In nuclei, the effects of the strong nuclear field on a bound nucleon are an interesting issue: do bound nucleons have the same properties as free ones?

Thus, measurements of the ratio of the polarization components of a proton which was knocked-out by a polarized electron (in quasi-free scattering) is a good tool to detect changes in the bound-proton structure, reflected in the ratio of the electric to magnetic form factors (FFs). The polarization transfer in the ²H(e,e'p) and ⁴He(e,e'p) reactions was measured, and observed deviations from calculation with free proton FFs were attributed to nuclear medium modifications. However, it is not clear what mainly affects these changes: is it the nuclear density, or is it due to the nucleon being off-shell in the nuclear medium.

In ¹²C the effective nuclear density for protons in s-shell is about twice as high as for p-shell protons. Polarization-transfer measurements by the reaction ¹²C(e,e'p), knocking out a proton from the s- or p-shell, can provide constraints on the in-medium FF ratio as a function of the local nuclear density. We performed a dedicated experiment at the Mainz Microtron (MAMI) measuring such effects. We present the analysis and the study of the nuclear density dependence of nuclear modifications, by comparing the ¹²C(e,e'p) results with those from the ²H(e,e'p) and ⁴He(e,e'p) reactions.