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Register a new userQuasi-elastic polarization-transfer measurements on the deuteron in anti-parallel kinematics
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We present measurements of the polarization-transfer components in the $^2$H$(vec e,evec p)$ reaction, covering a previously unexplored kinematic region with large positive (anti-parallel) missing momentum, $p_{rm miss}$, up to 220 MeV$/c$, and $Q^2=0.65$ $({rm GeV}/c)^2$. These measurements, performed at the Mainz Microtron (MAMI), were motivated by theoretical calculations which predict small final-state interaction (FSI) effects in these kinematics, making them favorable for searching for medium modifications of bound nucleons in nuclei. We find in this kinematic region that the measured polarization-transfer components $P_x$ and $P_z$ and their ratio agree with the theoretical calculations, which use free-proton form factors. Using this, we establish upper limits on possible medium effects that modify the bound protons form factor ratio $G_E/G_M$ at the level of a few percent. We also compare the measured polarization-transfer components and their ratio for $^2$H to those of a free (moving) proton. We find that the universal behavior of $^2$H, $^4$He and $^{12}$C in the double ratio $frac{(P_x/P_z)^A}{(P_x/P_z)^{^1rm H}}$ is maintained in the positive missing-momentum region.
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We report the first measurements of the transverse ($P_{x}$ and $P_{y}$) and longitudinal ($P_{z}$) components of the polarization transfer to a bound proton in the deuteron via the $^{2}mathrm{H}(vec{e},evec{p})$ reaction, over a wide range of missing momentum. A precise determination of the electron beam polarization reduces the systematic uncertainties on the individual components, to a level that enables a detailed comparison to a state-of-the-art calculation of the deuteron that uses free-proton electromagnetic form factors. We observe very good agreement between the measured and the calculated $P_{x}/P_{z}$ ratios, but deviations of the individual components. Our results cannot be explained by medium modified electromagnetic form factors. They point to an incomplete description of the nuclear reaction mechanism in the calculation.
We report measurements of the induced polarization $vec P$ of protons knocked out from $^2$H and $^{12}$C via the $A(e,evec p,)$ reaction. We have studied the dependence of $vec P$ on two kinematic variables: the missing momentum $p_{rm miss}$ and the off-coplanarity angle $phi_{pq}$ between the scattering and reaction planes. For the full 360$degree$ range in $phi_{pq}$, both the normal ($P_y$) and, for the first time, the transverse ($P_x$) components of the induced polarization were measured with respect to the coordinate system associated with the scattering plane. $P_x$ vanishes in coplanar kinematics, however in non-coplanar kinematics, it is on the same scale as $P_y$. We find that the dependence on $phi_{pq}$ is sine-like for $P_x$ and cosine-like for $P_y$. For carbon, the magnitude of the induced polarization is especially large when protons are knocked out from the $p_{3/2}$ shell at very small $p_{rm miss}$. For the deuteron, the induced polarization is near zero at small $|p_{rm miss}|$, and its magnitude increases with $|p_{rm miss}|$. For both nuclei such behavior is reproduced qualitatively by theoretical results, driven largely by the spin-orbit part of the final-state interactions. However, for both nuclei, sizeable discrepancies exist between experiment and theory.
We report the measurements of the transverse ($Px$) and longitudinal ($Pz$) components of the polarization transfer to a bound proton in carbon via the quasi-free $^{12}{rm C}(vec e,evec p)$ reaction, over a wide range of missing momenta. We determine these polarization-transfers separately for protons knocked out from the $s$- and $p$-shells. The electron-beam polarization was measured to determine the individual components with systematic uncertainties which allow a detailed comparison with theoretical calculations.
A comparison between polarization-transfer to a bound proton in quasi-free kinematics by the A$(vec{e},evec p)$ 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 $^2textrm{H}(vec{e},evec p)textrm{n}$, to those of elastic scattering off a moving proton, assuming the protons initial (Fermi) 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 $frac{(P_x/P_z)^A}{(P_x/P_z)^{^1!textrm{H}}}$ is largely canceled out, as shown for both $^2$H and $^{12}$C data. This implies that the kinematics is not the primary cause for the deviations between quasi-elastic and elastic scattering reported previously.
Possible differences between free and bound protons may be observed in the ratio of polarization-transfer components, $P_x/P_z$. We report the measurement of $P_x/P_z$, in the $^2textrm{H}(vec{e},e^{prime}vec{p})n$ reaction at low and high missing momenta. Observed increasing deviation of $P_x/P_z$ from that of a free proton as a function of the virtuality, similar to that observed in hefour, indicates that the effect in nuclei is due to the virtuality of the knock-out proton and not due to the average nuclear density. The measured differences from calculations assuming free-proton form factors ($sim10%$), may indicate in-medium modifications.
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