  The invariant mass spectra of e+e- pairs produced in 12-GeV proton-induced
nuclear reactions are measured at the KEK Proton-Synchrotron. On the low-mass
side of the omega meson peak, a significant enhancement over the known hadronic
sources has been observed. The mass spectra, including the excess, are well
reproduced by a model that takes into account the density dependence of the
vector meson mass modification, as theoretically predicted.
