Hadrons in the Nuclear Medium

Nucleon properties are modified in the nuclear medium. To under-stand 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, e′~p)3H reaction to test these predictions. These results, with the most precise data at Q2 = 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 Q2 = 1.3 (GeV/c)2 the measured ratios of polarization-transfer are successfully described in a fully relativistic calculation when in-cluding a medium modification of the proton form factors or, alter-natively, by strong charge-exchange final-state interactions. We also discuss possible extensions of these studies with measurements of the 4He(~e, e′~p)3H and 2H(~e, e′~p)n reactions as well as with the neutron knockout in 4He(~e, e′~n)3He.