Observation of rho/omega meson modification in nuclear matter.

We measured the invariant mass spectra of electron-positron pairs in the target rapidity region of 12-GeV $p+A$ reactions. We have observed a significant difference in the mass spectra below the $\ensuremath{\omega}$ meson between $p+\mathrm{C}$ and $p+\mathrm{Cu}$ interactions. This difference indicates that the spectral shape of mesons is modified at normal nuclear-matter density.