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Register a new userFirst measurement of protons charge form factor at very low $Q^2$ with initial state radiation
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We report on a new experimental method based on initial-state radiation (ISR) in e-p scattering, in which the radiative tail of the elastic e-p peak contains information on the proton charge form factor ($G_E^p$) at extremely small $Q^2$. The ISR technique was validated in a dedicated experiment using the spectrometers of the A1-Collaboration at the Mainz Microtron (MAMI). This provided first measurements of $G_E^p$ for $0.001leq Q^2leq 0.004 (GeV/c)^2$.
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We present new data on the Bjorken sum $overline Gamma_1^{p-n}(Q^2)$ at 4-momentum transfer $ 0.021 leq Q^2 leq 0.496$ GeV$^2$. The data were obtained in two experiments performed at Jefferson Lab: EG4 on polarized protons and deuterons, and E97110 on polarized $^3$He from which neutron data were extracted. The data cover the domain where chiral effective field theory ($chi$EFT), the leading effective theory of the Strong Force at large distances, is expected to be applicable. We find that our data and the predictions from $chi$EFT are only in marginal agreement. This is somewhat surprising as the contribution from the $Delta(1232)$ resonance is suppressed in this observable, which should make it more reliably predicted by $chi$EFT than quantities in which the $Delta$ contribution is important. The data are also compared to a number of phenomenological models with various degrees of agreement.
We report a new, high-precision measurement of the proton elastic form factor ratio mu_p G_E/G_M for the four-momentum transfer squared Q^2 = 0.3-0.7 (GeV/c)^2. The measurement was performed at Jefferson Lab (JLab) in Hall A using recoil polarimetry. With a total uncertainty of approximately 1%, the new data clearly show that the deviation of the ratio mu_p G_E/G_M from unity observed in previous polarization measurements at high Q^2 continues down to the lowest Q^2 value of this measurement. The updated global fit that includes the new results yields an electric (magnetic) form factor roughly 2% smaller (1% larger) than the previous global fit in this Q^2 range. We obtain new extractions of the proton electric and magnetic radii, which are <r^2_E>^(1/2)=0.875+/-0.010 fm and <r^2_M>^(1/2)=0.867+/-0.020 fm. The charge radius is consistent with other recent extractions based on the electron-proton interaction, including the atomic hydrogen Lamb shift measurements, which suggests a missing correction in the comparison of measurements of the proton charge radius using electron probes and the recent extraction from the muonic hydrogen Lamb shift.
The charge form factor of $^$4He has been extracted in the range 29 fm$^{-2}$ $le Q^2 le 77$ fm$^{-2}$ from elastic electron scattering, detecting $^4$He nuclei and electrons in coincidence with the High Resolution Spectrometers of the Hall A Facility of Jefferson Lab. The results are in qualitative agreement with realistic meson-nucleon theoretical calculations. The data have uncovered a second diffraction minimum, which was predicted in the $Q^2$ range of this experiment, and rule out conclusively long-standing predictions of dimensional scaling of high-energy amplitudes using quark counting.
We report the first measurement of the parity-violating asymmetry in elastic electron scattering from the proton. The asymmetry depends on the neutral weak magnetic form factor of the proton which contains new information on the contribution of strange quark-antiquark pairs to the magnetic moment of the proton. We obtain the value $G_M^Z= 0.34 pm 0.09 pm 0.04 pm 0.05$ n.m. at $Q^2=0.1$ (GeV/c)${}^2$.
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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