DOI 10.1140/epja/i2002-10191-8 Eur. Phys. J. A 17, 451–455 (2003) T HE EUROPEAN P HYSICAL JOURNAL A Is the proton electromagnetic form factor modified in nuclei? J. Morgenstern 1, a and Z.-E. Meziani 2 1 CEA Saclay DSM/DAPNIA/SPhN, F91191, Gif-sur-Yvette Cedex, France 2 Temple University, Philadelphia, PA 19122, USA Received: 1 November 2002 / Published online: 15 July 2003 – c  Societ` a Italiana di Fisica / Springer-Verlag 2003 Abstract. Guided by the recent experimental confirmation of the validity of the Effective Momentum Approximation (EMA) in quasi-elastic scattering off nuclei, we have re-examined the extraction of the Longitudinal and Transverse Response Functions in medium-weight and heavy nuclei. In the EMA we have performed a Rosenbluth separation of the available world data on 40 Ca, 48 Ca, 56 Fe and 208 Pb. We find that the Longitudinal Response Function for these nuclei is “quenched” and that the Coulomb sum is not saturated, at odds with recent claims in the literature. PACS. 25.30.Fj Inelastic electron scattering to continuum Quasi-elastic electron scattering off nuclei has permit- ted to investigate the properties of nucleons in nuclei. As the Coulomb response depends quasi-exclusively on nu- cleonic degrees of freedom, it was proposed that a Rosen- bluth separation of the Coulomb and magnetic responses of a nucleus (R L and R T , respectively) could test a model- independent property known as the Coulomb Sum Rule (CSR), S L (q) [1]: S L (q)= 1 Z  ∞ 0 + R L (q,ω) ˜ G E 2 dω. (1) Here ˜ G E =(G p E +N/ZG n E )ζ takes into account the nu- cleon charge form factor inside the nucleus (which is usu- ally taken to be equal to that of a free nucleon) as well as a relativistic correction (ζ ) suggested by de Forest [2]. The lower limit of integration 0 + excludes the elastic peak. In the limit of large q, C(q), which depends on correlations, is predicted to vanish and consequently S L (q) to be equal to unity. There is a general agreement in non-relativistic theories, that beyond |q| = q ∼ 500 MeV/c, twice the Fermi momentum, C L (q) is not bigger than a few % (see review paper [3]). In the last twenty years a large experimental program has been carried out at Bates [4–12], Saclay [13–17] and SLAC [18–20] aimed at the extraction of R L and R T for a variety of nuclei. Unfortunately, in the case of medium- weight and heavy nuclei, conclusions reached by different experiments ranged from a full saturation of the CSR to its violation by 30%. As a result, a spectrum of explana- tions has emerged ranging from questioning the validity a e-mail: morgen@hep.saclay.cea.fr of the experiments (i.e., experimental backgrounds), in- adequate Coulomb corrections (especially for heavy nu- clei) to suggesting a picture of a “swollen nucleon” in the nuclear medium due to a partial deconfinement [21–25]. Another approach with the relativistic σ - ω model initi- ated by Walecka [26,27] has been applied to Nuclear Mat- ter calculations with further improvements including RPA correlations [28], to finite nuclei without RPA correla- tions [29], to finite nuclei with Relativistic RPA correla- tions (RRPA) and local density approximations [30–33]. Recently, for Nuclear Matter, the σ - ω model has been extended [34] to take into account the internal nucleon structure using the Quark Meson Coupling (QMC) model of Guichon [35]. In these relativistic models the nucleon form factor is changed in the medium due to vacuum po- larization with N N pairs. Up to now the Coulomb corrections for inclusive ex- periments have been evaluated theoretically by two in- dependent groups, one from Trento University [36,37] and the other from Ohio University [38]. The Trento group found that the Effective Momentum Approxima- tion (EMA) agrees with DWBA with an accuracy better than 1%, while the Ohio group derived significant correc- tions beyond EMA. All useful quantities and equations are defined in [36,37,39,40]. A detailed discussion of the differ- ent theoretical approaches can be found in [37]. Previous extractions of R L and R T were performed either without Coulomb corrections in [14,15] or by applying the Trento group calculations [17], or by applying the Ohio group calculations [12,41]. This led to questionable results even when Coulomb corrections from either groups were ap- plied, particularly in the region beyond the quasi-elastic peak known as the “dip region”, since meson exchange