First observation of ϕ-meson mass modification in nuclear medium

We have measured e(+)e(-) invariant mass spectra in 12 GeV p + A interactions at the KEK Proton-Synchrotron. The aim of the experiment is to detect possible in-medium modification of vector mesons at normal nuclear density. We used carbon and copper targets to study the nuclear size dependence of the e(+)e(-) invariant mass spectra. A significant enhancement on the low-mass side of the phi meson peak was observed in the low beta gamma region (beta gamma(phi) < 1.25) in the copper data. On the other hand, in the higher beta gamma region (beta gamma(phi) < 1.25), both the spectral shapes obtained from carbon and copper targets were consistent with the expected one from the simulation. This observation is consistent with a picture of the phi modification in a nucleus, i.e. normal nuclear density, because such an effect should be visible only for slowly moving mesons produced in a larger nucleus.