispersion analysis [33] (gray band), a quark-diquark model with a pion cloud [34] (dashed line), and the extended Lomon-Gari-Krümpelmann model of nucleon electromagnetic FF [35] (dotted lines, for two different parametrizations of resonance widths, timelike proton form factor data from BABAR included).