Modification of Vector Mesons in Nuclear Matter measured in 12 GeV p+A reactions at KEK-PS

摘要

We have measured invariant mass spectra of electron-positron pairs and K+ K- pairs in 12 GeV p+A interactions at the KEK Proton-Synchrotron. The aim of the experiment is to detect in-medium modification of vector mesons, which is theoretically predicted as a consequence of partially-restored chiral symmetry. We have observed clear peaks of phi -->K+K- and w -->e(+)e(-). The invariant mass spectra of phi -->K+K- have been well described with known physical processes. In the e(+)e(-) invariant-mass spectra, we have observed a statistically significant excess for the copper target. below the w peak over the known physical processes, which indicates that the spectral shape of meson is modified at normal nuclear-matter density. [References: 13]