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Register a new userPolarization-transfer measurement to a large-virtuality bound proton in the deuteron
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A1 Collaboration: I. Yaron
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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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We measured the ratio $P_{x}/P_{z}$ of the transverse to longitudinal components of polarization transferred from electrons to bound protons in $^{12}mathrm{C}$ by the $^{12}mathrm{C}(vec{e},evec{p})$ process at the Mainz Microtron (MAMI). We observed consistent deviations from unity of this ratio normalized to the free-proton ratio, $(P_{x}/P_{z})_{^{12}mathrm{C}}/(P_{x}/P_{z})_{^{1}mathrm{H}}$, 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 $^{2}mathrm{H}$ and $^{4}mathrm{He}$, suggesting a universal behavior. It further implies no dependence on average local nuclear density.
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.
Recoil proton polarization observables were measured for both the p($vec {rm e}$,e$^primevec{rm p},$) and d($vec {rm e}$,e$^primevec{rm p},)$n reactions at two values of Q$^2$ using a newly commissioned proton Focal Plane Polarimeter at the M.I.T.-Bates Linear Accelerator Center. The hydrogen and deuterium spin-dependent observables $D_{ellell}$ and $D_{{ell}t}$, the induced polarization $P_n$ and the form factor ratio $G^p_E/G^p_M$ were measured under identical kinematics. The deuterium and hydrogen results are in good agreement with each other and with the plane-wave impulse approximation (PWIA).
Compton scattering from the proton was investigated at s=6.9 (GeV/c)**2 and t=-4.0 (GeV/c)**2 via polarization transfer from circularly polarized incident photons. The longitudinal and transverse components of the recoil proton polarization were measured. The results are in excellent agreement with a prediction based on a reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton and in disagreement with a prediction of pQCD based on a two-gluon exchange mechanism.
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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