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Register a new userExtraction of the Neutron Electric Form Factor from Measurements of Inclusive Double Spin Asymmetries
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$[Background]$ Measurements of the neutron charge form factor, $G^n_E$, are challenging due to the fact that the neutron has no net charge. In addition, measurements of the neutron form factors must use nuclear targets which require accurately accounting for nuclear effects. Extracting $G^n_E$ with different targets and techniques provides an important test of our handling of these effects. $[Purpose]$ The goal of the measurement was to use an inclusive asymmetry measurement technique to extract the neutron charge form factor at a four-momentum transfer of $1~(rm{GeV/c})^2$. This technique has very different systematic uncertainties than traditional exclusive measurements and thus serves as an independent check of whether nuclear effects have been taken into account correctly. $[Method]$ The inclusive quasi-elastic reaction $^3overrightarrow{rm{He}}(overrightarrow{e},e)$ was measured at Jefferson Lab. The neutron electric form factor, $G_E^n$, was extracted at $Q^2 = 0.98~(rm{GeV/c})^2$ from ratios of electron-polarization asymmetries measured for two orthogonal target spin orientations. This $Q^2$ is high enough that the sensitivity to $G_E^n$ is not overwhelmed by the neutron magnetic contribution, and yet low enough that explicit neutron detection is not required to suppress pion production. $[Results]$ The neutron electric form factor, $G_E^n$, was determined to be $0.0414pm0.0077;{(stat)}pm0.0022;{(syst)}$; providing the first high precision inclusive extraction of the neutrons charge form factor. $[Conclusions]$ The use of the inclusive quasi-elastic $^3overrightarrow{rm{He}}(overrightarrow{e},e)$ with a four-momentum transfer near $1~(rm{GeV/c})^2$ has been used to provide a unique measurement of $G^n_E$. This new result provides a systematically independent validation of the exclusive extraction technique results.
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The spin polarizabilities of the nucleon describe how the spin of the nucleon responds to an incident polarized photon. The most model-independent way to measure the nucleon spin polarizabilities is through polarized Compton scattering. Double-polarized Compton scattering asymmetries on the proton were measured in the $Delta(1232)$ region using circularly polarized incident photons and a transversely polarized proton target at the Mainz Microtron. Fits to asymmetry data were performed using a dispersion model calculation and a baryon chiral perturbation theory calculation, and a separation of all four proton spin polarizabilities in the multipole basis was achieved. The analysis based on a dispersion model calculation yields $gamma_{E1E1} = -3.5 pm 1.2$, $gamma_{M1M1}= 3.16 pm 0.85$, $gamma_{E1M2} = -0.7 pm 1.2$, and $gamma_{M1E2} = 1.99 pm 0.29$, in units of $10^{-4}$ fm$^4$.
Measurements of the electric and the magnetic neutron form factors have been performed at the Mainz Microtron for more than 20 years. These MAMI experiments are reviewed in the context of measurements from other groups, and future measurements at MAMI are outlined.
We report the measurement of beam-target double-spin asymmetries ($A_text{LT}$) in the inclusive production of identified hadrons, $vec{e}~$+$~^3text{He}^{uparrow}rightarrow h+X$, using a longitudinally polarized 5.9 GeV electron beam and a transversely polarized $^3rm{He}$ target. Hadrons ($pi^{pm}$, $K^{pm}$ and proton) were detected at 16$^{circ}$ with an average momentum $<$$P_h$$>$=2.35 GeV/c and a transverse momentum ($p_{T}$) coverage from 0.60 to 0.68 GeV/c. Asymmetries from the $^3text{He}$ target were observed to be non-zero for $pi^{pm}$ production when the target was polarized transversely in the horizontal plane. The $pi^{+}$ and $pi^{-}$ asymmetries have opposite signs, analogous to the behavior of $A_text{LT}$ in semi-inclusive deep-inelastic scattering.
The electric form factor of the neutron was determined from studies of the reaction He3(e,en)pp in quasi-elastic kinematics in Hall A at Jefferson Lab. Longitudinally polarized electrons were scattered off a polarized target in which the nuclear polarization was oriented perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons that were registered in a large-solid-angle detector. More than doubling the Q2-range over which it is known, we find GEn = 0.0225 +/- 0.0017 (stat) +/- 0.0024 (syst), 0.0200 +/- 0.0023 +/- 0.0018, and 0.0142 +/- 0.0019 +/- 0.0013 for Q2 = 1.72, 2.48, and 3.41 GeV2, respectively.
The ratio of the electric and magnetic form factor of the proton, $mu_p G_E^p/G_M^p$, has been measured for elastic electron-proton scattering with polarized beam and target up to four-momentum transfer squared, $Q^2=5.66$ (GeV/c)$^2$ using the double spin asymmetry for target spin orientation aligned nearly perpendicular to the beam momentum direction. This measurement of $mu_p G_E^p/G_M^p$ agrees with the $Q^2$ dependence of previous recoil polarization data and reconfirms the discrepancy at high $Q^2$ between the Rosenbluth and the polarization-transfer method with a different measurement technique and systematic uncertainties uncorrelated to those of the recoil-polarization measurements. The form factor ratio at $Q^2$=2.06 (GeV/c)$^2$ has been measured as $mu_p G_E^p/G_M^p = 0.720 pm 0.176_{stat} pm 0.039_{sys}$, which is in agreement with an earlier measurement with the polarized target technique at similar kinematics. The form factor ratio at $Q^2$=5.66 (GeV/c)$^2$ has been determined as $mu_p G_E^p/G_M^p=0.244pm0.353_{stat}pm0.013_{sys}$, which represents the highest $Q^2$ reach with the double spin asymmetry with polarized target to date.
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