Study Of In-Medium Meson Modification In 12 Gev P+A Reactions

Invariant mass spectra of low- mass vector mesons in the e(+) e(-) channel are measured in 12 GeV p + C and p + Cu reactions (root s = 5.1 GeV) at the KEK Proton Synchrotron. Significant excess is observed at the low- mass side of omega meson peak over the known hadronic sources both in the C and Cu target data. Also below the phi meson peak, an excess is found in the Cu target data for slowly moving mesons whose beta gamma is less than 1.25. They can be interpreted as signatures of the spectral modi. cation of vector mesons in cold nuclear matter.