Observation of Ρ/ω Meson Modification in Nuclear Matter

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 Present Address: Center for Nuclear Study, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan, email: [email protected] Present Address: ICEPP, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan Present Address: Physics Department, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan Present Address: Fujitsu Corporation, 4-1-1, Kamikodanaka, Nakahara, Kawasaki, Kanagawa 211-8588, Japan Present Address: RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Present Address: Xaxon Corporation,1-3-19, Tanimachi, Chu-ou, Osaka, Japan Present Address: Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 , Japan