Medium modification of the proton form factor

I argue that the double ratio of proton-recoil polarization-transfer coefficients, P′ x and P′ z , of the quasielastic reaction with respect to the elastic reaction is sensitive to possible medium modifications of the proton form factor in 4He. Recent measurements at both Mainz and Jefferson Lab of this double ratio at four-momentum transfers squared between between 0.4 and 2.6 are discussed. I show that the data challenge state-of-the-art conventional meson-nucleon calculations, as these are unable to describe the results. The data hint at the need to include medium modifications of the proton form factor, as predicted by a quark-meson-coupling model, in the calculations. A recently approved follow-up experiment at a Q 2 of 0.8 and 1.3 with unprecedented precision will provide one of the most stringent tests of the applicability of various calculations.