Nucleon properties are modified in the nuclear medium. To understand these modifications and their origin is a central issue in nuclear physics. For example, a wide variety of QCD-based models, including quark-meson coupling and chiral-quark soliton
models, predict that the nuclear constituents change properties with increasing density. These changes are predicted to lead to observable changes in the nucleon structure functions and electromagnetic form factors. We present results from a series of recent experiments at MAMI and Jefferson Lab, which measured the proton recoil polarization in the 4He(e,ep)3H reaction to test these predictions. These results, with the most precise data at Q^2 = 0.8 (GeV/c)^2 and at 1.3 (GeV/c)^2 from E03-104, put strong constraints on available model calculations, such that below Q^2 = 1.3 (GeV/c)^2 the measured ratios of polarization-transfer are successfully described in a fully relativistic calculation when including a medium modification of the proton form factors or, alternatively, by strong charge-exchange final-state interactions. We also discuss possible extensions of these studies with measurements of the 4He(e,ep)3H and 2H(e,ep)n reactions as well as with the neutron knockout in 4He(e,en)3He.
Polarization transfer in quasi-elastic nucleon knockout is sensitive to the properties of the nucleon in the nuclear medium. In experiment E03-104 at Jefferson Lab we measured the proton recoil polarization in the 4He(e,ep)3H reaction at a Q^2 of 0.8
(GeV/c)^2 and 1.3 (GeV/c)^2 with unprecedented precision. The measured polarization-transfer coefficients transverse and longitudinal to the momentum-transfer direction are well described by a fully relativistic calculation when a density-dependent medium modification of the nucleon form factors is included in the model. Results of an alternative model show that the ratio of these observables is also well described through strong charge-exchange final-state interactions. The induced polarization in the (e,ep) reaction is sensitive to the final-state interactions and the data from E03-104 will further constrain these models.
Polarization transfer in quasi-elastic nucleon knockout is sensitive to the properties of the nucleon in the nuclear medium, including possible modification of the nucleon form factor and/or spinor. In our recently completed experiment E03-104 at Jef
ferson Lab we measured the proton recoil polarization in the 4He(e,ep)3H reaction at a Q^2 of 0.8 (GeV/c)^2 and 1.3 (GeV/c)^2 with unprecedented precision. These data complement earlier data between 0.4 and 2.6 (GeV/c)^2 from both Mainz and Jefferson Lab. The measured ratio of polarization-transfer coefficients differs from a fully relativistic calculation, favoring either the inclusion of a medium modification of the proton form factors predicted by a quark-meson coupling model or strong charge-exchange final-state interactions. The measured induced polarizations agree well with the fully relativistic calculation and indicate that these strong final-state interactions may not be applicable.
