Experimental signature of in-medium mass modification of vector mesons at normal nuclear density

We measured invariant mass spectra of e+e− pairs produced in 12 GeV p + A interaction to investigate in-medium mass modification of vector mesons. We observed an enhancement over the known hadronic sources on the low-mass side of the ω meson mass peak. The obtained ρ/ω ratio indicates that this enhancement mainly results from ρ mesons whose mass was modified in nuclear matter. We performed a toy model calculation based on the prediction using the QCD sum rule. It reproduced well the tendency of our data.