We measured the ratio P x /P z of the transverse to longitudinal components of polarization transferred from electrons to bound protons in 12 C by the 12 C( e, e p) process at the Mainz Microtron (MAMI). We observed consistent deviations from unity of this ratio normalized to the free-proton ratio, (P x /P z )12 C /(P x /P z )1 H , for both s-and p-shell knocked out protons, even though they are embedded in averaged local densities that differ by about a factor of two. The dependence of the double ratio on proton virtuality is similar to the one for knocked out protons from 2 H and 4 He, suggesting a universal behavior. It further implies no dependence on average local nuclear density.
We report on the first Q^{2}-dependent measurement of the beam-normal single spin asymmetry A_{n} in the elastic scattering of 570 MeV vertically polarized electrons off ^{12}C. We cover the Q^{2} range between 0.02 and 0.05 GeV^{2}/c^{2} and determine A_{n} at four different Q^{2} values. The experimental results are compared to a theoretical calculation that relates A_{n} to the imaginary part of the two-photon exchange amplitude. The result emphasizes that the Q^{2} behavior of A_{n} given by the ratio of the Compton to charge form factors cannot be treated independently of the target nucleus.
A comparison between polarization-transfer to a bound proton in quasi-free kinematics by the A( e, e p) knockout reaction and that in elastic scattering off a free proton can provide information on the characteristics of the bound proton. In the past the reported measurements have been compared to those of a free proton with zero initial momentum. We introduce, for the first time, expressions for the polarization-transfer components when the proton is initially in motion and compare them to the 2 H data measured at the Mainz Microtron (MAMI). We show the ratios of the transverse (P x ) and longitudinal (P z ) components of the polarization transfer in 2 H( e, e p)n, to those of elastic scattering off a "moving proton", assuming the proton's initial (Fermi-motion) momentum equals the negative missing momentum in the measured reaction. We found that the correction due to the proton motion is up to 20% at high missing momentum. However the effect on the double ratio (Px/Pz) A (Px/Pz) 1 H is largely canceled out, as shown for both 2 H and 12 C data. This implies that the difference between the restingand the moving-proton kinematics is not the primary cause for the deviations between quasi-elastic and elastic scattering reported previously.
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