Observation of meson modification in nuclear matter at KEK-PS

We have measured invariant mass spectra of e+e- pairs and K+K- pairs in 12-GeV p+A interactions at KEK 12-GeV Proton-Synchrotron (PS). The aim of the present experiment is to detect in-medium modification of vector mesons, which is theoretically predicted as a consequence of a partial restoration of the chiral symmetry in nuclear matter. We have observed clear peaks of φ→K+K-, ω→e+e- and φ→e+e- in the invariant mass spectra. The φ→K+K- invariant mass spectra have been well described with known physical processes. The nuclear mass-number dependence of the production cross section of φ→K+K- follows Aα (α=1.01±0.09). In the e+e- invariant mass spectra, we have observed a statistically significant excess for the copper target below the ω peak over the known physical processes. The excess indicates that the spectral shape of meson is modified at the normal nuclear matter density