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 [InlineMediaObject not available: see fulltext.]reaction with respect to the elastic [InlineMediaObject not available: see fulltext.]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 [InlineMediaObject not available: see fulltext.]and 2.6 [InlineMediaObject not available: see fulltext.]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 [InlineMediaObject not available: see fulltext.]and 1.3 [InlineMediaObject not available: see fulltext.]with unprecedented precision will provide one of the most stringent tests of the applicability of various calculations