Polarization-transfer measurement to a large-virtuality bound proton in the deuteron

Abstract

Possible differences between free and bound protons may be observed in the ratio of polarizationtransfer components, P x /P z . We report the measurement of P x /P z , in the 2 H( e, e 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 4 He, 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 (∼ 10%), may indicate in-medium modifications.

Quasi-elastic polarization-transfer measurements on the deuteron in anti-parallel kinematics

“…The larger crosssection in setting F allowed us to obtain large statistics, and to compare the measurements with those at a similar kinematic setting (A) in Refs. [13,14] as a cross-check.…”

“…2 These data sets have the same Q 2 and cover the same range in p miss , but had different beam energies: 600 (690) MeV/c for setting A (F). The dependence of the polarization transfer components and their ratios on Q 2 (due to the form factors) and other kinematic effects are reduced by dividing the measured values by the expected ones for elastic ep scattering with similar kinematics [13][14][15]27]. We use the "moving-proton" prescription [27,28], rather than the expressions for elastic scattering off a resting proton [10].…”

“…Following the convention used in [13][14][15]27] we use the struck-proton virtuality ("off-shellness"), ν, as our parameter of choice. The virtuality is defined as [13]:…”

“…Quasi-free polarization-transfer experiments have been carried out on 2 H and 12 C at the Mainz Microtron (MAMI) [13][14][15], as well as on 2 H, 4 He and 16 O at Jefferson Lab (JLab) [16][17][18][19], in search of medium modification in the proton's internal structure. These include measurements on far off-shell nucleons, characterized by high missing momentum, which is equivalent (neglecting final-state interactions (FSI)) to the struck protons having high initial momentum.…”

“…These include measurements on far off-shell nucleons, characterized by high missing momentum, which is equivalent (neglecting final-state interactions (FSI)) to the struck protons having high initial momentum. For 2 H in parallel kinematics (negative p miss ), it was shown that the deviations in P x /P z from that of elastic ep scattering can be explained by nuclear effects without the necessity of introducing modified FFs [13,14]. The individual components P x and P z were also measured.…”