We have measured invariant mass spectra of electron-positron pairs in the
target rapidity region of 12GeV p+A reactions. We have observed a significant
difference in the mass spectra below the ω meson between p+C and p+Cu
interactions. The difference is interpreted as a signature of the ρ/ω
modification at normal nuclear-matter density.Comment: 8 pages, 3 figures, submitted to Phys. Rev. Letters. all the figures
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