Effect of nucleon structure variation on the longitudinal response function

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Using the quark-meson coupling (QMC) model, we study the longitudinal response function for quasielastic electron scattering from nuclear matter. In QMC the coupling constant between the scalar (σ) meson and the nucleon is expected to decrease with increasing nuclear density, because of the self-consistent modification of the structure of the nucleon. The reduction of the coupling constant then leads to a smaller contribution from relativistic RPA than in the Walecka model. However, since the electromagnetic form factors of the in-medium nucleon are modified at the same time, the longitudinal response function and the Coulomb sum are reduced by a total of about 20% in comparison with the Hartree contribution. We find that the relativistic RPA and the nucleon structure variation both contribute about fifty-fifty to the reduction of the longitudinal response.