Evidence for in-medium modification of the meson at normal nuclear density

Invariant mass spectra of e+e- pairs have been measured in 12 GeV p+A reactions to detect possible in-medium modification of vector mesons. Copper and carbon targets are used to study the nuclear-size dependence of e+e- invariant mass distributions. A significant excess on the low-mass side of the meson peak is observed in the low βγ(=β/1-β2) region of mesons (βγ<1.25) with copper targets. However, in the high βγ region (βγ>1.25), spectral shapes of mesons are well described by the Breit-Wigner shape when experimental effects are considered. Thus, in addition to our earlier publications on ρ/ω modification, this study has experimentally verified vector meson mass modification at normal nuclear density.