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Register a new userNeutron form factor measurements at MAMI
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Added by
Bj\\\"orn S\\\"oren Schlimme
Publication date
2021
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and research's language is
English
Authors
B. S. Schlimme
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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.
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The distribution of the parton content of nuclei, as encoded via the generalized parton distributions (GPDs), can be accessed via the deeply virtual Compton scattering (DVCS) process contributing to the cross section for leptoproduction of real photons. Similarly to the scattering of light by a material, DVCS provides information about the dynamics and the spatial structure of hadrons. The sensitivity of this process to the lepton beam polarization allows to single-out the DVCS amplitude in terms of Compton form factors that contain GPDs information. The beam spin asymmetry of the $^4$He($vec {mathrm e}$,e$ gamma ^4$He) process was measured in the experimental Hall B of the Jefferson Laboratory to extract the real and imaginary parts of the twist-2 Compton form factor of the $^4$He nucleus. The experimental results reported here demonstrate the relevance of this method for such a goal, and suggest the dominance of the Bethe-Heitler amplitude to the unpolarized process in the kinematic range explored by the experiment.
$[Background]$ 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.
In 1963, a proton radius of $0.805(11)~mathrm{fm}$ was extracted from electron scattering data and this classic value has been used in the standard dipole parameterization of the form factor. In trying to reproduce this classic result, we discovered that there was a sign error in the original analysis and that the authors should have found a value of $0.851(19)~mathrm{fm}$. We additionally made use of modern computing power to find a robust function for extracting the radius using this 1963 datas spacing and uncertainty. This optimal function, the Pad{e} $(0,1)$ approximant, also gives a result which is consistent with the modern high precision proton radius extractions.
The possibility of measuring the proton electromagnetic form factors in the time-like region at FAIR with the PANDA detector is discussed. Detailed simulations on signal efficiency for the annihilation of $bar p +p $ into a lepton pair as well as for the most important background channels have been performed. It is shown that precision measurements of the differential cross section of the reaction $bar p +p to e^++ e^-$ can be obtained in a wide angular and kinematical range. The individual determination of the moduli of the electric and magnetic proton form factors will be possible up to a value of momentum transfer squared of $q^2simeq 14$ (GeV/c)$^2$. The total $bar p +pto e^++e^-$ cross section will be measured up to $q^2simeq 28$ (GeV/c)$^2$. The results obtained from simulated events are compared to the existing data. Sensitivity to the two photons exchange mechanism is also investigated.
The electric form factor of the neutron, G_En, has been measured at the Mainz Microtron by recoil polarimetry in the quasielastic D(e_pol,en_pol)p reaction. Three data points have been extracted at squared four-momentum transfers Q^2 = 0.3, 0.6 and 0.8 (GeV/c)^2. Corrections for nuclear binding effects have been applied.
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