Medium Modi cation on Vector Mesons Observed in 12 GeV p + A Reactions

The invariant mass spectra of e+e− pairs produced in 12 GeV p+A reactions are measured at the KEK‐PS. We observed a significant enhancement over the known hadronic sources on the low‐mass side of the ω meson peak. The 95 % C.L. allowed parameter regions for ρ/ω ratio are obtained as ρ/ω < 0.15 and ρ/ω < 0.31 for C and Cu targets, respectively. As for the φ meson, the data obtained with a Cu target revealed a significant excess on the low‐mass side of the φ meson peak mainly in the βγφ < 1.25 region.