In-medium behaviour of vector mesons and the longitudinal and transverse response functions in (e, e'p) reactions

Abstract

The electromagnetic form factors of nucleons appear dominated by vector mesons at momentum transfers smaller than ∼1 |GeV/ c. It is therefore expected that measurements of quantities involving the electromagnetic form factors of nucleons embedded in nuclei will be sensitive to changes in vector-meson properties arising from their interaction with the medium. Longitudinal and transverse response functions measured in quasi-elastic (e, e'p) reactions provide such data for two very different operators, the charge and the current densities. We show that a decrease of vector-meson masses in the medium, consistent with present expectations about chiral-symmetry restoration in nuclei, produces the quenching observed in the longitudinal response of light systems ( 3He, 4He) and part of this quenching for heavier nuclei ( 40Ca) where nuclear correlations are expected to generate an additional suppression of the longitudinal response. The transverse response is almost unchanged, which is in agreement with the data. Difficulties in extracting very quantitative information on the in-medium behaviour of vector mesons from (e, e'p) data are pointed out.